Security feature, value document and printing pigment

Abstract

A security feature for protecting documents of value, comprising a luminescent material based on an inorganic host lattice doped with Mn (V) as luminophore according to one of formulae (I), (II), (III) or (IV): A5 (XO4) 3Hal: Doty (I), A2XO4Hal: Doty (II), A3 (XO4) 2: Doty (III), EZO4: Doty (IV), where y represents the degree of doping of the luminophore and is at 0 lt; y < lt >; 0.1, and the elements for the host lattice and dopant are selected from the following list: A = Ca, Sr, Ba, and mixtures thereof; e = Sr, Ba and mixtures thereof; x = P, V, As and mixtures thereof; z = S, Se, and mixtures thereof; hal is F, Cl and a mixture thereof; and Dot = Mn (V).

Description

Technical Field

[0001] This invention relates to a security feature for protecting valuable documents, wherein the security feature exists in the form of a luminescent material based on an inorganic main lattice doped with Mn(V) as a luminescent element. The invention also relates to a valuable document having the aforementioned security feature, particularly banknotes. Furthermore, the invention relates to a printing pigment for printing valuable documents having this security feature. Background Technology

[0002] To protect and verify valuable documents such as banknotes, bank cards, identification documents, and passports, as well as other items requiring anti-counterfeiting measures, such as pharmaceuticals, luminescent materials are often incorporated. These luminescent materials are typically printed onto the corresponding valuable items or incorporated into the (paper) substrate within the valuable items. Excitation with light of a suitable wavelength causes the emission of light of another wavelength, which is used to verify the value of the item. Depending on the application, luminescent materials with different excitation and emission properties have been developed, which can be visually identifiable and / or machine-readable. Visually imperceptible, i.e., security features that are invisible to the naked eye (also known as "covert features") provide enhanced security. In particular, luminescent materials that are excited and emitted in visually imperceptible areas serve as visually imperceptible security features, making their presence visually undetectable or at least difficult to detect. Furthermore, ideally, the particles constituting these visually imperceptible security features should be undetectable or at least difficult to detect by the naked eye or auxiliary techniques such as magnification. Therefore, luminescent materials must possess special properties in terms of body color, luminescence intensity, and pigment particle size, properties that cannot be achieved without special selection of the main lattice and elemental composition. Summary of the Invention

[0003] The technical problem to be solved by this invention is to provide suitable luminescent materials for the verification of valuable articles, especially valuable documents. These luminescent materials, despite having low luminescent concentration, possess extremely high luminescence intensity and, additionally, improved chemical stability. In particular, the luminescent materials should have at most a faint body color and a small particle size, thus being suitable for processing as visually imperceptible security features.

[0004] The technical problem described herein is solved by a combination of features defined in the independent claim. Extended designs of the invention are the subject of the dependent claims. Invention Overview

[0006] 1. (First aspect of the invention) A security feature for protecting valuable documents, said security feature comprising a luminescent material based on an inorganic main lattice doped with Mn(V) as a luminescent element, according to one of formulas (I), (II), (III) or (IV):

[0007] A5(XO4)3Hal:Dot y (I),

[0008] A2XO4Hal:Dot y (II),

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

[0010] EZO4:Dot y (IV),

[0011] Where y represents the doping degree of the luminescent material and is selected in the range of 0 < y < 0.1, and the elements used for the main lattice and dopant are selected from the following list:

[0012] A = Ca, Sr, Ba and their mixtures;

[0013] E = Sr, Ba and their mixtures;

[0014] X = P, V, As and their mixtures;

[0015] Z = S, Se and their mixtures;

[0016] Hal = F, Cl and their mixtures;

[0017] Dot = Mn(V).

[0018] 2. (Preferred embodiment) According to the safety feature of item 1, wherein the safety feature, when excited by radiation in the VIS / IR range, has characteristic radiative emission outside the visible light wavelength range, especially in the IR range.

[0019] 3. (Preferred embodiment) According to the safety features of item 1 or 2, the luminescent material exists in the form of particles with a particle size of D99 < 20 µm, preferably D99 < 12 µm.

[0020] 4. (Preferred embodiment) According to the safety feature of item 3, the particles of the luminescent material also have a particle size of D50 < 5 µm, preferably D50 < 3 µm.

[0021] 5. (Preferred embodiment) According to any one of the safety features of items 1 to 4, wherein the molar fraction of the luminescent Mn(V) in the main lattice is less than 2%.

