Security feature, printing ink, document of value, and authenticity checking method

Abstract

Security feature for protecting a document of value based on a transition-metal-containing ESR-active material that has an alkaline-earth-metal-containing or zinc-containing host lattice with a garnet structure or perovskite structure and has the following general molecular formula EaRbTcXdQeZfO12, where E is at least one alkaline earth metal chosen from the elements Mg, Ca, Sr, Ba or is Zn and a meets the condition 0 < a ≤ 4; R is at least one rare earth metal chosen from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and b meets the condition 0 ≤ b ≤ 4; T is at least one transition metal chosen from the elements Cr, Mn, Fe, Cu and c meets the condition 1 ≤ c ≤ 5; X is at least one trivalent codopant chosen from the elements B, Al, Ga, Bi and d meets the condition 0 ≤ d ≤ 4; Q is at least one tetravalent codopant chosen from the elements Si, Ge, Sn, Ti, Zr, Hf and e meets the condition 0 ≤ e ≤ 4; Z is at least one pentavalent codopant chosen from the elements P, As, Sb, V, Nb, Ta and f meets the condition 0 ≤ f ≤ 4.

Description

[0001]Security feature, printing ink, valuable document and authenticity verification method The invention relates to a security feature for securing a valuable document, wherein the security feature contains an electron spin resonance active substance, hereinafter also referred to as an ESR-active substance or ESR substance. The invention further relates to a valuable document provided with the security feature, in particular a banknote. The invention also relates to a printing ink having the security feature for printing a valuable document and to a method for verifying the authenticity of a valuable document. In particular, the invention deals with security features which are suitable for high-speed detection. In the context of the invention, high-speed detection is understood to mean the detection and evaluation of the security feature in high-speed machines in which the valuable documents, e.g.Banknotes move at high speeds, e.g., 10 m / s or more. Here, often only a few milliseconds are available for a measurement instead of several seconds, which means that the security features are subject to special requirements. In the following, the term "high-speed processing" is used synonymously for the measurement of valuable documents using high-speed detection. To secure and authenticate valuable documents, such as banknotes, bank cards, ID cards, or passports, product packaging, and other products to be secured, such as medications, these are usually provided with machine-detectable security features, such as magnetic materials. For this purpose, ESR-active materials can be used, for example. ESR-active materials are understood to be substances that contain one or more unpaired electrons.In ESR-active materials, increased microwave absorption can be measured when a suitable magnetic field is applied at suitable wavelengths. In the following, the abbreviations ESR and EPR (electron paramagnetic resonance, also referred to as paramagnetic electron resonance) are used synonymously. An ESR measurement typically determines the so-called g-value (or effective g-value) of a signal, where g is also known as the gyromagnetic factor or Landé factor and has a value of approximately 2.0023 for an unaffected free electron. Depending on the electron's environment and other influencing factors (e.g., interaction with local or neighboring electron spins and nuclear spins, spin-orbit coupling, crystal field, etc.), the measured g-value can exhibit significant deviations from g = 2, allowing conclusions to be drawn about the nature of the spin-bearing center and its surroundings.In the context of this invention, “spin-carrying center” refers in particular to ESR-active transition metal cations with unpaired electrons, which generate the detected ESR signal; the synonymous term “spin-carrying transition metal center” is therefore also used. Various inorganic substances with ESR signals at different g-values ​​are known to the person skilled in the art. For example, substances with so-called axial or rhombic centers of the ESR-active element typically have g-values ​​in a range of 4 to 6, while weakly influenced spin centers often have g-values ​​in a range of 1.9 to 2.1. Substances with more complex band patterns and / or multiple signal lines in the microwave absorption spectrum are also easy for the person skilled in the art to produce. For example, it is known that substances which contain Mn in low concentrations 2+ show a spectral splitting of the ESR signal. For example, in the case of Mn 2+-salts in aqueous solution, where the ESR signal is caused by interaction of the group spin S = 5 / 2 and the nuclear spin I = 5 / 2 of the Mn 2+is split into 30 individual lines. It is known to dope inorganic host lattices with small amounts of ESR-active cations, which then exhibit a measurable ESR signal. However, in this form, they are not suitable as ESR substances for use as security features in high-speed detection due to the weak ESR signal intensities. However, increasing the concentration of the spin-carrying center leads to changes in the ESR spectrum. Starting with an inorganic host lattice, which displays a complex, albeit weak, signal pattern at a low concentration of the spin-carrying center, this original signal pattern disappears at higher concentrations, and instead, in most cases, a single strong signal is measured at a g-value of approximately 2.Figure 1 shows, using an yttrium aluminum garnet as an example, how the originally complex and characteristic ESR spectrum increasingly loses detail with increasing iron concentrations, ultimately exhibiting only a single, broad ESR signal at g = 2 at even higher iron concentrations. This is disadvantageous for measurements that are not subject to the limitations of high-speed detection, such as a detailed analysis of an ESR spectrum recorded over several minutes. As the concentration increases, the originally complex ESR spectrum with multiple signals disappears, which can be used to distinguish between different substances.Since, at high transition metal concentrations, generally only a simple ESR spectrum with a single signal at g = 2 is obtained, different substances can no longer be easily distinguished unless special selections are made. Surprisingly, the substances according to the invention described here still generate easily recognizable, characteristic ESR signals even at very high transition metal concentrations. Since the detectable intensity of an ESR signal correlates with the number of measurable spins in the substance, the highest possible concentration of the spin-bearing center in the host lattice is essential for high-speed detection. Otherwise, the signal is not strong enough to reliably identify the ESR security feature of the valuable document within the very short measurement times of high-speed detection.Furthermore, the detection and analysis of a complex ESR spectrum consisting of multiple narrowband individual signals, such as those exhibited by prior-art substances, is not readily technically feasible in high-speed detection, whereas substances according to the invention, with their simple ESR spectra, offer metrological advantages here, as will be explained below. The poor suitability of prior-art ESR-active substances with low transition metal concentrations for high-speed detection generally cannot be compensated for by greatly increased application quantities without incurring significant technical disadvantages. If a significantly higher amount of the ESR-active substance is incorporated into the substrate or printing ink, this changes the properties of the printing ink (e.g., viscosity) or the security document produced with it.For example, the higher proportion of the ESR-active substance can lead to the application site being visually recognizable due to its body color and no longer camouflaged, or the mechanical properties of the substrate can be negatively affected. A high concentration of the spin-carrying center in the host lattice of the substances according to the invention is therefore necessary to allow advantageous applicability as a security feature in high-speed processing. The associated loss of the complex spectrum is particularly advantageous with a g-value other than 2 or the presence of additional signal parameters specific to the respective substance, as explained below. As described above, a g-value of approximately 2 corresponds to the natural g-value of a free electron without further influences.State-of-the-art inorganic host lattices generally exhibit only such an ESR signal at sufficiently high concentrations of the spin-carrying center, since most influences on electrons are balanced or disappear on average due to exchange interactions. For simplicity, the following explanation will be based on "an ESR signal at g ≈ 2," since the ESR signal of such substances can deviate slightly from 2, e.g., g = 2.03. It is not readily possible to use such substances as a security feature, since the same or highly consistent and uncharacteristic ESR spectra are obtained with most host lattices.Surprisingly, it has been shown that with the substances according to the invention, with a suitable choice of stoichiometry, ESR signals at g ≈ 2 can be generated with specific signal shapes that differ significantly in a multi-parameter space and are therefore very well suited for securing valuable documents. Furthermore, with a suitable choice of stoichiometry, even ESR signals can be generated whose position differs significantly from g ≈ 2, e.g., g ≈ 3. The formulation g ≈ 2 means that minor deviations are permissible; in particular, a g value in a range of 1.95 to 2.05 is meant. The deviation ± 0.05 applies analogously in the case of g ≈ 3 and the like. Three preferred embodiments are now described which are possible with the ESR-active substances according to the invention: In a first preferred embodiment, the ESR-active substance has an ESR signal at a g value between 1.8 and 2.2, preferably at g ≈ 2.In this case, the substance has an ESR signal with a characteristic signal shape that can be clearly identified using parameters such as line width or asymmetry. This has the advantage that the ESR signal can be clearly assigned to the security feature, but is not immediately recognized as such due to the "usual" g value, and is therefore inconspicuous. In a second preferred embodiment, the ESR-active substance has an ESR signal that differs significantly from g ≈ 2, i.e. g > 2.2 or g < 1.8. This simplifies detection, among other things, because interfering signals from additional substances containing transition metals, which may be present in valuable documents, for example as impurities, color pigments, fillers, security features, etc., have less of an impact on the measurement.The substance preferably has an ESR signal with a characteristic signal shape that can be uniquely identified using parameters such as line width or asymmetry. In a third preferred embodiment, several different ESR substances are combined. For example, if two substances whose ESR signals strongly overlap can be used to generate a new