Security feature, value document, and printing ink

Abstract

The invention relates to a security feature for safeguarding a value document, comprising a luminescent substance based on an inorganic host lattice, which is doped with Mn(V) as a luminophore, according to one of the formulas (I), (II), (III) or (IV): A5(XO4)3Hal:Doty (I), A2XO4Hal:Doty (II), A3(XO4)2:Doty (III), EZO4:Doty (IV), wherein y indicates the degree of doping of the luminophore and is selected from the range 0 < y < 0.1, and the elements for the host lattice and dopants are selected from the following list: A = Ca, Sr, Ba and the mixtures thereof; E = Sr, Ba and the mixtures thereof; X = P, V, As and the mixtures thereof; Z = S, Se and the mixtures thereof; Hal = F, CI and the mixtures thereof; and Dot = Mn(V).

Description

[0001] Security feature, value document and printing ink

[0002] The invention relates to a security feature for protecting a valuable document, wherein the security feature is in the form of a luminescent substance based on an inorganic host lattice doped with Mn(V) as a luminophore. The invention further relates to a valuable document provided with the security feature, in particular a banknote. The invention further relates to a printing ink having the security feature for printing a valuable document.

[0003] To secure and authenticate valuable documents, such as banknotes, bank cards, ID cards, and passports, and other products requiring protection against counterfeiting, such as medicines, these are often provided with luminescent substances. The luminescent substances are typically printed onto the valuable item in question or incorporated into a (paper) substrate present in the valuable item. Excitation with light of a suitable wavelength results in the emission of light of a different wavelength, whereby the light emission serves to authenticate the valuable item in question. Depending on the application, luminescent substances with different excitation and emission properties have been developed that are visually detectable and / or machine-readable. Higher security is offered by security features that are invisible to the naked eye (also known as "covert features").In particular, luminescent substances are used as visually imperceptible security features. These substances are excited and emit light in the visually imperceptible range, and their presence is therefore not visually detectable, or at least very difficult to detect. Furthermore, it is desirable that the particles underlying the visually imperceptible security features are not, or at least very difficult to detect, with the naked eye or with technical aids such as a magnifying glass. To achieve this, however, the luminescent substances must exhibit special properties with regard to body color, luminescence intensity, and pigment size, which are unattainable without a specific selection of the host lattice and elemental composition.

[0004] The present invention is based on the object of providing suitable luminescent substances for the authentication of valuables, in particular valuable documents, which, despite a low concentration of the luminophore, exhibit a particularly high luminescence intensity and also possess improved chemical resistance. In particular, the luminescent substances should have at most a faint body color and a small particle size, making them suitable for processing as a visually imperceptible security feature.

[0005] This object is achieved by the combinations of features defined in the independent claims. Further developments of the invention are the subject of the dependent claims.

[0006] Summary of the invention

[0007] 1. (First aspect of the invention) Security feature for securing a value document, comprising a luminescent substance based on an inorganic host lattice doped with Mn(V) as a luminophore according to one of the formulas (I), (II), (III) or (IV):

[0008] As(XO4)3Hal:Doty (I), A2XO4Hal:Dot y (II),

[0009] A3(XO4)2:Dot y (III),

[0010] EZO4:Dot y (IV), where y denotes the doping level of the luminophore and is chosen in a range 0 < y < 0.1 and the elements for the host lattices and dopants are selected from the following lists:

[0011] A = Ca, Sr, Ba and their mixtures;

[0012] E = Sr, Ba and their mixtures;

[0013] X = P, V, As and their mixtures;

[0014] Z = S, Se and their mixtures;

[0015] Hal = F, CI and their mixtures;

[0016] Dot = Mn(V).

[0017] 2. (Preferred embodiment) Security feature according to clause 1, wherein the security feature, when excited with radiation in the VIS / IR range, exhibits a characteristic emission of radiation outside the visible wavelength range, in particular in the IR range.

[0018] 3. (Preferred embodiment) Security feature according to clause 1 or 2, wherein the luminescent substance is in the form of particles with a particle size D99 < 20 μm, preferably D99 < 12 μm.

[0019] 4. (Preferred embodiment) Security feature according to clause 3, wherein the particles of the luminescent substance additionally have a particle size D50 <

[0020] 5 |im, preferably D50 < 3 μm. 5. (Preferred embodiment) Security feature according to one of clauses 1 to 4, wherein the mole fraction of the luminophore Mn(V) in the host lattice is less than 2%.