[0022] 6. (Second aspect of the invention) A valuable document comprising security features according to any one of items 1 to 5.

[0023] 7. (Preferred embodiment) The valuable document according to item 6, wherein the valuable document is banknotes.

[0024] 8. (Third aspect of the invention) A printing pigment for printing valuable documents, wherein the printing pigment includes a security feature according to any one of items 1 to 5.

[0025] Detailed description of preferred embodiments

[0026] The subject of this invention is a safety feature, in the form of a luminescent material based on an inorganic host lattice doped with Mn(V) as a luminescent agent. This luminescent material preferably has a characteristic emission spectrum in the IR range, outside the visually perceptible wavelength range. The IR range hereinforcingly refers to the range from 700 nm to 2500 nm. The luminescent material is preferably exciteable at least one wavelength in the VIS / IR range, and particularly preferably exciteable at least one wavelength in the IR range. The VIS / IR range hereinforcingly refers to the range between 400 nm and 2500 nm. By selectively choosing the host lattice in conjunction with Mn(V) as the luminescent agent, luminescent materials can be obtained that exhibit unexpectedly high luminescence intensity and advantageously possess a faint body color, even at extremely low luminescent agent concentrations (i.e., the molar fraction of Mn(V) in the host lattice is less than 2 wt%). Furthermore, selectively choosing the host lattice and its elemental composition improves the chemical stability of these luminescent materials in further processing in aqueous systems and in their introduction into paper and polymer substrates. The improved chemical stability also enables the use of luminescent pigments with smaller particle sizes (i.e., D99 < 12 µm and D50 < 3 µm) without adverse effects from excessive surface area and increased reactivity. Furthermore, the small particle size reduces the visual and analytical recognizability (e.g., by microscopy) of the luminescent materials, making them particularly suitable for generating invisible security features in valuable documents.

[0027] The selection according to the present invention is based on EP 1 370 424 B1, which discloses a large number of luminescent materials having 17 preferred principal lattices and five 3D lattices. 2Luminescent materials. Regarding luminescent materials, Ti(II), V(III), Cr(IV), Mn(V), and Fe(VI) are mentioned. Preferred host lattices are selected from, for example, apatite, phosphatic aluminate, potassium phosphate, forsterite, perovskite, carbon apatite, silica apatite, fluorapatite, triclinic calcium apatite, fluoroapatite, sulforaphane, hydroxyapatite, pyrophosphate, garnet, perovskite, olivine, silicates, titanates, vanadates, phosphates, sulfates, aluminates, and zirconates.

[0028] Within the scope of this invention, the following selected luminescent materials according to one of formulas (I) to (IV) have proven particularly advantageous, and this selection is described below with regard to their elemental composition and advantageous properties:

[0029] A5(XO4)3Hal:Dot y (I),

[0030] A2XO4Hal:Dot y (II),

[0031] A3(XO4)2:Dot y (III),

[0032] EZO4:Dot y (IV),

[0033] The elements used for the main lattice and dopants are selected from the following list:

[0034] A = Ca, Sr, Ba and their mixtures, that is, A is especially not Mg, Pb, Cd;

[0035] E = Sr, Ba and their mixtures, that is, E is not Mg, Ca, Pb, Cd;

[0036] X = P, V, As and their mixtures, i.e. X is not Si, S, or Cr in particular;

[0037] Z = S, Se and their mixtures, that is, Z is not Cr, Mo, W;

[0038] Hal = F, Cl and their mixtures, that is, Hal is not Br;

[0039] Dot = Mn(V), meaning that Dot is not Ti(II), V(III), Cr(IV), or Fe(VI).

[0040] It should be noted that the term "master lattice" refers to undoped material, while the doped master lattice is referred to here as luminescent material.

[0041] Regarding the selection of the aforementioned elements, the term "not" means that, in order to synthesize the doped host lattice, no initial material containing these elements is added to affect the properties of the host lattice. However, the elements may exist as impurities in the initial material (content < 0.1 atomic%).

[0042] Regarding halogens (Hal), it should be noted that under specific synthesis conditions and with the selection of initial materials, hydroxide ions (OH) may enter the main lattice in trace amounts and replace one of the halogens. Therefore, it should be pointed out that hydroxide ions (OH) may enter as undesirable impurities. Preferably, less than 10%, particularly preferably less than 5%, of the halogen is replaced by hydroxide ions, and more preferably, no hydroxide ions enter the main lattice. Excessively high hydroxide ion content may adversely affect chemical stability and luminescent properties.