ESR signal with defined parameters. If, however, two substances whose ESR signals do not overlap are combined, the two substances can be detected separately, for example to increase the number of possible codings. WO 2020 / 245280 A1 describes a product with an ESR-active substance for authentication and identification, as well as the production and method for authentication. In addition to organic radicals such as TEMPO, inorganic substances with Fe-O compounds and paramagnetic mineral substances and salts are described as substances.Based on the prior art, the present invention is based on the object of providing suitable inorganic ESR-active substances with high intensities and specific ESR signal shapes, in particular at different g-values, which are particularly well suited for use as security features. The present invention is particularly based on the object of providing suitable ESR-active substances that are characterized by high chemical, thermal, and optical stability, while at the same time exhibiting sufficiently intense signals and can be distinguished from ESR-active substances with a "standard signal" at g-values ​​of approximately 2. The invention is further based on the object of providing a valuable document provided with the above security feature, a printing ink having the above security feature, and a method for verifying the authenticity of a valuable document.This object is achieved by the feature combinations defined in the independent claims. Further developments of the invention are the subject of the dependent claims. Summary of the Invention 1. (First aspect of the invention) Security feature for securing a valuable document based on a transition metal-containing ESR-active substance or ESR substance that has an alkaline earth metal-containing or zinc-containing host lattice with a garnet structure or perovskite structure and the following general empirical formula (I): E. a R b T c X d Q e Z f O 12(I); where - E is at least one alkaline earth metal or Zn selected from the elements Mg, Ca, Sr, Ba and a satisfies the condition 0 < a ≤ 4; - R is at least one rare earth metal selected from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and b satisfies the condition 0 ≤ b ≤ 4; - T is at least one transition metal selected from the elements Cr, Mn, Fe, Cu and c satisfies the condition 1 ≤ c ≤ 5; - X is at least one trivalent codopant selected from the elements B, Al, Ga, Bi and d satisfies the condition 0 ≤ d ≤ 4; - Q is at least one tetravalent codopant selected from the elements Si, Ge, Sn, Ti, Zr, Hf, and e satisfies the condition 0 ≤ e ≤ 4; - Z is at least one pentavalent codopant selected from the elements P, As, Sb, V, Nb, or Ta, and f satisfies the condition 0 ≤ f ≤ 4. 2.(Preferred embodiment) Security feature according to clause 1, wherein the sum a+b+c+d+e+f assumes a value in a range from 7.5 to 8.5 and preferably the condition a+b+c+d+e+f = 8 is met. 3. (Preferred embodiment) Security feature according to clause 1 or 2, wherein with respect to the ESR-active substance having the general empirical formula (I), at least one alkaline earth metal E selected from the elements Mg, Ca, Sr, Ba is contained and a meets the condition 0.5 ≤ a ≤ 3.5, wherein the alkaline earth metal E is preferably Ca or Sr. 4. (Preferred embodiment) Security feature according to one of clauses 1 to 3, wherein, with respect to the ESR-active substance having the general empirical formula (I), at least one rare earth metal R selected from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu is contained and b satisfies the condition 0.5 ≤ b ≤ 3.5, wherein the rare earth metal R is preferably Y, Gd, Er, or Yb. 5.(Preferred embodiment) Security feature according to one of clauses 1 to 4, wherein with regard to the ESR-active substance having the general empirical formula (I), the transition metal T is at least one of the elements Cr, Mn, Fe, Cu and c satisfies the condition 2 ≤ c ≤ 5 and preferably satisfies the condition 3 ≤ c ≤ 5. 6. (Preferred embodiment) Security feature according to one of clauses 1 to 5, wherein with regard to the ESR-active substance having the general empirical formula (I), the condition X = Al applies for the trivalent codopant X and d satisfies the condition 0.5 ≤ d ≤ 4. 7. (Preferred embodiment) Security feature according to one of clauses 1 to 6, wherein, with respect to the ESR-active substance having the general empirical formula (I), the tetravalent codopant Q is at least one of the elements Sn, Ti, and Zr, and e satisfies the condition 0.5 ≤ e ≤ 4. 8.(Preferred embodiment) Security feature according to one of clauses 1 to 7, wherein with respect to the ESR-active substance having the general empirical formula (I), the condition Z = Nb applies to the pentavalent codopant Z and f satisfies the condition 0.25 ≤ f ≤ 4. 9. (Preferred embodiment) Security feature according to one of clauses 1 to 8, wherein with respect to the ESR-active substance having the general empirical formula (I), the condition T = Fe applies to the transition metal T. 10. (Preferred embodiment) Security feature according to one of clauses 1 to 9, wherein the security feature has a spin density of at least 1.5 mol / kg, preferably at least 5 mol / kg, where spin density means the number of unpaired transition metal electrons per unit mass of the substance having the general empirical formula (I) multiplied by a factor of 1 / 2. 11.(Preferred embodiment) Security feature according to one of clauses 1 to 10, wherein the ESR-active substance is in powder form and has a grain size D99 in the range from 0.5 to 30 µm, preferably in the range from 1 to 30 µm and particularly preferably in the range from 3 to 20 µm. 12. (Preferred embodiment) Security feature according to one of clauses 1 to 11, wherein the ESR-active substance is based on a transition metal-containing host lattice with a perovskite structure. 13. (Preferred embodiment) Security feature according to one of clauses 1 to 11, wherein the ESR-active substance is based on a transition metal-containing host lattice with a garnet structure. 14. (Preferred embodiment) Security feature according to one of clauses 1 to 13, wherein the ESR-active substance is non-luminescent. 15. (Preferred embodiment) A security feature according to any of clauses 1 to 13, wherein the ESR-active substance is luminescent. 16.(Preferred embodiment) Security feature according to one of clauses 1 to 15, wherein the ESR-active substance has an ESR signal g > 2.2 or g < 1.8. 17. (Preferred embodiment) Security feature according to one of clauses 1 to 16, wherein the security feature is based on a composition of two or more different ESR-active substances, each of which has the general molecular formula (I) and has different signal shapes in the ESR spectrum, in order to form a coding in this way. 18. (Second aspect of the invention) Value document with a security feature according to one of clauses 1 to 17. 19. (Preferred embodiment) Value document according to clause 18, wherein the security feature is incorporated into the substrate of the value document. 20. (Preferred embodiment) A security document according to Clause 18, wherein the security feature is printed on the security document in the form of a printing ink. 21.(Preferred embodiment) A value document according to any one of clauses 18 to 20, wherein the value document is a banknote. 22. (Third aspect of the invention) Printing ink comprising the security feature according to any one of clauses 1 to 17. 23. (Preferred embodiment) Printing ink according to clause 22, wherein the ESR-active substance is in the form of particles with a grain size D99 in the range of 0.5 to 5 µm. 24. (Fourth aspect of the invention) A method for checking the authenticity of a value document according to any one of clauses 18 to 21, comprising a) the step of carrying out a measurement on the value document using electron spin resonance (ESR) spectroscopy in order to generate ESR measurement values; b) the step of assessing the authenticity of the value document based on the ESR measurement values ​​obtained in step a).Detailed Description of the Preferred Embodiments The present invention relates to a security feature for securing valuable documents, wherein the security feature contains an ESR-active substance which is based in particular on a transition metal-containing, alkaline earth metal-containing, or zinc-containing host lattice with a garnet or perovskite structure. For the sake of simplicity, the term "ESR substance" is used herein instead of the phrase "ESR-active substance." By a suitable choice of stoichiometry, different spectral signal positions and signal shapes with high intensities are generated. The high intensity enables the ESR substance to be detected mechanically when introduced into the substrate of a valuable document, for example, into paper or polymer, preferably into paper, or when a valuable document is printed with a printing ink containing the ESR substance, even at high processing speeds.In particular, machine detection can be achieved over the entire surface. The present invention is based on the surprising discovery that ESR materials according to the present invention, in contrast to conventional ESR materials, exhibit very specific ESR spectra even at very high concentrations of spin-carrying transition metal centers, the signals of which are particularly advantageous for high-speed processing. The invention particularly relates to inorganic ESR materials from the material classes of transition-metal-containing garnets and perovskites, which are suitable as security features (hereinafter also referred to as ESR features or ESR security features) for authenticating and securing valuable documents using ESR techniques.The ESR materials from the aforementioned material classes have the following advantageous properties: - A high number of spins from unpaired transition metal electrons per mass fraction of the ESR material, hereinafter also referred to as "spin density", and thus high intensities of the detected ESR signal. - Special ESR spectra suitable for high-speed detection, usually with only a single main signal, which have a suitable signal width range. - The setting of different g-values ​​is possible, preferably g-values ​​in the range 1.2 to 7, particularly preferably in the range 1.5 to 5, such as in the range 1.5 to 4 or in the range 1.5 to 3.5 or in the range 1.5 to 3.0, in particular g-values ​​> 2.2 or g-values ​​< 1.8. - The setting of different signal shapes is possible, in particular the setting of parameters for the asymmetry of the signal shape and the line width.To achieve the above properties, the ESR materials according to the invention preferably have a high molar fraction of transition metals of 50,000 ppm or more, more preferably 100,000 ppm or more, and especially preferably 150,000 ppm or more. The ESR materials preferably contain at least one alkaline earth metal with a molar fraction of 25,000 ppm or more. The ESR materials preferably contain at least one rare earth metal with a molar fraction of 25,000 ppm or more. The transition metal content and its limits mentioned here refer only to the ESR-active transition metal. Other, non-ESR-active transition metals, which may also be present in the material, are not considered for the transition metal content. Preferred compositions and element selections for the advantageous construction of the transition metal-containing garnet and perovskite structures are listed below.The preferred ESR materials of the transition metal-containing garnet and perovskite structures have the general molecular formula EaRbTcXdQeZfO12, which satisfies the following conditions: - E is at least one alkaline earth metal or Zn selected from the elements Mg, Ca, Sr, Ba and a satisfies the condition 0 < a ≤ 4; - R is at least one rare earth metal selected from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and b satisfies the condition 0 ≤ b ≤ 4, preferably 0 < b ≤ 4; even more preferably 0 < b ≤ 3; - T is at least one transition metal selected from the elements Cr, Mn, Fe, Cu and c satisfies the condition 1 ≤ c ≤ 5; - X is at least one trivalent codopant selected from the elements B, Al, Ga, Bi, and d satisfies the condition 0 ≤ d ≤ 4, preferably 0 < d ≤ 4; more preferably 0 < d ≤ 3, particularly preferably 0 < d ≤ 2.