[0021] 6. (Second aspect of the invention) A valuable document comprising the security feature according to any one of clauses 1 to 5.

[0022] 7. (Preferred arrangement) A security document as defined in Clause 6, wherein the security document is a banknote.

[0023] 8. (Third aspect of the invention) Printing ink for printing a value document, wherein the printing ink comprises the security feature according to any one of clauses 1 to 5.

[0024] Detailed description of the preferred embodiments

[0025] The present invention relates to a security feature in the form of a luminescent substance which is based on an inorganic host lattice doped with Mn(V) as a luminophore. The luminescent substance has a characteristic emission spectrum outside the visually perceptible wavelength range, preferably in the IR range. The IR range here means in particular the range from 700 nm to 2500 nm. The luminescent substance can preferably be excited at at least one wavelength in the VIS / IR range, particularly preferably at at least one wavelength in the IR range. The VIS / IR range means in particular the range between 400 nm and 2500 nm. By targeted selection of the host lattices in combination with Mn(V) as a luminophore, luminescent substances are obtained which, despite an extremely low concentration of the luminophore (i.e. less than 2 wt.-% mole fraction of Mn(V) in the host lattice) exhibit a surprisingly high luminescence intensity and possess an advantageous, weakly pronounced body color. In addition, the luminescent substances exhibit improved chemical stability due to a targeted selection of the host lattices and their elemental compositions with regard to further processing in aqueous systems and incorporation into paper and polymer substrates. The improved chemical stability also allows the luminescent pigments to be used with a smaller particle size, i.e. D99 < 12 gm and D50 < 3 gm, without adverse effects due to an excessively large surface area and increased reactivity. In addition, visual detectability and analytical detectability (e.g.using a microscope) of the luminescent substances is reduced due to the small particle size, which makes them particularly suitable for creating invisible security features in valuable documents.

[0026] The basis for the selection according to the invention is EP 1 370424 B1, in which a high variety of luminescent substances with 17 preferred host lattices and five 3d 2 -Luminophores are disclosed. With reference to luminophores, Ti(II), V(III), Cr(IV), Mn(V), and Fe(VI) are mentioned. The preferred host lattices are selected, for example, from the classes of apatite, spodiosite, palmierite, forsterite, brushite, dahllite, ellestadite, francolite, monetite, morinite, whitlockite, wilkeite, voelckerite, pyromorphite, garnet, perovskite, olivine, silicates, titanates, vanadates, phosphates, sulfates, aluminates, and zirconates.

[0027] In the context of the present selection invention, the following selected luminescent substances according to one of the formulas (I) to (IV) have proven to be particularly advantageous and this selection is described below with regard to the elemental composition and its advantageous properties:

[0028] A5(XO4)3Hal:Dot y (I),

[0029] A2XO4Hal:Dot y (II),

[0030] A3(XO4)2:Dot y (III),

[0031] EZO4:Dot y (IV), where the elements for the host lattices and dopants are selected from the following lists:

[0032] A = Ca, Sr, Ba and their mixtures, ie A is in particular not Mg, Pb, Cd;

[0033] E = Sr, Ba and their mixtures, ie E is in particular not Mg, Ca, Pb, Cd;

[0034] X = P, V, As and their mixtures, ie X is in particular not Si, S, Cr;

[0035] Z = S, Se and their mixtures, ie Z is in particular not Cr, Mo, W;

[0036] Hal = F, CI and their mixtures, ie Hal is in particular not Br;

[0037] Dot = Mn(V), ie Dot is in particular not Ti(II), V(III), Cr(IV), Fe(VI).

[0038] It should be noted that the term "host lattice" refers to the undoped material, while the doped host lattice is referred to herein as the luminescent material.

[0039] The above term "not" means, with reference to the above element selection, that no starting materials containing these elements are added to the synthesis of the doped host lattices to influence the properties of the host lattice. However, the said elements may be present in the starting materials as impurities (in an amount < 0.1 atom%).

[0040] Regarding the halogens (Hal), it should be noted that, under certain synthesis conditions and starting materials, small traces of hydroxide ions (OH) may be incorporated into the host lattice and replace one of the halogens. Therefore, it should be noted that hydroxide ions (OH) may be incorporated as undesirable impurities. Preferably, less than 10% of the halogen content is replaced by hydroxide ions, particularly preferably less than 5%, and further preferably, no hydroxide ions are incorporated into the host lattice. Excessively high a proportion of hydroxide ions can have a detrimental effect on chemical stability and luminescence properties.