[0043] The doping degree y of the luminescent material is in the range of 0 < y < 0.1 according to the total chemical formula, therefore the molar fraction of the luminescent material in the main lattice is < 2%, wherein the weight of the initial material used for the luminescent material (e.g., Mn₂O₃, MnO, MnO₂, MnCO₃, MnCl₂, or KMnO₄) is less than 2% by weight relative to the total weight of the initial material. Here, the term "luminescent material" should be understood as the emission of the doped main lattice originating from this element (here, Mn(V)). Here, "luminescent material" is the preferred name; other equivalent terms include luminescent center, activator, or dopant.

[0044] The luminescent materials (1) to (4) according to the invention are characterized in particular by their high luminescence intensity, despite the use of the luminescent material Mn(V) in only a low molar fraction as described. Within this concentration range, the luminescence intensity of the luminescent material unexpectedly decreases not proportionally to the luminescent material concentration. This unexpected effect is achieved through the targeted selection of the host lattice and luminescent-related elements. In contrast, prior art EP 1 370 424 B1 describes a large number of luminescent materials that generally have a higher luminescent material fraction of up to 20% by weight (relative to reactants), while the material of the invention has a maximum fraction of less than 2% by weight. Furthermore, the targeted selection of the luminescent material according to the invention provides sufficient chemical stability even with small particle sizes, thus ensuring processability in aqueous media. In contrast, the highly diverse luminescent materials described in prior art EP 1 370 424 B1 have disadvantages in a wide range. On the one hand, only a few host lattices are able to stabilize Mn(V) in a low amount as required by the material according to the invention. Furthermore, the main lattice capable of successfully stabilizing Mn(V) in low amounts typically lacks sufficient chemical stability, thus potentially leading to a loss of luminescence intensity depending on processing conditions (i.e., temperature, pH, time, pressure, etc.) when the particle size is correspondingly reduced. Surprisingly, these drawbacks can be overcome by specific choices made in this invention.

[0045] Compared to the prior art EP 1 370 424 B1, another advantage of using a smaller amount of luminescent material is that manganese is preferably introduced into the lattice as Mn(V), thereby preventing the formation of other manganese valence states and undesirable manganese-containing secondary phases (such as dark-colored Mn-oxides such as MnO, Mn2O3, etc.).

[0046] The luminescent material according to the invention exhibits a pale blue hue characteristic of Mn(V) in powder form, which may shift to the range of blue-green or blue-violet depending on matrix interactions. With increasing proportions of different valence states of manganese, both the luminescence and blue hue impression of Mn(V) are lost, and in extreme cases, colorless or gray, green, yellow, or brown materials may be obtained. The pale blue coloration of the luminescent material according to the invention is not strong enough to allow for visual detection of safety features by body color, and this can only be achieved through considerable effort using technical methods. In contrast, in cases of high Mn(V) concentrations and / or the presence of dark-colored different valence states of manganese, safety features can be visually detected by body color.

[0047] Compared to other luminescent materials not selected according to the invention, manganese (V) has the advantage of exhibiting high luminescence intensity in the non-visible spectral range, even in small amounts. For example, the isoelectronic luminescent material Fe(VI) exhibits only a weak luminescence intensity. Furthermore, compared to Mn(V), Fe(VI) is more easily converted to a lower valence Fe state through defect introduction, impurities, and increased temperature, thus reducing the luminescent material concentration is disadvantageous. This also applies to other luminescent materials not selected according to the invention, Ti(II), V(III), and Cr(IV), which are more difficult or impossible to stabilize sufficiently at low concentrations. Through the specific selection of the main lattice, in the case of manganese, sufficient stabilization in the +V valence state is ensured, thus allowing its introduction even at low concentrations without loss of luminescence intensity.

[0048] Based on a specially selected combination of cations, anions, halogens, luminescent materials, and luminescent material concentrations, particularly advantageous luminescent materials have been obtained. These materials are particularly suitable as invisible security features, possess characteristic excitation and emission spectra, and are characterized by a high relative abundance of Mn(V) relative to the total manganese content, and the absence or presence of only low relative abundance (< 5%) of other low- or high-valence manganese oxide states. If manganese is introduced in oxidation states other than Mn(V), they reduce the luminescence intensity of Mn(V) and favor the formation of crystal structures other than the selected host lattice. Furthermore, the presence of other oxidation states of manganese may lead to emission in the visible region of the spectrum, as is known for Mn(II), thereby making the security features readily detectable by optical methods.