- Q is at least one tetravalent codopant selected from the elements Si, Ge, Sn, Ti, Zr, Hf and e satisfies the condition 0 ≤ e ≤ 4, preferably 0 < e ≤ 4; even more preferably 0 < e ≤ 3, particularly preferably 0 < e ≤ 2; - Z is at least one pentavalent codopant selected from the elements P, As, Sb, V, Nb, Ta and f satisfies the condition 0 ≤ f ≤ 4, preferably 0 < f ≤ 4; even more preferably 0 < f ≤ 3, particularly preferably 0 < f ≤ 2; - the relationship a+b+c+d+e+f = 8 preferably applies. The formulation “a+b+c+d+e+f = 8” means that minor, e.g. Deviations caused, for example, by imperfections in the lattice are permissible. In particular, the sum a+b+c+d+e+f can be selected within a range of 7.5 to 8.5, with the sum a+b+c+d+e+f preferably assuming the value 8.The following restrictions are suitable for generating particularly preferred ESR substances with greatly differing g-values ​​and characteristic ESR signal parameters: Particular preference is given to containing at least one alkaline earth metal (or Zn) E with 0.1 ≤ a ≤ 3.5, such as 0.3 ≤ a ≤ 3.5, 0.5 ≤ a ≤ 3.5, 0.5 ≤ a ≤ 3.0, 1.0 ≤ a ≤ 3.0 or 1.3 ≤ a ≤ 3.0, wherein E is preferably Ca or Sr. Particular preference is given to containing E in the ESR substance in the oxidation state +2. Particular preference is given to containing at least one rare earth metal R, ie 0 < b ≤ 3.5. In embodiments, a rare earth metal R is included where 0.1 ≤ b ≤ 3.5, such as 0.3 ≤ b ≤ 3.5, 0.5 ≤ b ≤ 3.5 or 0.5 ≤ b ≤ 3.0, wherein R is preferably Y, Gd, Er or Yb. R is particularly preferably Y or Gd. R is particularly preferably present in the ESR substance in the oxidation state +3. T is particularly preferably Fe.Particularly preferably, Fe is present in the ESR substance in the oxidation state +3. Particularly preferably, at least one of the following cases a), b) and / or c) applies: a) X = Al with d ≥ 0.5; b) Q = Sn, Ti or Zr with e ≥ 0.5; c) Z = Nb with f ≥ 0.25. Particularly preferably, X is present in the ESR substance in the oxidation state +3, Q is present in the oxidation state +4 and Z is present in the oxidation state +5. In some embodiments, the structure of the empirical formula (I) contains at least five different elements (including oxygen (O)). Several different elements from one or more of the groups as defined for E, R, T, X, Q and Z can also be present. In some embodiments, at least four of E, R, T, X, Q, and Z are present in the structure of molecular formula (I), ie, at least four of a, b, c, d, e, and f are greater than 0 (> 0). For example, in embodiments, at least E, R, T, and X; or at least E, R, T, and Q; or at least E, R, T, and Z are present.Preferably, at least E, R, T, and Q are present. It has been shown that the ESR substance for use as a machine-readable security feature, e.g., in banknote processing at high speeds, advantageously has spin densities of at least 1.5 mol / kg, preferably at least 5 mol / kg. Spin density here refers to the number of unpaired transition metal electrons per unit mass of the ESR substance multiplied by a factor of 1 / 2. This allows the substances to be used in suitable quantities and still be reliably detected. Any unpaired electrons from rare earth metal cations of the ESR substance that may be present are not taken into account in the invention for calculating the spin density, since ESR signals from rare earth metal cations are generally not measurable at room temperature.In the case of ESR materials according to the invention with perovskite and garnet structures of the form EaRbTcXdQeZfO12, spin-carrying centers T with five unpaired electron spins (S = 5 / 2), such as Mn. 2+ or Fe 3+Suitable spin densities are typically achieved from a transition metal content c > 0.5. To ensure reliable detection, c ≥ 1, preferably c ≥ 2, particularly preferably c ≥ 3. This corresponds to a molar fraction of the spin-carrying transition metal center T in the ESR substance of 50,000 ppm or more, preferably 100,000 ppm or more, particularly preferably 150,000 ppm or more. The ESR substances according to the invention therefore have a significantly higher transition metal content than known in the prior art, where concentrations in the range of 1,000 ppm or less are typically used to obtain narrowband spectra. At the spin densities or transition metal contents mentioned, garnet and perovskite structures containing transition metals normally form the typical pattern of a single signal with a g-value of approximately 2.Surprisingly, it has been found within the scope of this invention that, through targeted adaptation of the stoichiometry and selection of the matrix elements and codopants, special ESR substances are obtained which exhibit particular signal shapes or a g-value that deviates significantly from 2. Since this behavior only occurs in substances with specifically selected element combinations and stoichiometry ranges, they are particularly well suited as a forgery-proof ESR feature. Therefore, the ESR substances according to the invention are a transition metal-doped or transition metal-containing inorganic host lattice from the class of garnets or perovskites. In addition to the aforementioned advantage of the variability of the achieved g-value and the signal shape, these substances exhibit advantageously high chemical, thermal, and optical stability, particularly compared to the less suitable ESR-active organometallic complexes and organic radicals.For example, the substances according to the invention are significantly more stable against the influence of organic solvents as well as acids and / or bases (chemical stability), significantly more stable against temperature treatment up to several hundred degrees Celsius (thermal stability), and significantly more stable against the effects of sunlight and / or UV radiation (optical stability) than ESR-active organic or organometallic substances. The ESR substance according to the invention is preferably in powder form and preferably has a D99 grain size in the range of 1 to 30 µm, particularly preferably in the range of 3 to 20 µm. Depending on the application, for example for incorporation into printing inks, smaller D99 grain sizes such as 0.5 to 5 µm can also be advantageous. For use as a security feature, the ESR substance is preferably incorporated into the substrate of the value document, so that a security feature that can be detected over the entire surface is obtained.In a first preferred embodiment, the substrate of the value document is paper, i.e. the ESR substance is added to the sheet stock, for example, during production of the banknote substrate in a paper mill. In a further preferred embodiment, the substrate of the value document is a polymer, i.e. the ESR substance is added, for example, during film extrusion in order to incorporate it into a polymer body. However, the ESR substance does not necessarily have to be contained in the substrate, but can also be applied to the value document as a coating, e.g. as a varnish, or be located in a partial layer or intermediate layer of the value document, for example within the ink-accepting layer of polymer banknotes or within an adhesive layer in multi-layer hybrid banknotes (e.g. foil / paper / foil composite banknote or paper / foil / paper composite banknote) or (identity) cards.Furthermore, the ESR substance does not necessarily have to be applied over the entire surface of the valuable document, but can also be present only within certain partial areas, e.g. in the form of a barcode. In a further preferred embodiment, the ESR substance is added to a printing ink in order to generate full-surface or local areas with an ESR signal as required. Furthermore, the ESR substance can be integrated into any security elements, such as security threads, holograms and (security) patches or (security) labels. In this form, it is possible to equip only certain partial areas of a valuable document with an ESR signal and to form an ESR security feature. According to a first preferred embodiment, the ESR security feature is applied over the entire surface. This advantageously secures the valuable document as a whole and prevents manipulation or forgery of partial areas.According to a second preferred embodiment, the ESR security feature is present only on partial areas. Here, comprehensive protection is dispensed with in order to advantageously obtain increased variability or additional coding options (e.g., barcodes for denomination separation) or to advantageously protect only certain, particularly critical areas (e.g., precise detection of the removal or tampering of a patch or label to protect a valuable document). The ESR substances according to the invention have special spectroscopic features that increase their suitability as ESR security features. These arise from the requirements of high-speed detection, which requires short measurement times and therefore has difficulty resolving complex ESR spectra with multiple different signals or a signal split into multiple signal lines, making unambiguous authentication impossible.While a complex ESR spectrum with multiple lines is advantageous for detailed analysis with measurement times of several seconds or longer, as it increases specificity and thus reliability, such spectra tend to be disadvantageous for high-speed detection. For example, the signal from complex spectra is distributed across multiple detection channels, making it even more difficult to achieve the minimum intensity per channel required for reliable detection. Furthermore, such spectra are usually very narrowband, necessitating very precise, magnetic-field-selective or wavelength-selective detection, which is usually incompatible with the high processing speeds of 10 m / s or more.The ESR substances according to the invention therefore differ significantly in terms of their spectral properties from the ESR substances commonly used in the prior art to form security features. The ESR substances according to the invention preferably have ESR spectra with a maximum of three, particularly preferably a maximum of two ESR signals, and further preferably only a single ESR signal. This simplifies detection. Certain substances