[0041] The doping level of the luminophore is based on the molecular formula 0 < y < 0.1, and thus the mole fraction of the luminophore in the host lattice is < 2%, with the weight fraction of the starting material used for the luminophore (e.g., MmCl, MnO, MnCl, MnCOa, MnCl, or KMnCl) based on the total amount of starting materials being less than 2 wt.%. The term "luminophore" is understood to mean that the emission of the doped host lattice emanates from this element (here, Mn(V)). The term "luminophore" is the preferred designation chosen herein; other equivalent terms include, for example, luminescence center, activator, or dopant.

[0042] The luminescent substances (1) to (4) according to the invention are particularly characterized by a high luminescence intensity, although the luminophore Mn(V) is used only in small molar proportions, as described. Surprisingly, the luminescence intensity of the luminescent substance does not decrease proportionally to the concentration of the luminophore in the concentration range used. This surprising effect is achieved by the targeted selection of the elements with respect to the host lattice and the luminophore. In comparison, the prior art EP 1 370424 B1 describes a wide variety of luminescent substances, which consistently have higher amounts of luminophore, with up to 20 wt. % based on the reactants, whereas the substances according to the invention have a maximum proportion of less than 2 wt. %.Furthermore, the specifically selected luminescent substances according to the invention exhibit sufficient chemical stability even with a small particle size, thus ensuring further processability in aqueous media. In comparison, the high variety of luminescent substances described in the prior art EP 1 370424 B1 has a wide range of disadvantages. Firstly, only a few host lattices are capable of stabilizing Mn(V) in small amounts, as required for substances according to the invention. Furthermore, even with host lattices that can successfully stabilize Mn(V) in small amounts, sufficient chemical stability is often not present, so that, for example, with a correspondingly small particle size reduction, depending on the processing conditions (i.e. temperature, pH, time, pressure, etc.), losses in luminescence intensity occur.Surprisingly, these disadvantages can be solved by the specific selection of substances according to the invention.

[0043] The use of the luminophore in small amounts has the further advantage, compared to the prior art EP 1 370424 B1, that manganese is preferentially incorporated as Mn(V), thus preventing the formation of other Mn valences and undesirable Mn-containing secondary phases (e.g., strongly colored Mn oxides such as MnO, MmOi, or the like). In powder form, the luminescent substances according to the invention exhibit a pale bluish hue characteristic of Mn(V), which, depending on the matrix interaction, can be shifted into the blue-green or blue-violet range. With increasing proportions of other-valent manganese species, both the Mn(V) luminescence and the bluish color impression are lost; in extreme cases, colorless or gray, green, yellow, or brown colored substances are obtained.The pale blue coloration of the luminescent substances according to the invention is not intense enough, so that detection of the security feature due to its body color is not possible with the naked eye and is only possible with great effort using technical methods, which would be the case with high Mn(V) concentrations and / or the presence of dark-colored, other-valent manganese species.

[0044] In contrast to other luminophores not selected according to the invention, manganese(V) offers the advantage that it exhibits high luminescence intensities in the non-visible spectral range even in small amounts. The luminescence intensities are only weakly pronounced in the isoelectric luminophore Fe(VI), for example. In addition, Fe(VI) can be converted into low-valent Fe species more easily than is the case with Mn(V) due to the incorporation of defects, impurities, and elevated temperatures, so that a reduced luminophore concentration is disadvantageous. This also applies to the other luminophores Ti(II), V(III), and Cr(IV), which are more difficult or insufficient to stabilize in low concentrations. The specific selection of the host lattices ensures sufficient stabilization in the +V valence in the case of manganese, so that incorporation is possible even at low concentrations without any loss of luminescence intensity.Based on the combination of specially selected cations, anions, halogens, the luminophore and the luminophore concentration, particularly advantageous luminescent substances are obtained which are particularly suitable as non-visible security features, which have a characteristic excitation and emission spectrum and are characterized by a high relative proportion of Mn(V) based on the total amount of manganese and an absent or only low relative proportion (< 5%) of other, lower or higher valent, oxidation states of manganese. If manganese were to be incorporated in oxidation states other than Mn(V), these would reduce the luminescence intensity of Mn(V) and promote the formation of crystal structures other than those of the selected host lattices. Furthermore, the presence of manganese in other oxidation states could lead to emissions in the visible region of the spectrum, such as those found, for example, infor Mn(II) are known, thus making it easier to find the security feature using optical methods.