[0049] The following uses Mn(II) as an example to explain why the presence of manganese in oxidation states other than Mn(V) is not desired in the luminescent material according to the present invention. However, it should be noted that this undesirable presence is not limited to Mn(II), but applies to all manganese oxidation states below or above Mn(V). Mn is known to be commonly introduced into apatite in the form of Mn(II) and exhibits a yellow luminescence. Due to its (especially under UV light excitation) visible light emission, which is prevalent not only in apatite but also in luminescent materials, Mn(II) is widely used in luminescent materials. In the luminescent material of the present invention, less than 5% of the manganese is present in the form of Mn(II). Preferably, the luminescent material of the present invention does not contain Mn(II) but only Mn(V). The presence of Mn(II) can be determined not only by visible light emission but also, for example, by electron spin resonance (ESR or EPR), where Mn(II) has a signal with a g-factor of approximately 2, and can be measured as a broadened single signal or, under appropriate conditions, as six independent spectral lines. The luminescent material according to the invention preferably has no measurable signal at this position in the ESR spectrum, which is typical for substances containing Mn(II).

[0050] When used as an invisible security feature, it is advantageous that the particle size of the largest present particles is as small as possible, preferably D99 < 20 µm, so that the security feature becomes difficult to detect by optical methods. The term D99 refers to a particle size in which 99% by weight of the particles in the particle size distribution are smaller than that size. The term D50 refers to a particle size in which 50% by weight of the particles in the particle size distribution are smaller than that size. Particularly preferably, the particle size distribution of the luminescent material according to the invention has a value of D99 < 12 µm, meaning that a very small number of particles have a diameter greater than 12 µm exist. However, such a particle size distribution means that the average particle diameter D50 is typically in the range of 0.5 µm to 5 µm. If the particle size is too small, for example, D50 = 2 µm, the particles may not be stable enough during processing into a security feature due to increased reactivity and increased surface area; therefore, only particle sizes > 5 µm are reasonably processed. Therefore, the luminescent material selected according to the invention is chosen to ensure that it remains stable when processed into a security feature even with a small particle size of D50 < 5 µm, thus making it highly suitable as an invisible security feature. Particularly preferably, the particle size distribution of the luminescent material according to the invention has a value of D50 < 5 µm, and especially preferably D50 < 3 µm. Preferably, the particle size distribution of the luminescent material according to the invention has a value of D50 > 0.5 µm, and especially preferably D50 > 1 µm.

[0051] Regarding the further processing of the luminescent material according to the invention, the selections made, specifically the chosen combinations of cations, anions, halogens, luminescent elements, and luminescent element concentrations, bring particular advantages. Therefore, these selections ensure that the luminescent material according to the invention remains sufficiently stable even with small particle sizes during processing into safety features, for example, in aqueous media and acidic and / or alkaline media, and that potential strength loss upon contact with water or acidic or alkaline solutions is reduced. The improved stability during processing into safety features (e.g., wet milling, surface functionalization, core-shell particle formation, introduction of (thin film) safety elements, or different (paper) substrates, remains advantageous compared to non-inventive variations of the invention that differ from the selections made according to the invention (i.e., the specially chosen combinations of cations, anions, halogens, luminescent elements, and luminescent element concentrations), because the desired purity of Mn(V) is maintained, and no other low- or high-valence manganese forms are formed due to contact with different chemicals and / or aqueous media. The luminescent material according to the invention, after contact with a hot aqueous solution at 50°C and pH 9 for 30 minutes, exhibits a luminescence intensity loss of less than 10%, preferably none, while luminescent materials not according to the invention may show a significant loss of 20% or higher. This improved stability under the above conditions enhances its suitability in typical processing steps and procedures in aqueous media (e.g., applying an additional protective coating to the particles). Furthermore, even when using luminescent materials with very small particle sizes, the stability of the particles in the aqueous pulp of the paper machine for several hours upon introduction into the paper substrate is ensured. Similarly, the improved stability of fine-particle luminescent materials is also advantageous when introduced into or placed on polymer substrates, for example, in thermopolymer melts or in liquid coating formulations forming (printing) pigment receiving layers.