according to the invention display an ESR spectrum which, upon closer analysis, is composed of a superposition of two different ESR signals. However, the resulting superposition can be evaluated metrologically like a single ESR signal if deviations from the typical signal curve, such as additional smaller secondary maxima, are ignored and therefore does not lead to a significantly increased detection effort, such as, for example,with two separate, non-superimposed ESR signals, each of which must be evaluated individually. In a preferred embodiment, the ESR spectrum is constructed in such a way that it has an intense main signal and no separate secondary signals or only significantly less intense secondary signals, with the main signal accounting for > 90% of the total intensity of the ESR spectrum. Such an ESR spectrum appears relatively simple, but at the same time has the advantage that, when viewed appropriately, it is very specific for the respective ESR substance and can be detected mechanically at high processing speeds. Figure 2 shows a schematic ESR spectrum with a single ESR signal. This is an ESR spectrum as recorded by commercially available X-band laboratory spectrometers, for example when an ESR substance according to the invention is measured with them.The parameters for the ESR signal shape specified within the scope of the invention refer to such spectra recorded with an X-band ESR spectrometer, even if, in the case of high-speed detection, no complete ESR spectrum is measured, but rather, for example, only individual prominent points or regions of the spectrum, or if other microwave frequencies or measurement techniques are used. ESR spectra are typically represented as the first derivative of the microwave absorption at a fixed microwave frequency (in Figure 2: ordinate (y-axis): I) against the applied magnetic field (in Figure 2: abscissa (x-axis): B). The individual ESR signal thus shows a first region with a maximum and a second region with a minimum, which each correspond to the rise or fall of a microwave absorption band.At the maximum of the microwave absorption band, the derivative is zero; therefore, the magnetic field value used to calculate the g-value of an ESR signal lies where the signal intersects the x-axis (in Figure 2: g). The position of the g-value is usually not given as an absolute magnetic field strength, but rather normalized depending on the ESR spectrometer used, so that a characteristic ESR signal always has the same g-value regardless of the device used. Since the ESR resonance frequency of a substance depends linearly on the applied B-field, the representation of the absorption of a fixed microwave frequency as a function of the B-field is analogous to a representation of the frequency-dependent absorption for a fixed B-field, i.e., the microwave absorption spectrum. However, there are other relevant signal parameters besides the g-value, for example, the signal width (in Figure 2: W). It is determined by the distance between the two extremes of the ESR signal.The signal width can be normalized analogously to the g-value in order to obtain measured values ​​that are largely spectrometer-independent. For example, if the g-value 2 of the spectrometer used is 3600 Gauss, and the distance W is determined as 180 Gauss, the signal width W is 2*180 G / 3600 G = 0.1. The values ​​for W given here refer to such normalized values, which were determined with an X-band spectrometer. Furthermore, the symmetry of the ESR signal can be used to identify a substance, since the signals can be symmetrical or asymmetrical. The latter occurs, for example, when a microwave absorption has a steep and a more slowly falling edge. After differentiation, a difference in the absolute intensities of the signals at both extremes results (in Figure 2: p1 and p2). For example, a symmetrical signal has a ratio p1 / p2 = 1, while a strongly asymmetrical signal has a ratio p1 / p2 = 2.The relative ratio of the distances between the extrema and the g-value can also be used as a qualification parameter (in the image: p3 and p4). A strongly symmetrical signal has a ratio of p3 / p4 = 1, an asymmetrical signal of p3 / p4 < 1 or p3 / p4 > 1. The signal width W is preferably more than 0.02, particularly preferably more than 0.05. A certain minimum signal width is advantageous for high-speed detection in order to obtain a usable signal even in the event of deviations and interference, such as a variation in the distance to the sensor due to a bent or wavy value document or magnetic field inhomogeneities. Otherwise, signals that are too narrow can escape the measuring range even with small displacements, e.g. because the banknote is now exposed to a different magnetic field strength than that intended for the measurement due to a change in its spatial position in an area with an inhomogeneous magnetic field.The signal width W is preferably less than 1, particularly preferably less than 0.5. A maximum signal width is advantageous for high-speed detection because the signal becomes less specific with increasing width and, at a certain point, also becomes technically more difficult to detect, for example because only a small portion of the signal is within the sensor's measuring range. This makes it difficult to clearly identify a specific coding and separate it from other codings, or to distinguish the signal from background, interference, and measurement drift. While many ESR substances tend to have greatly broadened ESR signals when the concentration of transition metals is greatly increased, ESR substances according to the invention can exhibit a moderate signal width despite a very high transition metal concentration.This means that, despite a very high transition metal content, they still have a signal width W that is both greater than the minimum width described above and smaller than the maximum width described above, and are therefore particularly well suited for creating an ESR security feature. In a preferred embodiment, ESR substances with a strongly asymmetric ESR signal are used. Preferably, p1 / p2 is > 1.5, particularly preferably > 2. In a further preferred embodiment, p1 / p2 is < 0.8, particularly preferably < 0.5. This allows advantageous coding to be created. For example, by determining p1 and p2 at only two points on the ESR spectrum, a substance with a symmetrical signal with p1 / p2 = 1 can be distinguished from a similarly structured substance with the same g-value but an asymmetrical signal with p1 / p2 = 2.Analogously, substances with a similar structure but the same g-value but a different p3 / p4 ratio can be differentiated from one another by measuring at several points. In a preferred embodiment, p3 / p4 is > 2, particularly preferably > 5. In a further preferred embodiment, p3 / p4 is < 0.9. This allows advantageous coding to be created. Due to the advantageous ability of the ESR substances according to the invention to generate different signal shapes with different parameters p1, p2, p3, p4 and W depending on their structure, more coding can be created with the same detection effort (i.e. number of measuring points). Furthermore, safety is improved because measuring several parameters also increases substance specificity. For advantageous checking of the characteristic ESR signal shape orFor example, two or more measured values ​​can be generated for the described parameters of substances according to the invention in high-speed detection. This means that the absorption of the microwave radiation is determined for two or more different combinations of magnetic field and microwave frequency. For example, to determine the asymmetry of the ESR signal, only the points corresponding to the relative maxima and minima of the ESR signal can be evaluated. Preferably, at least two measured values ​​are determined to verify the ESR security feature, particularly preferably at least three measured values. Preferably, fewer than 20 measured values ​​are determined, particularly preferably fewer than 10 measured values. This strikes a compromise between the technical complexity of the detection or the available measurement time and the accuracy or specificity of the detection.To increase substance specificity, it is also very advantageous if the g-value of the main ESR signal differs significantly from g ≈ 2. As already mentioned, most prior art substances exhibit a signal in the range g ≈ 2 when the transition metal concentration is increased. As a result, this signal position is only slightly specific, and substances with a signal at g ≈ 2 without a characteristic signal shape are therefore less suitable for use as ESR features. However, combining them with other ESR substances or incorporating additional parameters, such as the aforementioned signal asymmetry, still allows the use of such substances as ESR security features. With the ESR substances according to the invention, it is surprisingly possible to generate different g-values ​​by varying their composition, which is advantageous in several respects.On the one hand, such signals can be differentiated from "normal" ESR-active substances with g ≈ 2 with less effort, and measurements with such substances are less susceptible to interference from impurities. On the other hand, substances with different g values ​​can be used to generate additional, separable codes by deliberately mixing them with substances that have a signal at g ≈ 2. To ensure reliable separation, the g value of the main ESR signal is preferably at least greater than 2.2 or less than 1.8. In a preferred embodiment, the g value is greater than 2.4, particularly preferably greater than 2.6. In another preferred embodiment, the g value is less than 1.7, particularly preferably less than 1.6. These increased distances are advantageous because they allow the signals to be clearly separated from one another during high-speed detection, even with poor resolution.Furthermore, material specificity is increased because it is easier to produce materials that differ only slightly from g = 2, whereas suitable ESR materials with high signal intensity, suitable spectral shape, and strongly differing g-values ​​require very specific compositions. This increases the reliability of the ESR characteristic. Larger changes in the g-value of a spin-carrying center are normally only possible through significant changes in symmetry and coordination sphere, e.g., when transitioning from a rhombic to an axial center. It is therefore surprising that the materials according to the invention exhibit strongly different g-values, even though they have the same structure type, e.g., a perovskite lattice, and the same transition metal, e.g., Fe, and differ only slightly in the type or amount of the selected co-dopants (see below). This behavior has not been observed for other materials or other structure types.Furthermore, even when the same stoichiometry is selected, deviations in the manufacturing process, such as a reaction temperature 100°C higher or lower, can lead to measurable deviations in the g-value and change other parameters, such as the signal width. This makes imitation difficult. Surprisingly, it was found that the g-value of the substances according to the invention also exhibits