[0045] Using Mn(II) as an example, the following describes why manganese in oxidation states other than Mn(V) is undesirable in the luminescent substances according to the invention. It should be noted, however, that the undesirable presence is not limited to Mn(II), but applies to all oxidation states of manganese that are lower than Mn(V) or higher than Mn(V). It is known that Mn is typically incorporated into apatites as Mn(II) and exhibits a yellow luminescence. Due to its VIS emission, not only in apatites but generally in luminescent substances (particularly upon excitation with UV light), the use of Mn(II) in phosphors is widespread. In the luminescent substances according to the invention, less than 5% of the manganese is present as Mn(II). The luminescent substances according to the invention preferably contain no Mn(II), but exclusively Mn(V). The presence of Mn(II) can be detected in addition to the VIS emission, e.g.This can also be determined by electron paramagnetic resonance (ESR or EPR). Mn(II) exhibits a signal with a g-factor of approximately 2, which can be measured either as a broadened single signal or, under suitable conditions, split into six separate lines. The luminescent substances according to the invention preferably do not exhibit any measurable signals at this point in the ESR spectrum, as would be typical for substances containing Mn(II).

[0046] For use as a non-visible security feature, it is advantageous that the particle size of the largest particles present is as small as possible, preferably D99 < 20 μm, in order to make it more difficult to find the security feature using optical methods. The term D99 refers to the particle size below which 99% by weight of the particles in a particle size distribution are smaller. The term D50 refers to the particle size below which 50% by weight of the particles in a particle size distribution are smaller. The particle size distribution of the luminescent substances according to the invention particularly preferably has a value of D99 < 12 μm, i.e. there are very few particles with a diameter greater than 12 μm. Such a particle size distribution, however, means that the average particle diameter D50 is typically within the range from 0.5 μm to 5 μm.If the particle size is too small, for example D50 = 2 pm, the particles may not be stable enough during processing to produce the security feature due to the increased reactivity and enlarged surface area, and can therefore only be processed effectively with particle sizes > 5 μm. The luminescent substances according to the invention are therefore selected such that the luminescent substances are stable during processing to produce the security feature even with small particle sizes D50 < 5 μm and are therefore well suited as invisible security features. The particle size distribution of the luminescent substances according to the invention particularly preferably has a value of D50 < 5 μm, especially preferably D50 < 3 μm. The particle size distribution of the luminescent substances according to the invention preferably has a value of D50 > 0.5 μm, especially preferably D50 > 1 μm.

[0047] With regard to the further processing of the luminescent substances according to the invention, the selection made, i.e. the combination of the specially selected cations, anions, halogens, the luminophore and the luminophore concentration, is associated with particular advantages. The selection made ensures that the luminescent substances according to the invention, even with small particle sizes, are sufficiently stable during processing to form a security feature in aqueous media and acidic and / or basic media, and possible losses of intensity upon contact with water or an acidic or basic solution are reduced. Compared to variants not according to the invention which differ from the selection made according to the invention (i.e. the combination of the specially selected cations, anions, halogens, the luminophore and the luminophore concentration), the increased stability during processing to form a security feature (e.g.Wet grinding, surface functionalization, formation of core-shell particles, incorporation into (foil) security elements or various (paper) substrates, etc.) remain advantageous because the desired purity of the Mn(V) is maintained and no other, lower-valent or higher-valent manganese species form through contact with different chemicals and / or aqueous media. For example, the luminescent substances according to the invention lose less than 10% of their luminescence intensity upon 30 minutes of contact with a hot aqueous solution (i.e., 50°C, pH 9), preferably no loss occurs, whereas significantly higher losses of 20% or more are possible with luminescent substances not according to the invention.The increased stability under the aforementioned conditions improves their applicability in typical processing steps and process steps in aqueous media, such as the application of an additional protective coating to the particles. Furthermore, the stability of the particles in the aqueous paper pulp of a paper machine is ensured for several hours when incorporated into a paper substrate, even when luminescent substances with very small particle sizes are used. The increased stability of the fine-grained luminescent substances is also advantageous when incorporated into or onto polymer substrates, e.g., into hot polymer melts or into liquid coating formulations to form a (printing) ink-receiving layer.

