WO2010058290A2

WO2010058290A2 - Lanthanide compounds

Info

Publication number: WO2010058290A2
Application number: PCT/IB2009/007599
Authority: WO
Inventors: Wen Tian Fu; Jan Reedijk
Original assignee: Universiteit Leiden
Priority date: 2008-11-19
Filing date: 2009-11-19
Publication date: 2010-05-27
Also published as: GB0821135D0; WO2010058290A3

Abstract

A compound, according to general formula 1: M( II )Ln(III) _1-p-q Ce(III) _p Tb(III) _q M'(III) ₃ O ₇ wherein M(II) represents a divalent metal selected from the group of Ca, Sr and mixtures thereof; Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and wherein p and q represent fractional numbers varying between 0 and 1, and1≥p+q>0 is provided. M' (III) may be substituted by a tetravalent component M''' (IV), in which case the trivalent metal M' (III) may be substituted with a divalent metal M'' (II) or the divalent metal M(II) may be substituted with a monovalent metal M'''' (I) to maintain charge balance. The compounds of formula 1 can find use as phosphors including in lighting applications.

Description

Lanthanide Compounds

Field of the invention

The present invention relates to the provision of novel lanthanide compounds and their use as phosphor materials, especially, but not exclusively, in ultraviolet activated light-emitting diodes.

Background of the Invention

Light emitting diodes (LEDs) are expected to provide the next generation of illuminating light sources. When compared to the conventional incandescent or fluorescent light sources, this so-called solid-state lighting (SSL) offers many advantages. For example, LEDs have higher energy conversion efficiency. It is estimated that the ultimate performance of LEDs might reach 200 lm/W. In addition, LEDs have very long lifetime (>100000 h) , which can effectively reduce the maintenance costs. Further, LEDs are solid-state devices and are therefore not susceptible to vibration and can withstand far more abuse than glass- based lamps. Finally, LEDs are regarded as environmentally friendly since they do not contain dangerous metals such as mercury or lead.

For general lighting purposes, "white LEDs" are required. One way to create white LEDs is the so-called multi-chip solution, which combines three LEDs emitting in the blue, green and red regions of the visible spectrum respectively. This solution, however, presents some difficulties. Current green/yellow LEDs have very poor efficiency in comparison with available blue or red LEDs. In addition, wavelengths of the emission spectra of compositions used in current green/yellow emitting compositions are not situated at suitable or required wavelengths to combine with the red and blue emitters to provide a bright white light. Furthermore because the three LED components used in multi chip devices have different voltage requirements, different degradation characteristics and different temperature dependencies, sophisticated control systems are required.

An alternative solution is to use a single LED chip combined with appropriate compositions, which exhibit suitable phosphorescence (phosphors) . Two alternatives have been suggested: combining a blue emitting LED with suitable phosphor (s) or combining an ultraviolet emitting LED with three suitable phosphors .

The first option was proposed and demonstrated by S. Nakamura (MRS Bulletin 22 (1997) 29) , and has been commercialized since then. This approach uses a blue LED, emitting at about 460 nm, and a single phosphor Y₃Al₅Oi₂: Ce (YAG: Ce) . Part of the blue light is converted by the phosphor to a yellow light, which is the complimentary color of blue, thus producing white light. An advantage of this method is the high conversion efficiency, due to relatively low quantum deficit during the conversion of blue to yellow light. However, this particular combination only provides lamps with a high color temperature, >4000 K, resulting in a relatively poor color rendering index (CRI) (-75) .

Some improvements have been made by introducing an additional red phosphor, e.g. CaS: Eu²⁺ to the compositions employed. This can give a lamp with a color temperature of about 3200 K and a CRI value exceeding 90. The efficiency of such devices drops, however, to about 2/3 due mainly to unfavorable spectral distribution of the red phosphor. The alternative approach uses a UV emitting LED (e.g. emitting at a wavelength of 380 nm) to excite three phosphors (red green, blue) , similar to those used in tricolor fluorescent lamps. Since the three colors are generated simultaneously, a high lumen output as well as a high CRI can be realized. However, when compared to the blue LED approach, UV LEDs have an additional quantum deficit (loss of efficiency of energy transfer) of about 15%. Accordingly, the UV LED approach is only suitable if this extra energy loss is sufficiently compensated by a high lumen equivalent from the phosphors employed in the device .

Lanthanide-activated phosphors have been successively applied in tricolor fluorescent lamps for more than three decades. Rare earth phosphors, especially Tb³⁺ and Eu³⁺- dopped materials, emit line spectra in the visible region, so that a high "light output", i.e. high lumen equivalent, is produced. This helps to compensate for the additional quantum deficit of the color conversion.