a strong dependence of the g-value on non-stoichiometric additives. If, for example, a component is added in slight excess when mixing the starting materials, the ESR spectrum of the resulting substance can exhibit strong deviations, even though other characterization parameters, e.g., grain size determination or X-ray diffraction, show no measurable changes. This behavior is particularly pronounced with the added amount of the transition metal starting material, e.g., Fe2O3 or Cr2O3.In a preferred embodiment, an excess of 10% or more of a starting material is used in the production of the ESR material, compared to the stoichiometric amount required for a garnet or perovskite structure. Particular preference is given to using an excess of 10% or more of the transition metal starting material. This allows the production of materials with particularly characteristic ESR spectra. The excess starting material can either be separated after synthesis (e.g., by mechanical methods such as sieving or chemical methods such as acid treatment), or it can remain in the product. In most cases, the excess transition metal starting material does not result in a detectable ESR signal after synthesis, or its contribution to the ESR signal is negligible, amounting to less than 5% of the total ESR signal.This property (dependence of the g-value on non-stoichiometric additives) of the ESR materials according to the invention significantly increases the security of ESR features and security features produced from them, since even with a known composition of the material, replication is not possible without knowledge of the exact manufacturing parameters. Since this behavior occurs particularly in perovskite structures, the ESR material in a preferred embodiment is a perovskite structure. The general molecular formula (I) of the materials according to the invention can be given as follows, whereby the idealized charge ratios of the cations and anions in the structure have been given for the following explanation: E. a R b T c X d Q e Z f O 12 (I), where E = alkaline earth cation selected from Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ or Zn 2+or mixtures thereof; R = rare earth metal cation selected from Sc 3+ , Y 3+ , La 3+ , Ce 3+ , Pr 3+ , Nd 3+ , PM 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , He 3+ , Tm 3+ , Yb 3+ , Lu 3+ or mixtures thereof; T = transition metal cation selected from Cr 3+ , Mn 2+ , Fe 3+ , Cu 2+ or mixtures thereof; X = trivalent cation selected from B 3+ , Al 3+ , Ga 3+ , Bi 3+ or mixtures thereof; Q = tetravalent cation selected from Si 4+ , Ge 4+ , Sn 4+ , Ti 4+ , Zr 4+ , Hf 4+ or mixtures thereof; Z = pentavalent cation selected from P 5+ , As 5+ , Sb 5+ , V 5+ , Nb 5+ , Ta 5+ or mixtures thereof; O = oxygen, ie O 2-. The following preferred ratios apply to idealized garnet and perovskite structures: a+b+c+d+e+f = 8. Furthermore, the following idealized charge ratios apply: 2*a + 3*b + L*c + 3*d + 4*e + 5*f = 24; where L = charge number of the transition metal cation. The person skilled in the art is aware that, especially in garnet and perovskite structures, significant deviations from these idealized ratios may occur in certain cases, e.g., in the case of defect structures (which will be discussed separately later) or in the case of the tendency of certain cations to assume completely or partially different charge states in the lattice, such as Pr 3+ / Pr 4+ , Sm 2+ / Sm 3+ or Nb 4+ / Nb 5+and others. In the context of this invention, “garnet structure” means that the crystal lattice of the ESR material is similar to that of a garnet, in which the cations and oxygen anions form specifically linked tetrahedra, octahedra and dodecahedra. This does not mean that the material must necessarily belong to the mineral class of garnets, or that the charge distribution and type of cations must be analogous to “classic” garnets. The person skilled in the art is aware that garnet structures with many different elements are possible. For example, rare earth garnets such as YAG:Nd (neodymium-doped yttrium aluminum garnet) are used in laser technology. The symmetry group of the crystal system of the garnet structure can, for example, B. through the occupation order or through distortion of the position and shape of coordination polyhedra compared to "classical" garnets, without losing the overarching affiliation as a garnet structure.Likewise, "perovskite structure" refers to a common structural type consisting of interconnected octahedra and cuboctahedra. Here, too, it is known that perovskite structures can be massively distorted, for example, depending on the type and occupancy of the cations. For example, even the mineral perovskite (CaTiO3), which gives the structure type its name, exhibits a very strong distortion compared to the idealized structure. Here, too, it is known to those skilled in the art that, due to the great flexibility of the perovskite structure, many different elements can be used to construct it. For example, rare-earth perovskites such as cerium-doped yttrium aluminum perovskite (YAP) are used industrially as scintillators. In this case, too, the symmetry group of the crystal system can change compared to "classical" perovskites due to distortion of the position and shape of coordination polyhedra, without losing the overarching affiliation as a perovskite structure.For the sake of consistency, the stoichiometries of the ESR materials for both garnet and perovskite structures are given in this document with the same cation / anion ratio. e.g., an yttrium aluminum perovskite (YAP) is not represented as YAlO3 as usual, but as Y4Al4O12, and an yttrium aluminum garnet (YAG) is represented as Y3Al5O12 as usual. This allows garnets and perovskites to be represented by the same general formula EaRbTcXdQeZfO12. Whether a garnet or perovskite structure forms depends on the elements selected, their relative size ratios, their charges and coordination preferences, as well as the reaction conditions. For example, garnet structures generally have a 3:5 ratio of “large” cations (e.g., alkaline earth metals, rare earth metals, bismuth, etc.) to “small” cations, while perovskites have a 1:1 ratio.However, certain cations can change their positions and occupy both "large" and "small" sites; therefore, a precise assignment of lattice sites is dispensed with in this invention. In some cases, it is even possible to produce both garnet and perovskite structures with the same stoichiometry by varying the reaction conditions, e.g., by changing the reaction temperature. In a preferred application, substances with exact stoichiometric ratios are produced. The advantage is that such substances are easy to produce and deliver stable signals, thereby improving their use as a security feature. However, perovskite structures in particular tend to generate defects; therefore, deviations from the idealized stoichiometry can occur here. For example, charge balancing of cations can occur via a deficiency or excess of oxygen anions incorporated into the lattice.In this case, the number of oxygen cations in the molecular formula would be E. a R b T c X d Q e Z f O 12formally, for example, no longer exactly 12, but rather 11.6 or 12.8. Such structures are still generally referred to as perovskites or perovskite lattices or perovskite structures. It should therefore be noted that the information given here refers to idealized stoichiometries, which, depending on the manufacturing conditions and elements used in the application, may also tend to show non-stoichiometric deviations (e.g., due to a partial change in the oxidation numbers of the cations). However, the resulting materials are regarded as perovskite structures within the meaning of the invention as long as the basic perovskite crystal structure described above is formed, even if the stoichiometric ratio of the number of cations to the number of oxygen should not be exactly 8:12 upon closer examination. The same applies to superstoichiometric incorporation of cations into lattice gaps orCation defects, the extent of which is often technically difficult to control and determine. The ratio is preferably between 7.5:12 and 8.5:12, particularly preferably 8:12, since a high proportion of defects, e.g. due to structural inhomogeneities, can have a negative effect on the ESR signal, e.g. by excessively increasing the line width. The advantages of selecting certain types, amounts, and combinations of elements in order to obtain ESR materials with garnet and perovskite structures according to the invention are discussed below. The alkaline earth metal cation E is in particular a divalent alkaline earth metal cation selected from Mg, Ca, Sr, Ba, preferably selected from Ca and Sr, particularly preferably Ca. E is particularly preferably present in the ESR material in the +2 oxidation state. The ESR substance according to the invention preferably contains alkaline earth metal cations, ie a > 0. Particularly preferably 0.5 ≤ a ≤ 3.5.By incorporating significant amounts of alkaline earth metal cations (particularly in combination with another element Q or Z for charge balance), ESR materials can be produced which exhibit particularly specific signal shapes and particularly specific g-values, thus differing significantly from materials with g ≈ 2. It has been shown that Zn can be used as E in a similar way to alkaline earth metals, with alkaline earth metals being preferred due to their better incorporation behavior and lower tendency to form secondary phases. The rare earth metal cation R is a di-, tri-, or tetravalent rare earth metal cation. These are the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Trivalent rare earth metal cations are preferred. Particularly preferably, the rare earth metal cations are selected from Y, La, Nd, Gd, Er, Ho, Tm, Yb and more preferably from Y, Gd, Yb and Er.Particularly preferably, R in the ESR substance is in the oxidation state +3. With these rare earth cations, specific g-values ​​≠ 2 and characteristic signal shapes can be achieved (see examples). In a preferred embodiment, the ESR substance according to the invention contains a rare earth metal cation, i.e. b > 0. Particularly preferably, 0 < b ≤ 3.5 applies. Although many rare earth metal cations formally have an electron spin, they are not the detectable spin-bearing center within the meaning of this invention. Due to their physical properties, the ESR signals of rare earth metal cations are difficult or impossible to measure at room temperature using conventional methods. They therefore do not contribute to the calculation of the spin density for the substances according to the invention and are treated as spinless cations within the scope of the invention. Rare earth metal cations are also known for their luminescence properties.Their incorporation can, under certain conditions, lead to luminescent ESR substances. However, this is not always desired. In a first preferred embodiment, the ESR substance is a non-luminescent substance. As a result, the ESR substance cannot be detected by optical methods, and this advantageous property allows the security feature to be detected exclusively using ESR methods. In a second preferred embodiment, the ESR substance is