[0052] Valuable documents within the scope of this invention include banknotes, checks, stocks, value tokens, certificates, passports, credit cards, certificates and other documents, labels, seals, and items requiring protection such as CDs and packaging. A preferred application area is banknotes, particularly paper-based banknotes.

[0053] Other embodiments and advantages of the present invention will be described below with reference to the accompanying drawings. Attached Figure Description

[0054] In the attached diagram:

[0055] Figure 1 The luminescent material (Ba3(PO4)2:Mn) according to the present invention, having a low concentration of Mn(V) as the luminescent agent, is shown. y (y = 0.02) and non-inventive luminescent materials with high concentrations of luminescent material (Ba3(PO4)2:Mn) y The excitation spectrum of (y = 0.2). Detailed Implementation

[0056] Figure 1 A luminescent material according to the invention, having a low concentration of Mn(V) as the luminescent agent, is shown (see [reference]). Figure 1 (solid line in Ba3(PO4)2:Mn) y (y = 0.02) and non-inventive luminescent materials with high concentrations of luminescent elements (see [reference]). Figure 1 (dashed line in Ba3(PO4)2:Mn) y The excitation spectrum (y = 0.2). The luminescent material not described in this invention corresponds to the material according to prior art EP 1 370 424 B1, which contains less than 4% by weight of initial Mn material. This demonstrates that, under the same principal lattice, a low concentration of luminescent material is sufficient to achieve high luminescence intensity. In some cases, it is even possible to increase luminescence intensity by reducing the luminescent material concentration.

[0057] The specific selection of elements A, E, X, Z, and Hal provides the aforementioned unexpected advantages, which will be illustrated by the following examples and comparative examples. Generally, the selection of the principal lattice elements aims to produce a luminescent material with favorable luminescence intensity, minimizing the formation of manganese in valence states other than Mn(V), and ensuring the chemical stability of the luminescent material allows for good processing into safe features. Furthermore, the luminescent material in powder form exhibits only a faint blue body color, and its additional advantage—that it is imperceptible without technical aids due to its small particle size and can only be detected through significant expenditure—can be specifically utilized.

[0058] The following examples a), b), c), and d) illustrate the rationale for the selection of luminescent materials according to equations (I) to (IV):

[0059] a) Luminescent material: A5(XO4)3Hal:Dot y (I)

[0060] The selection of elements Ca, Sr, and Ba for A promotes the preferred introduction of Mn(V), thereby obtaining a luminescent material with high luminescence intensity at low manganese content. Ba5(PO4)3Cl:Mn, as an example according to the invention, exhibits high luminescence intensity at low manganese content compared to non-inventive Pb5(PO4)3Cl:Mn. Comparing luminescent materials based on Ba5(PO4)3Cl:Mn and Pb5(PO4)3Cl:Mn with similar particle size and similar low manganese content (< 2%), it can be found that the luminescence intensity of Mn(V) in Ba5(PO4)3Cl:Mn is more than twice that of Pb5(PO4)3Cl:Mn, and no indication of the presence of other manganese forms was found, while Pb5(PO4)3Cl:Mn typically contains other manganese forms.

[0061] The elemental choices for X = P, V, and As contribute to improved chemical stability during the processing described above. This applies if processing non-inventory Sr5(SiO4). 1.5 (SO4) 1.5 While F:Mn, as a powder, may possess similar luminescent properties (e.g., intensity, emission spectrum) to its isomorphic counterpart Sr5(PO4)3F:Mn, it exhibits significantly different properties after processing. For instance, if equal amounts of two luminescent materials with the same intensity and average particle size D50 = 3 µm are introduced into a paper substrate, the resulting material will contain Sr5(SiO4). 1.5 (SO4) 1.5 The strength of F:Mn substrates is reduced by more than 20% compared to the corresponding substrates containing Sr5(PO4)3F:Mn.

[0062] Conversely, using X = Cr may lead to a decrease in luminescence intensity because Cr(V) can also function as a luminescent material, and may interfere with the luminescence of Mn(V) when Mn(V) is present in only a low molar fraction in the luminescent material. Furthermore, X = Cr results in strong coloration of the luminescent material itself.

[0063] The element choice for Hal is to stabilize Mn(V) as well as possible, because Hal = Br may be unstable due to its size, especially when OH ions from moisture or the initial material used are introduced simultaneously (unintentionally), which may allow manganese to enter the lattice in oxidation states other than Mn(V).