However, not all phosphors currently used in mercury discharge lamps absorb UV light efficiently in the wavelength range close to 380 nm, i.e. the output range of a typical UV LED. This is particularly the case for Tb³⁺- activated (green) phosphors, although some Eu³⁺-activated phosphors that do absorb satisfactorily at 380 nm with high quantum efficiencies are known (Y₂O₂SrEu³⁺ and Y (V, P, B) O₄: Eu³⁺, for example) .

It is the object of the present invention to provide phosphor materials for use in lighting applications that can avoid or at least mitigate at least one of the aforementioned problems. Summary of the Invention

According to a first aspect the present invention provides a compound, according to general formula 1:

M(JI)LΩ(JIJ)₁__p__ςrCe(JJl)_pT_Jb(JJI)_gM'(JJJ)₃O₇ (1)

wherein M(II) represents a divalent metal selected from the group of Ca, Sr and mixtures thereof; Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and wherein p and q represent fractional numbers varying between 0 and 1, and l≥p+q>0.

The compounds of the invention contain cerium (Ce³⁺) and/or terbium (Tb³⁺) . Preferably p + q >0.1.

The trivalent metal M' (III) of formula 1 may be substituted to some extent by a tetravalent component M''' (IV) . The ratio of M''' (IV) component to M' (III) in a compound may be at up to 1:1. The tetravalent component M''' (IV) may be selected from Si(IV), Ge(IV) and mixtures thereof.

In order to maintain the necessary charge balance in the compound, the additional + charge provided by the M''' (IV) component, over that provided by M' (III), may be balanced by further substitution of the trivalent metal M' (III) with a divalent metal M'' (II). Alternatively the charge balance of the compound may be maintained by the substitution of the divalent metal M(II) of formula 1 with a monovalent metal M'''' (I) . As a yet further alternative the charge balance of the compound may be maintained by substitution in formula 1 with both divalent metal M'' (II) and monovalent metal M'''' (I) in selected amounts sufficient to maintain charge balance of the compound. The divalent metal M'' (II) may be selected from the group consisting of Zn(II), Mg(II) and mixtures thereof. The monovalent metal M'''' (I) may be selected from the group consisting of K, Na and mixtures thereof.

For example, where the charge balance is achieved by replacing M' (III) with a divalent metal M'' (II) the compounds of the invention may take the form of formula 2 :

M(II)Ln(ffl)₁__p__qCe(in)_pTb(in)_qM'(in)₃__2rM"(n)_rM'"(IV)_rO₇ (2)

wherein M(II), Ln(III), M' (III), p and q have the same meaning as before; M''' (IV) is a tetravalent component selected from the group consisting of Si(IV), Ge (IV) and mixtures thereof; M'' (II) is a divalent metal; and r is a fractional number having the value l≥r>0.

For example, where the charge balance is achieved by replacing M(II) with a monovalent metal M'''' (I) the compounds of the invention may take the form of formula 3 :

M(II)_l__rM""(l)_rLn(lIl)_l__p__gCe(III)_pTb(lII)_qM'(lIl)^_rM'"(lV)_rO₁ (3)

wherein M(II), Ln(III), M' (III), p and q have the same meaning as before; M''' (IV) is a tetravalent component selected from the group consisting of Si(IV), Ge (IV) and mixtures thereof; M'''' (I) is a monovalent metal; and r is a fractional number having the value l>r>0.

Preferred compounds of the invention may have the structure of the mineral melilite as discussed with reference to specific examples below.

Preferred compounds of formula 1 include but are not limited to: CaYi-_pCe_pAl₃θ₇, CaYi-_qTb_qAl₃0₇ and CaY_1-p-qCe_pTb_qAl₃θ₇. More preferred compounds of formula 1 include but are not limited to:

CaY₀.₈Ce₀.2Al₃O₇ , CaY₀.₈Tb₀.₂A1₃O₇ , CaY₀.₇Ce₀.15Tb₀.15Al₃O₇ , CaCe₀.₇₅Tb_0.25Al₃O₇ and CaCeAl₃O₇.

Compounds according to the invention have been found to emit light in the blue (with Ce³⁺ containing or Ce³⁺ doped compounds) green (Tb³⁺ containing or Tb³⁺ doped compounds) or in both the green and blue parts of the visible spectrum (compounds containing both Ce³⁺ and Tb³⁺) when excited by an appropriate UV light. These emissions make compounds according to the invention suitable for use as phosphors in lighting applications.