a luminescent substance. This allows the ESR substance to be analyzed using optical methods, and, for example, the spectral signature of the luminescence emission can be used as an additional property of the ESR feature for verification. This is an advantageous property if a combined system of optical and ESR detection is to be used, for example, to generate additional coding.The transition metal cation T is the detectable spin-carrying center of the ESR substance. It thus forms the basis for detecting the feature. T can be any mono-, di-, tri-, tetra-, penta-, or hexavalent transition metal cation with unpaired electron spins. T is preferably a so-called Kramers system, i.e. it has an odd group spin S of 1 / 2, 3 / 2, or 5 / 2. Only such systems can be measured without major additional effort and have intense signals. S = 5 / 2 is particularly preferred, as particularly high spin densities can be achieved here, which improves detection. T is preferably selected from the transition metal cations Cr, Mn, Fe, and Cu, with the transition metal cations in the ESR substance being in the oxidation states Cr. 3+ , Cr 5+ , Mn 2+ , Mn 4+ , Fe 3+ , Cu 2+ Particularly preferably, T is selected from Mn 2+ , Cr 3+ and Fe 3+and more preferably T is Fe 3+. T is preferably present in the ESR substance in its high-spin configuration. The ESR substance necessarily contains a transition metal cation T, ie c > 0, such as c > 0.1 or c > 0.5. Preferably c ≥ 1, more preferably c ≥ 2, particularly preferably c ≥ 3. Further elements which do not have to be spin-carrying, but are required due to their charge and / or size to produce the substances according to the invention or their g-values, are the following cations: X = trivalent cations; Q = tetravalent cations; Z = pentavalent cations. Preferably, X is selected from B, Al, Ga and Bi or a mixture of two or more of the aforementioned elements. Preferably, X is present in the ESR substance in the oxidation state +3. Particularly preferably, X is Al. In a preferred embodiment, trivalent cations are used, ie d > 0, such as d > 0.1, particularly preferably d > 0.25, such as d > 0.5.Preferably, Q is selected from Si, Ge, Sn, Ti, Zr and Hf or a mixture of two or more of the above-mentioned elements. Preferably, Q is present in the ESR substance in the oxidation state +4. More preferably, Q is Sn, Ti or Zr or a mixture of two or all of the above-mentioned elements. In a particularly preferred embodiment, tetravalent cations are used, ie e > 0, such as e > 0.1, more preferably e > 0.25, such as e > 0.5. Preferably, Z is selected from P, As, Sb, V, Nb and Ta or a mixture of two or more of the above-mentioned elements. Preferably, Z is present in the ESR substance in the oxidation state +5. Z is particularly preferably Nb or Ta. Preferably, at least one of these elements is used for the construction of the structure, ie d+e+f > 0, particularly preferably d+e+f ≥ 0.5.In some embodiments, 0 < d+e+f < 4, such as 0.1 < d+e+f < 4 or 0.5 < d+e+f < 4, preferably 0.5 < d+e+f < 3.5, such as 0.5 < d+e+f < 3 or 0.5 < d+e+f < 2.5 or 0.5 < d+e+f < 2.0. This allows substances with specific g-values ​​and ESR signal parameters to be produced. In some embodiments, e+f > 0, such as e+f > 0.1, particularly preferably e+f ≥ 0.5. In some embodiments, 0 < e+f < 4, such as 0.1 < e+f < 4 or 0.5 < e+f < 4, preferably 0.5 < e+f < 3.5, such as 0.5 < e+f < 3 or 0.5 < e+f < 2.5 or 0.5 < e+f < 2.0. In a preferred embodiment, pentavalent cations are used, ie f > 0, such as f > 0.1, particularly preferably f ≥ 0.25, such as f > 0.5. This allows substances with specific g values ​​and ESR signal parameters to be produced.In addition to the tri-, tetra-, and pentavalent cations mentioned above, it is of course also possible to incorporate monovalent cations, such as alkali metal cations, or higher-valent cations, such as Mo or W in the +6 oxidation state, instead or in addition. While it cannot be ruled out that small, undefined traces of alkali metal may be incorporated into the structure, e.g., from the synthesis flux or from impurities, the intentional incorporation of significant amounts of alkali metal into the structure almost always leads to inferior chemical stability. Their incorporation is therefore not preferred. Likewise, higher-valent cations can theoretically be incorporated into the lattice, but do not lead to specific advantages that cannot be better achieved by the preferred tri-, tetra-, and pentavalent cations, whose incorporation is technically easier. Their incorporation is therefore also not preferred.Particularly preferred embodiments: In a particularly preferred embodiment, the ESR material EaRbTcXdQeZfO12 according to the invention is a rare earth-containing Ca-Nb-Fe perovskite with the following composition: E = Ca and a ≥ 2, R = Er or Y and b ≥ 0.5, T = Fe and c ≥ 2, Z = Nb and f ≥ 1. Such compositions can, among other things, achieve particularly high g-values ​​with g > 2.5. As shown in the exemplary embodiments, the achieved g-value also depends on certain reaction conditions. In a further particularly preferred embodiment, the ESR material EaRbTcXdQeZfO12 according to the invention is a rare earth-containing Ca-Nb-Fe-Al perovskite with the following composition: E = Ca and a ≥ 2, R = Er, Yb or Y and b ≥ 0.5, T = Fe and c ≥ 2, X = Al and d ≥ 0.1, Z = Nb and f ≥ 1. Such compositions can, among other things, result in particularly low g values ​​with g < 1.6.In a further particularly preferred embodiment, the ESR material EaRbTcXdQeZfO12 according to the invention is a garnet with at least one alkaline earth metal E = Ca or Sr and a ≥ 0.5, at least one rare earth metal R = Y, La, Gd, Yb, or Er and b ≥ 0.5, T = Fe and c ≥ 2 and at least one tetravalent cation Q = Sn, Ti, Zr with e ≥ 0.5. Such compositions can, among other things, form characteristic asymmetric ESR signals with particularly high or low p1 / p2 and p3 / p4 ratios. In a further particularly preferred embodiment, the ESR material EaRbTcXdQeZfO12 according to the invention is a garnet with at least one alkaline earth metal E = Ca or Sr and a ≥ 0.5, at least one rare earth metal R = Y, La, Gd, Yb, or Er and b ≥ 0.5, T = Fe and c ≥ 2 and at least one pentavalent cation Z = Nb or Ta with f ≥ 0.25.Such compositions can, among other things, form characteristic asymmetric ESR signals with particularly high or low p1 / p2 and p3 / p4 ratios, respectively. The ESR substances according to the invention are also particularly well suited for securing value document systems. For example, a value document class (e.g., a first denomination of a banknote currency) contains a first ESR substance with a g-value of 2.5, a second value document class (e.g., a second denomination of a banknote currency) contains a second ESR substance with a g-value of 2, and a third value document class (e.g., a third denomination of a banknote currency) contains a third ESR substance with a g-value of 1.5. As shown in the exemplary embodiments (e.g. Examples 1 to 5 or Examples 12 to 18), when using the ESR substances according to the invention, the first, second and third ESR substances of the value document system can have a very similar composition, ieThey exhibit only slight differences in their stoichiometry and yet have significantly different g-values ​​or other signal parameters. This makes analysis and imitation of the ESR substances used extremely complex and thus increases the security of the feature system. Furthermore, due to their largely identical composition, the substances also display comparable processing behavior when incorporated into the banknote (i.e., comparable grindability, comparable grain sizes, comparable surfaces, comparable printability, comparable density, comparable incorporation behavior, etc.). In addition to the small amounts required, this is a further advantage over the state of the art, since structurally significantly different substances would have to be used here, which could then differ greatly in their physical properties (e.g.Different wetting behavior, different stability of organometallic iron complex compared to inorganic iron pigment). Preferably, a value document system is produced with at least two value document classes, wherein the first value document class contains a first ESR substance and the second value document class contains a second ESR substance. Preferably, both the first and the second ESR substance have a perovskite or garnet structure. Since the physical properties of perovskites and garnets do not differ greatly from one another with a similar element composition, a first preferred embodiment is that the first ESR substance is a substance with a perovskite structure and the second ESR substance is a substance with a garnet structure. This facilitates the creation of coding with differently designed signals.However, the correspondence of the physical properties can be increased even further if both materials have a perovskite or a garnet structure. In a second preferred embodiment, both the first and the second ESR material have the same structure, i.e. both the first and the second ESR material have a garnet structure or both the first and the second ESR material have a perovskite structure. Particularly preferably, the deviation in the elemental composition between the first and the second ESR material is less than 20%, i.e. to generate the stoichiometry of the first ESR material from the second ESR material, fewer than 20% of the atoms present need to be changed.This makes it more difficult to distinguish between the two substances and, on the one hand, increases the security feature's security against counterfeiting. On the other hand, it ensures comparable applicability of the ESR substances used for coding. In a preferred embodiment, the first and second ESR substances have the same spin-active center T; this is particularly preferably Fe. 3+ . Da Fe 3+-Compounds are often used to secure valuable documents (e.g. magnetic pigments, security threads, color pigments or the like), the ESR features are better camouflaged or more difficult to detect and the feature security is increased. Valuable documents within the scope of the invention are objects such as banknotes, checks, shares, tokens, ID cards, passports, credit cards, certificates and other documents, labels, seals, and objects to be secured such as CDs, packaging and the like. The preferred field of application is banknotes, which are based in particular on a paper substrate or a polymer substrate. Further embodiments and advantages of the invention are explained below with reference to the figures.They show: Figure 1 schematic ESR spectra of Y3Al5-cFecO12 with increasing iron concentration c, with the corresponding magnetic field strengths for a signal with g = 2 and g = 3 marked on the x-axis; Figure 2 a schematic ESR signal; and Figure 3 a curve shape of the ESR spectra for embodiments 1, 3, 5, 7, 11, 12, 13 and 14; the section shown shows the signal range between g = 7 and g = 1.45. The present invention is explained in more detail below using specific embodiments. Example 1: Approx. 