[0064] b) Luminescent material: A2XO4Hal:Dot y (II)

[0065] The advantages of the selection of elements A, X, and Hal mentioned in paragraph a) of formula (I) above also apply to these luminescent materials. Therefore, as an example according to the invention, Ca2PO4Cl:Mn possesses the aforementioned advantageous properties in terms of luminescence and stability. As a counterexample, Cd2AsO4F, due to its lower degree of stabilization of manganese in the +V oxidation state, may introduce a larger proportion of low-valence manganese forms; therefore, this luminescent material exhibits significantly lower Mn(V) luminescence than the example according to the invention. Similarly, the luminescent material Ca2(SiO4) not of the present invention... 0.5 (SO4) 0.5 Cl, due to the use of Si and S, exhibits lower stability in aqueous media at small particle sizes (D50 < 5 µm) compared to the example of this invention, and therefore the luminescence intensity may decrease after processing.

[0066] According to the present invention, a mixed form having Hal=Cl, F, such as Ca2PO4Cl 0.9 F 0.1 Mn does not cause Mn(V) to become unstable, as does, for example, in Ca2PO4Cl. 0.9 Br 0.1 The gradually increasing Br content in Mn (not in this invention) will have an adverse effect on lattice stability and thus on the introduction of Mn(V).

[0067] c) Luminescent material: A3(XO4)2:Dot y (III)

[0068] The advantages of choosing elements A and X mentioned in paragraph a) of formula (I) above also apply to these luminescent materials. As an example of the invention, Ba... 2.7 Sr 0.3 (PO4)2:Mn possesses the aforementioned advantageous properties in terms of luminescence and stability. As a counterexample, Cd3(PO4)2:Mn, in which manganese may be introduced in other oxidation states besides the preferred +V oxidation state, thus exhibiting reduced luminescence intensity.

[0069] d) Luminescent material: EZO4:Dot y (IV)

[0070] The favorable elemental choices E = Sr, Ba and Z = S, Se mentioned for this main lattice differ from the choices of A and X in the aforementioned categories, but produce the same beneficial properties, such as high luminescence intensity despite low luminescent concentration, and good chemical stability for processing into, for example, safety features. As an example of the invention, BaSO4:Mn possesses the aforementioned favorable properties in terms of luminescence intensity and chemical stability. Here, through appropriate synthetic control, the formation of Mn(VI) which normally tends to be strong in this main lattice is suppressed, so that in the emission spectrum, the narrow band of Mn(V) emission dominates relative to the broad Mn(VI) emission (<5%).

[0071] As a counterexample, in CaSO4:Mn, manganese may be introduced in other oxidation states, especially lower oxidation states, in addition to the preferred +V oxidation state, thus resulting in reduced luminescence intensity. Furthermore, compared to the example of the present invention, this counterexample exhibits reduced chemical stability with an average particle size D50 < 5 µm, and is therefore less suitable as a safety feature after processing, for example, in an aqueous medium.

Claims

1. A security feature for protecting valuable documents, comprising a luminescent material based on an inorganic principal lattice doped with Mn(V) as a luminescent agent, according to one of formulas (I), (II), (III), or (IV): A5(XO4)3Hal:Dot y (I), A2XO4Hal:Dot y (II), A3(XO4)2:Dot y (III), EZO4:Dot y (IV), in, y represents the doping degree of the luminescent material and is selected in the range of 0 < y < 0.1, and the elements used for the main lattice and dopant are selected from the following list: 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, Cl and their mixtures; Dot = Mn(V).

2. The security feature according to claim 1, wherein, The security feature is characterized by radiation emission outside the visible light wavelength range, particularly in the IR range, when excited by radiation in the VIS / IR range.

3. The security feature according to claim 1 or 2, wherein, The luminescent material exists in the form of particles with a particle size of D99 < 20 µm, preferably D99 < 12 µm.

4. The security feature according to claim 3, wherein, The luminescent material particles additionally have a particle size of D50 < 5 µm, preferably D50 < 3 µm.

5. The security feature according to any one of claims 1 to 4, wherein, The amount of Mn(V) in the main lattice is less than 2%.

6. A valuable document, said valuable document comprising the security features according to any one of claims 1 to 5.

7. The valuable document according to claim 6, wherein, The valuable document in question is banknotes.

8. A printing pigment for printing valuable documents, wherein, The printing pigment contains the safety features according to any one of claims 1 to 5.

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