Thus according to a second aspect the present invention provides the use of a compound according to general formula 1:

M{ll)Ln{III)_λ__p__qCe{iτi)_pTb{Hl)_qW(IIJ)₃O₇ ( 1 ) wherein M(II) represents a divalent metal selected from the group of Ca, Sr or mixtures thereof; Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and wherein p and q represent fractional numbers varying between 0 and 1 and l≥p+q>0; as a phosphor.

As described above the trivalent metal M' (III) of the composition of formula 1 may be substituted by a tetravalent component M'' (IV), with appropriate adjustments to the composition to balance the charges.

Combining both Ce³⁺ and Tb³⁺ in the same compound presents some additional advantages, especially in regard to obtaining efficient emission of green light. According to quantum selection rules, for Tb³⁺ intra 4f-4f transitions are both spin and parity forbidden. Furthermore the allowed Tb³⁺ transitions (4f-5d) are of a much higher energy. Therefore the efficient population of the light emitting 4f excited state for Tb³⁺ can only be achieved, through use of an appropriate sensitizer. The Ce³⁺ ion is known to be an excellent sensitizer for Tb³⁺. For example the use of Ce³⁺ as sensitizer has been successfully applied to the green phosphors used in current mercury discharge lamps.

However, the strong optical absorption bands of the allowed Ce³⁺ 4f-5d transitions in known oxide compositions are usually observed in the far UV, at less than about 320 nm. For example, the green phosphor (Ce, Tb, Gd) MgB5O10 absorbs only to about 10% at 380 nm (T. Jύstel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084) .

To be able to use Tb^{3+ 5}D₄-⁷Fj (J=2-6) transitions as the green primary color in a UV-LED based lamp (where the UV emission is around 380 nm) , one needs to displace the Ce³⁺ 4f-5d absorption bands towards the near-UV region while still maintaining the efficient energy transfer from Ce³⁺ to Tb³⁺.

Compounds of the present invention comprising both Ce³⁺ and Tb³⁺ can provide such new phosphor materials wherein the Tb³⁺-emission is sensitized by Ce³⁺-ions, and the latter ions are capable of absorbing the near-UV light close to 380 nm. In addition, the phosphor materials described can emit two different colors simultaneously and are therefore ideally suited as both blue and green light providers in lighting applications.

Where the compounds of the invention have a melilite structure or a structure related to melilite, it is believed that the compounds of the present invention provide these advantages by virtue of their crystal structure as discussed in more detail below. Such a structure creates a strong crystal field around the Ce³⁺ ions displacing the Ce³⁺ 4f-5d absorption bands towards the near-UV region. At the same time the distance between the lanthanide ions in the structure is not too large, facilitating effective energy transfer between Ce³⁺ and Tb³⁺ ions .

Phosphor materials of the invention that contain both Ce³⁺ and Tb³⁺ therefore have two emission centers, thus they can emit different colors (blue and green) . The relative concentrations of the activator components, i.e. Ce³⁺ and Tb³⁺, can be adjusted at the atomic level simply by adjusting the amount of each cation present in a mixture used to form the compound. A desired output color, for between blue and green, can readily be obtained more precisely than by mixing two separate phosphor compounds. In this respect, the compounds of the invention containing both Ce³⁺ and Tb³⁺ can be considered as "two prime color phosphors" .

The luminescent compounds of the invention containing Ce³⁺ exhibit high levels of absorption when subject to near-UV light, exceeding 95% at 380 nm. They also show intense emissions with a high quantum efficiency. Combined with their excellent stability these properties make them ideal candidates for lighting devices especially near-UV LED- based lighting devices. According to a third aspect the present invention provides a light comprising a compound according to general formula 1: M(Il)Ln(lIl)_x__p__qCe(III)₉Tb(111)_qM'(JIJ)₃O₇ (1) wherein M(II) represents a divalent metal selected from the group of Ca, Sr or mixtures thereof; Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and wherein p and q represent fractional numbers varying between 0 and 1 and l≥p+q>0; as a phosphor.

Preferably the light further comprises a UV LED as exciter for the phosphor. Other types of light making use of an emitter in the near-UV region to excite the phosphor may make use of the compounds of the invention.

The compounds of the present invention may be manufactured by intimately mixing (preferably by grinding) appropriate metal carbonates and/or metal oxides and then sintering the resultant mixture in a reducing atmosphere or under vacuum conditions. Where a reducing atmosphere is employed it may be, for example, a hydrogen-containing atmosphere, as described hereafter with reference to specific embodiments. Other precursor compounds containing the chosen metals that will thermally decompose under appropriate conditions to the metal oxides as required may be employed. For example, nitrates oxalates and other organic salts.