0,5 Y 2,5 Fe 4,5 Sn 0,5 O 120.652 g CaCO3, 3.680 g Y2O3, 4.684 g Fe2O3, and 0.982 g SnO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields an ESR substance 1, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​from Table 1. A corresponding ESR spectrum is shown in Figure 3 as spectrum 1. If such an ESR substance 1 is used in a valuable document as an ESR security feature, the g-value of 2.22, for example, can be used to distinguish it from other ESR substances with, for example, a g-value of 2.0.Likewise, the asymmetry of the signal, which is indicated by the p1 / p2 ratio of 1.68 and the p3 / p4 ratio of 0.89, can be used to distinguish it from other ESR substances, e.g. with a highly symmetrical signal with p1 / p2 = 1 and p3 / p4 = 1. Furthermore, the substance has a signal width W of 0.49 and can thus be distinguished from other substances with broader ESR signals, e.g. W = 1, and those with narrower ESR signals, e.g. W = 0.25. In the X-ray powder diffractogram, the ESR substance shows the reflections typical of garnet structures. For this and the following examples, the resulting structure is noted in Table 1. For use as an ESR security feature, a grain size D99 of 5 µm is produced by wet grinding in a ball mill. Alternatively, the grain size can be adjusted by dry grinding with a jet mill. The ESR material can then, for example,embedded in small quantities into the paper or polymer substrate of a valuable document, or, for example, added to a printing ink and then printed, for example, in the form of a barcode or fully onto the surface of a valuable document. Using an appropriate ESR sensor, the presence of the specific ESR security feature on the valuable document can now be verified. Example 2: CaY2Fe4SnO12 1.257 g CaCO3, 2.836 g Y2O3, 4.012 g Fe2O3, and 1.893 g SnO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water and dried at 80 °C.An ESR substance 2 is obtained which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​from Table 1. Use as a security feature is possible in a similar way to Example 1. Although ESR substance 2 is similar to ESR substance 1 from Example 1 in terms of element selection and composition, it exhibits a significantly different ESR spectrum and can therefore be clearly distinguished from it when used as an ESR security feature. For example, ESR substance 2 has a different g-value of 2.02 and a strong signal asymmetry with p1 / p2 = 0.68 and p3 / p4 = 7.29, which is thus opposite to the asymmetry of ESR substance 1. Furthermore, the signal width W is noticeably narrower at 0.31. Since even small deviations in the composition lead to large differences in the resulting ESR signal, analysis of the ESR security feature is made more difficult.Example 3: SrY2Fe4SnO12 1.750 g SrCO3, 2.677 g Y2O3, 3.786 g Fe2O3, and 1.786 g SnO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 is added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. An ESR material 3 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible, analogous to Example 1. ESR substance 3 is characterized in particular by a low ratio of p1 / p2 = 0.36, meaning that the relative heights of the left and right extrema of the signal are very different. The corresponding ESR spectrum is shown in Figure 3 as spectrum 3.Example 4: Ca2YSn2Fe3O. 122.344 g CaCO3, 1.322 g Y2O3, 2.805 g Fe2O3, and 3.529 g SnO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields an ESR substance 4, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible analogously to Example 1. ESR substance 4 is particularly characterized by a high ratio of p3 / p4 = 9.28. Example 5: Ca2ErSn2Fe3O12 2.147 g CaCO3, 2.051 g Er2O3, 2.569 g Fe2O3, and 3.233 g SnO2 are thoroughly mixed in an agate mortar.10 g of Na2SO4 are added as flux and mixed in an agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields ESR material 5, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible analogously to Example 1. ESR material 5 is characterized in particular by a low g-value of 1.75. The corresponding ESR spectrum is shown in Figure 3 as spectrum 5. Example 6: CaY2Fe4ZrO12 1.302 g of CaCO3, 2.938 g of Y2O3, 4.155 g of Fe2O3, and 1.603 g of ZrO2 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar.The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields ESR substance 6, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible analogously to Example 1. ESR substance 6 exhibits a similar ESR signal to ESR substance 2, but can be distinguished from it, for example, by the lower p3 / p4 ratio of 4.7. Example 7: Ca2YFe3Zr2O12 2.505 g CaCO3, 1.413 g Y2O3, 2.998 g Fe2O3, and 3.084 g ZrO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h.The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields an ESR substance 7, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible in a similar way to Example 1. ESR substance 7 is characterized in particular by its narrow signal width W of 0.07. The corresponding ESR spectrum is shown in Figure 3 as spectrum 7. Example 8: CaY2Cr2Ga2TiO. 121.343 g CaCO3, 3.030 g Y2O3, 2.040 g Cr2O3, 2.515 g Ga2O3, and 1.072 g TiO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 is added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. An ESR material 8 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible, analogous to Example 1. ESR substance 8 exhibits a similar ESR signal to ESR substance 10 and ESR substance 15b, but its stoichiometry differs significantly. This makes it possible, for example,Mixtures of ESR substance 10, ESR substance 15b, and ESR substance 8 can be used to adjust processing parameters such as the bulk density of the powder, etc., without significantly altering the resulting ESR signal. Furthermore, this complicates analysis of the security feature. Example 9: Sr2Y2Fe2Zr2O12 3.184 g SrCO3, 2.435 g Y2O3, 1.722 g Fe2O3, and 2.657 g ZrO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 is added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. An ESR substance 9 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible analogously to Example 1.Example 10: Ca0.5Yb3.5Fe3.5Zr0.5O12 0.463 g CaCO3, 6.381 g Yb2O3, 2.586 g Fe2O3, and 0.570 g ZrO2 are thoroughly mixed in an agate mortar. 10 g Na2SO4 is added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 h. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. An ESR substance 10 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​from Table 1. Use as a security feature is possible analogously to Example 1. Example 11: CaEr3Fe3.5Nb0.5O12: 1.315 g CaCO3, 7.539 g Er2O3, 3.672 g Fe2O3, and 0.873 g Nb2O5 are thoroughly mixed in an agate mortar. 10 g Na2SO4 are added as a flux and mixed in the agate mortar.The mixture is transferred to a corundum crucible and calcined at 1200 °C for 10 hours. The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. An ESR substance 11 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​from Table 1. Use as a security feature is possible analogously to Example 1. The corresponding ESR spectrum is shown in Figure 3 as spectrum 11. Example 12: Ca3.33Er0.67Fe2.33Nb1.67O12 5.145 g of CaCO3, 1.966 g of Er2O3, 2.873 g of Fe2O3, and 3.416 g of Nb2O5 are thoroughly mixed in an agate mortar. 10 g of Na2SO4 are added as a flux and mixed in the agate mortar. The mixture is transferred to a corundum crucible and annealed at 1200 °C for 10 h.The sintered cone is suspended in water (2 liters, 40 °C) to dissolve the sodium sulfate, filtered, washed with water, and dried at 80 °C. This yields an ESR substance 12 which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​shown in Table 1. Use as a security feature is possible in a similar way to Example 1. The corresponding ESR spectrum is shown in Figure 3 as spectrum 12. Example 13: Ca3.33Er0.67Fe2.33Nb1.67O12 with Fe2O3 excess This example demonstrates the use of an intentional Fe2O3 excess. The same batch as for Example 12 is prepared, but a relatively arbitrary amount of Fe2O3 is added (but at least 0.8 g additional Fe2O3), in this case the amount of 3.3 g Fe2O3 was chosen as the additional excess.An ESR substance 13 is obtained, which, when measured on a commercially available X-band ESR spectrometer, exhibits an ESR signal with the characteristic values ​​from Table 1. Use as a security feature is possible analogously to Example 1. The properties of the resulting ESR signal are essentially identical, regardless of the exact amount of Fe2O3 excess, but differ greatly from the ESR signal of the equivalent substance without excess (ESR substance 12). For example, the g-value shifts significantly from 2.01 to 2.86. The corresponding ESR spectrum is shown in Figure 3 as spectrum 13. The excess Fe2O3 can be detected as a secondary phase in the X-ray powder diffractogram after synthesis, but has no significant interference with the ESR signal, since the signal of the ESR substance is significantly stronger.Example 14: Ca3.33Er0.67Fe2.33-xAlxNb1.67O12 The same batch is prepared as in Example 13, but 0.5 g of Al2O3 is added. An ESR substance 14 is obtained which, when measured on a commercially available X-band ESR spectrometer, exhibits two ESR signals, with the main signal having the characteristic values ​​from Table 1. Use as a security feature is possible analogously to Example 1. The corresponding ESR spectrum is shown in Figure 3 as spectrum 14. This example is intended to demonstrate how it is possible to generate completely different ESR signals with a significant shift in the g-value in the highly suitable Ca-Er-Fe-Nb system from the same material base (Example 12) by making small changes such as excess Fe2O3 (Example 13) or adding small amounts of Al2O3 (Example 14).This makes it significantly more difficult to replicate a specific substance within this system, as the ESR spectrum depends heavily on the precise manufacturing conditions and stoichiometry. This increases the security of the corresponding ESR security feature. Example 15a: Approx. 3,33 Y 0,67 Fe 2,33 Nb 1,67 O 12 see example 15d. Example 15b: Ca 3,33 La 0,67 Fe 2,33 Nb 1,67 O 12See Example 15d. Example 15c: Ca3.33Gd0.67Fe2.33Nb1.67O12 See Example 15d. Example 15d: Ca3.33Yb0.67Fe2.33Nb1.67O12 The same batch as in Example 13 is prepared, but instead of 1.966 g Er2O3, the equimolar amount of another rare earth oxide is used: 1.160 g Y2O3 for Example 15a, 1.674 g La2O3 for Example 15b, 1.863 g Gd2O3 for Example 15c, and 2.025 g Yb2O3 for Example 15d. ESR materials are obtained which, when measured on a commercially available X-band ESR spectrometer, exhibit an ESR signal with the characteristic values ​​shown in Table 1. This example demonstrates how the resulting ESR spectrum can be influenced by varying the rare earth metal, even if it only accounts for a relatively small proportion of the total stoichiometry. Table 1: Characteristic values ​​of the ESR signals for the ESR materials of the exemplary embodiments 1-15d.Example g-value p1 / p2 p3 / p4 W Structure* 1 2.22 1.68 0.89 0.49 G 2 2.02 0.68 7.29 0.31 G 3 2.02 0.36 5.30 0.21 G 4 2.01 0.87 9.28 0.38 G 5 1.75 1.30 1.01 0.42 G 6 2.03 0.74 4.70 0.24 G 7 2.01 0.82 0.99 0.07 G 8 1.98 0.93 1.11 0.23 G 9 2.08 1.13 1.21 0.42 P 10 1.98 1.10 1.19 0.24 P 11 1.68 1.15 1.09 0.54 P 12 2.01 0.90 1.18 0.47 P 13 2.86 2.14 1.13 0.31 P 14 1.54 0.83 0.86 0.30 P 15a 2.82 2.69 1.33 0.35 P 15b 2.01 0.85 1.17 0.24 P 15c 2.05 0.81 1.36 0.51 P 15d 2.81 2.24 1.01 0.22 P * G = garnet structure, P = perovskite structure.