[0048] Within the scope of the invention, valuable documents include items such as banknotes, checks, stocks, tokens, ID cards, passports, credit cards, certificates and other documents, labels, seals, and items to be secured, such as CDs, packaging, and the like. The preferred area of ​​application is banknotes, particularly those based on a paper substrate.

[0049] Further embodiments and advantages of the invention are explained below with reference to the figure.

[0050] It shows:

[0051] Figure 1 Excitation spectra of a luminescent substance according to the invention with Mn(V) as luminophore in low concentration (Ba3(PO4)2:Mn y with y = 0.02) and a non-inventive luminescent substance with a high luminophore concentration (Ba3(PO4)2:Mn y with y = 0.2).

[0052] Figure 1 shows the excitation spectra of a luminescent substance according to the invention (see the solid line in Figure 1) with Mn(V) as luminophore in low concentration (Ba3(PO4)2:Mn y with y = 0.02) and a non-inventive luminescent substance (see the dashed line in Figure 1) with high luminophore concentration (Ba3(PO4)2:Mn y with y = 0.2). The luminescent substance not according to the invention, with < 4 wt. % Mn-containing starting materials, corresponds to a substance according to the prior art EP 1 370424 B1. This serves to illustrate that, with the same host lattice, a low concentration of the luminophore is sufficient to achieve high luminescence intensities. In some cases, the luminescence intensity can even be increased by a reduced concentration of the luminophore.

[0053] The special selection of elements A, E, X, Z, and Hal offers the aforementioned surprising advantages, which are explained using the following examples and comparative examples. In general, the elements for the host lattices were selected such that the composition results in luminescent substances with a favorable luminescence intensity, as little manganese as possible is formed in valences other than Mn(V), and the chemical stability of the luminescent substances ensures good processing into security features. Furthermore, the luminescent substances in powder form have only a weakly pronounced blue body color and can be used specifically with the additional advantage that, due to their small particle size, they are not detectable without the aid of technical aids and only with considerable effort.

[0054] Based on the selected examples a), b), c) and d), the selection made for the luminescent substances according to formulas (I) to (IV) is explained below: a) Luminescent substance: As(XO4)3Hal:Doty (I)

[0055] The elements Ca, Sr, and Ba selected for A promote the preferential incorporation of Mn(V), resulting in luminescent substances with high luminescence intensity at low manganese content. An example according to the invention is Ba5(PO4)3Cl:Mn, which, in contrast to the non-inventive Pb5(PO4)3Cl:Mn, exhibits high luminescence intensity at low manganese content. Comparing luminescent substances based on Bas(PO4)3Cl:Mn and Pbs(PO4)3Cl:Mn with similar particle sizes and similarly low manganese contents (< 2%), it is found that the intensity of the Mn(V) luminescence in Bas(PO4)3Cl:Mn is more than twice as high, and no evidence of additional Mn species is found, whereas Pbs(PO4)3Cl:Mn typically also contains additional Mn species.

[0056] Considering the selection of elements for X = P, V and As, these are selected in such a way that they contribute to an improvement in chemical stability during processing as described above. If the non-inventive Sr5(SiO4)i,5(SO4)i / 5F:Mn, it would have similar luminescence properties (e.g., intensity, emission spectrum) as a powder to the isostructural Sr5(PO4)3F:Mn, but would have significantly different properties after processing. For example, if the same amount of each of the two equally intense luminescent substances with an average particle size D50 = 3 μm is embedded in a paper substrate, the substrate with Srs^iCkjiXSChXsFiMn exhibits a more than 20% reduced intensity compared to the corresponding substrate with Srs(PO4)3F:Mn.

[0057] However, the use of X = Cr can lead to reduced luminescence intensity because Cr(V) also acts as a luminophore and can impair the luminescence of Mn(V) if Mn(V) is present only in small amounts in the luminescent substance. Furthermore, X = Cr causes a strong intrinsic coloration of the luminescent substance.