Claims

Patent Claims 1. Security feature for securing a valuable document based on a transition metal-containing ESR-active substance having an alkaline earth metal-containing or zinc-containing host lattice with a garnet structure or perovskite structure and the following general molecular formula (I): E a R b T c X d Q e Z f O 12(I); where - E is at least one alkaline earth metal or Zn selected from the elements Mg, Ca, Sr, Ba and a satisfies the condition 0 < a ≤ 4; - R is at least one rare earth metal selected from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and b satisfies the condition 0 ≤ b ≤ 4; - T is at least one transition metal selected from the elements Cr, Mn, Fe, Cu and c satisfies the condition 1 ≤ c ≤ 5; - X is at least one trivalent codopant selected from the elements B, Al, Ga, Bi and d satisfies the condition 0 ≤ d ≤ 4; - Q is at least one tetravalent codopant selected from the elements Si, Ge, Sn, Ti, Zr, Hf, and e satisfies the condition 0 ≤ e ≤ 4; - Z is at least one pentavalent codopant selected from the elements P, As, Sb, V, Nb, or Ta, and f satisfies the condition 0 ≤ f ≤ 4. 2.Security feature according to claim 1, wherein the sum a+b+c+d+e+f assumes a value in a range from 7.5 to 8.5 and preferably the condition a+b+c+d+e+f = 8 is met.

3. Security feature according to claim 1 or 2, wherein with respect to the ESR-active substance having the general empirical formula (I), at least one alkaline earth metal E selected from the elements Mg, Ca, Sr, Ba is contained and a meets the condition 0.5 ≤ a ≤ 3.5, wherein the alkaline earth metal E is preferably Ca or Sr.

4. Security feature according to one of claims 1 to 3, wherein, with respect to the ESR-active substance having the general empirical formula (I), at least one rare earth metal R selected from the elements Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu is contained, and b satisfies the condition 0.5 ≤ b ≤ 3.5, wherein the rare earth metal R is preferably Y, Gd, Er, or Yb. 5.Security feature according to one of claims 1 to 4, wherein with respect to the ESR-active substance having the general empirical formula (I), the transition metal T is at least one of the elements Cr, Mn, Fe, Cu and c satisfies the condition 2 ≤ c ≤ 5 and preferably satisfies the condition 3 ≤ c ≤ 5.

6. Security feature according to one of claims 1 to 5, wherein with respect to the ESR-active substance having the general empirical formula (I), the condition X = Al applies for the trivalent codopant X and d satisfies the condition 0.5 ≤ d ≤ 4.

7. Security feature according to one of claims 1 to 6, wherein, with respect to the ESR-active substance having the general empirical formula (I), the tetravalent codopant Q is at least one of the elements Sn, Ti, and Zr, and e satisfies the condition 0.5 ≤ e ≤ 4. 8.Security feature according to one of claims 1 to 7, wherein, with respect to the ESR-active substance having the general empirical formula (I), the condition Z = Nb applies to the pentavalent codopant Z and f satisfies the condition 0.25 ≤ f ≤ 4.

9. Security feature according to one of claims 1 to 8, wherein, with respect to the ESR-active substance having the general empirical formula (I), the condition T = Fe applies to the transition metal T.

10. Security feature according to one of claims 1 to 9, wherein the security feature has a spin density of at least 1.5 mol / kg, preferably at least 5 mol / kg, wherein spin density means the number of unpaired transition metal electrons per unit mass of the substance having the general empirical formula (I) multiplied by a factor of 1 / 2.Security feature according to one of claims 1 to 10, wherein the ESR-active substance is in powder form and has a grain size D99 in the range from 0.5 to 30 µm, preferably in the range from 1 to 30 µm and particularly preferably in the range from 3 to 20 µm.

12. Security feature according to one of claims 1 to 11, wherein the ESR-active substance is based on a transition metal-containing host lattice with a perovskite structure.

13. Security feature according to one of claims 1 to 11, wherein the ESR-active substance is based on a transition metal-containing host lattice with a garnet structure.

14. Security feature according to one of claims 1 to 13, wherein the ESR-active substance is non-luminescent.

15. Security feature according to one of claims 1 to 13, wherein the ESR-active substance is luminescent.

16. A security feature according to any one of claims 1 to 15, wherein the ESR-active substance has an ESR signal g > 2.2 or g < 1.

8. 17.A security feature according to any one of claims 1 to 16, wherein the security feature is based on a composition of two or more different ESR-active substances, each having the general molecular formula (I) and exhibiting different signal forms in the ESR spectrum, in order to thereby form a code.

18. A value document having a security feature according to any one of claims 1 to 17.

19. A value document according to claim 18, wherein the security feature is incorporated into the substrate of the value document.

20. A value document according to claim 18, wherein the security feature is printed on the value document in the form of a printing ink.

21. A value document according to any one of claims 18 to 20, wherein the value document is a banknote.

22. A printing ink comprising the security feature according to any one of claims 1 to 17.

23. Printing ink according to claim 22, wherein the ESR-active substance is in the form of particles with a grain size D99 in the range of 0.5 to 5 µm.

24. A method for verifying the authenticity of a value document according to any one of claims 18 to 21, comprising a) the step of performing a measurement on the value document using electron spin resonance (ESR) spectroscopy to thereby generate ESR measurement values; b) the step of evaluating the authenticity of the value document based on the ESR measurement values ​​obtained in step a).

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