[0058] The elements for Hal are selected to ensure the best possible stabilization of Mn(V). Hal = Br, due to its size, can lead to instabilities, particularly with the simultaneous (unintentional) incorporation of OH ions from moisture or the starting materials used, which can allow the additional incorporation of Mn in oxidation states other than Mn(V). b) Luminescent substance: AzXO / iHahDoty (II)

[0059] The advantages of the element selections for A, X, and Hal mentioned in paragraph a) above for formula (I) also apply to these luminescent substances. An example according to the invention is therefore Ca2PO4Cl:Mn, which exhibits the aforementioned advantageous properties with regard to luminescence and stability. A counterexample is Cd2AsO4F, where, due to the lower stabilization of the manganese in the +V oxidation state, larger proportions of low-valent manganese species can be incorporated, so that the luminescent substance exhibits significantly lower Mn(V) luminescence than the example according to the invention. Likewise, the non-inventive luminescent substance Ca2(SiO4)o / 5(S04)o / Due to the use of Si and S, sCl has a low stability in aqueous media compared to the example according to the invention at small particle sizes D50 < 5 jun, so that the luminescence intensity may be reduced after processing.

[0060] While inventive mixed forms with Hal = Cl, F such as CazPCbCWFojiMn do not lead to any destabilization of Mn(V), a non-inventive increasing Br content such as CazPChCWBro,!: Mn has a detrimental effect on the lattice stability and thus on the incorporation of Mn(V). c) Luminescent substance: As(XO4)2:Dot y (III)

[0061] The advantages of the element selections for A and X mentioned in paragraph a) above for formula (I) also apply to these luminescent substances. An example according to the invention is Ba^SroXPCh^Mn, which exhibits the aforementioned advantageous properties with regard to luminescence and stability. A counterexample is Cd3(PO4)2:Mn, where manganese can be incorporated in other oxidation states in addition to the preferred +V oxidation state and therefore exhibits a reduced luminescence intensity. d) Luminescent substance: EZO4:Dot y (IV)

[0062] The advantageous element selection mentioned for this host lattice, E = Sr, Ba and Z = S, Se, differs from that for A and X from the previous classes, but here it results in the same advantageous properties such as high luminescence intensity despite low luminophore concentration and good chemical stability for processing, e.g., in a security feature. An example according to the invention is BaSChiMn, which exhibits the aforementioned advantageous properties with regard to luminescence intensity and chemical stability. In this case, the formation of Mn(VI), which is normally strongly favored in this host lattice, was suppressed by suitable synthesis procedures, so that in the emission spectrum, the narrowband luminescence emission of Mn(V) dominates over the broad Mn(VI) emission (<5%).As a counterexample, CaSCUMn can be mentioned, where manganese can be incorporated into other, particularly low-valent, oxidation states in addition to the preferred +V oxidation state, and which therefore exhibits reduced luminescence intensity. Furthermore, at average particle sizes of D50 < 5 μm, the counterexample exhibits reduced chemical stability compared to the inventive example, making it less suitable as a security feature after processing, e.g., in aqueous media.

Claims

Patent claims 1. A security feature for securing a value document, comprising a luminescent substance based on an inorganic host lattice doped with Mn(V) as a luminophore according to one of the formulas (I), (II), (III) or (IV): A 5 (XO 4 ) 3 Hal:Dot y (I), A 2 XO 4 Hal:Dot y (II), A 3 (XO 4 ) 2 :Dot y (III), EZ0 4 :Dot y (IV), where y denotes the doping level of the luminophore and is chosen in a range 0 < y < 0.1 and the elements for the host lattices and dopants are selected from the following lists: A = Ca, Sr, Ba and their mixtures; E = Sr, Ba and their mixtures; X = P, V, As and their mixtures; Z = S, Se and their mixtures; Hal = F, CI and their mixtures; Dot = Mn(V).

2. Security feature according to claim 1, wherein the security feature, when excited with radiation in the VIS / IR range, has a characteristic emission of radiation outside the visible wavelength range, in particular in the IR range.

3. Security feature according to claim 1 or 2, wherein the luminescent substance is in the form of particles with a particle size D99 < 20 μm, preferably D99 < 12 μm.

4. Security feature according to claim 3, wherein the particles of the luminescent substance additionally have a particle size D50 < 5 µm, preferably D50 < 3 µm.

5. Security feature according to one of claims 1 to 4, wherein the mole fraction of the luminophore Mn(V) in the host lattice is less than 2%.

6. A value document comprising the security feature according to any one of claims 1 to 5.

7. A value document according to claim 6, wherein the value document is a banknote.

8. Printing ink for printing a value document, wherein the printing ink comprises the security feature according to one of claims 1 to 5.

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