Thus according to a fourth aspect the present invention provides a method for the preparation of a compound according to general formula 1 :

M(ll)Ln(lIl)_λ__p__qCe(lIl)_pTb(Hl)_q M'(HI)₃O₇ ( 1 ) wherein M(II) represents a divalent metal selected from the group of Ca, Sr or mixtures thereof; Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and wherein p and q represent fractional numbers varying between 0 and 1 and l≥p+q>0; as a phosphor;

the method comprising:

intimately mixing together metal oxides, or metal compounds decomposable under the reaction conditions to give metal oxide in calculated proportions to give a mixture having the desired relative compositions of the metal ions, M(II), M' (III), Ln(III), Ce(III) and Tb(III); and sintering the mixture under a reducing atmosphere, or under vacuum conditions, to produce the compound of general formula 1.

An appropriate flux material such as are well known in the art may be added to the mixture to assist forming the compound.

Preferably, when used, the reducing atmosphere is a hydrogen-containing atmosphere. Alternatively a carbon monoxide (CO) atmosphere may be employed. Preferably the sintering is carried out at a temperature in excess of 1000 ⁰C, more preferably in excess of 1300 ⁰C. A hydrogen containing reducing atmosphere is not suitable when Ga, Ge or Zn oxides are present in the mixtures, as they will tend to be reduced directly to their metals under these conditions .

The compounds of the invention may be prepared by other means such as are well known in the art, for example by co- precipitation or sol-gel techniques. Brief description of the Drawings

Fig. 1 is a schematic drawing of the crystal structure of CaLnAl₃O₇; and Figs. 2 and 3 show emission and excitation spectra of compounds of the invention.

Description of the Invention with reference to the preparation of compounds of the invention and results of tests

The following examples illustrate the preparation and the characteristics of the phosphor compounds of the invention.

Example 1

A compound corresponding to the formula CaY_o.8oCe_o.2oAl₃ O₇ was prepared from the following mixture:

Starting Quantity materials ( g)

CaCO₃ 3 . 013

Y₂O₃ 2 . 719

CeO₂ 1 . 036

Al₂O₃ 4 . 604

The powders were intimately mixed by grinding together. The resulting mixture was placed in an alumina or a platinum crucible, and sintered at 1450 °C in a diluted hydrogen flow for 15 hours .

The product is a white crystalline powder, which is insensitive to added water. It shows an intense blue emission when irradiated by 365 nm UV radiation. Example 2

A compound corresponding to the formula CaCe_0-75Tb_0.25Al₃ O₇ was prepared from the following mixture:

Starting Quantity materials (g)

CaCO₃ 2 . 649

CeO₂ 3 . 416

Tb₄O₇ 1 . 236

Al₂O₃ 4 . 047

The powders were intimately mixed by grinding together. The resulting mixture was placed in an alumina or a platinum crucible, and sintered at 1450 ⁰C in a diluted hydrogen flow for 15 hours .

The product obtained is a white crystalline powder, which is insensitive to water. It shows an intense green emission when irradiated by 365 nm UV radiation.

Structural characterization

All compounds prepared using the method described above were found to be crystalline powders. X-ray diffraction study shows that they all have the melilite structure having tetragonal symmetry with the space group P42,m . Structural refinement, using the Rietveld method, was carried out on two representative compounds and the refined results are listed below:

CaCeAl₃O₇

Table 1. Refined lattice constants, atomic positions and thermal parameters of CaCeAl₃O₇ in the space group P42,m . CaCeAl₃O₇

^~a 7.79190(7) A c 5.13908(6) A

x y z B(A)²

Ca/Ce 4e 0.1607(1) 0.6607(1) 0.5071 (3) 0.81(2)

Al(I) 2a 0 0 0 0.75( (9)

Al(2) 4e 0.3560(3) 0.8560(3) 0.9611(6) 0.42(6)

0(1) 2c 0 0.5 0.1704(17) 0.22(10)

0(2) 4e 0.3547(7) 0.8547(7) 0.2897(10) 0.85(15)

0(3) Sf 0.3446(6) 0.4139(5) 0.1899(7) 0.07(6)

R_wp=13.61%, R_p=IO.00%, χ²=2.31

CaCe₀.₇₅Tb_0-2SAl₃O₇

Table 2. Refined lattice constants, atomic positions and thermal parameters of CaCe₀.₇₅Tb₀.₂₅Al₃O₇ in the space group

P42,m .

CaCe₀.₇₅Tb₀.₂₅Al₃O₇ ~a 7.77142(9) A

C 5.11829(8) A

x y z B(A)²

Ca/Ce/Tb 4e 0.1603(1) 0.6603(1) 0.5095 (3) 0.94(2)

Al(I) 2a 0 0 0 0.65( (1O)

Al(2) 4e 0.3555(3) 0.8555(3) 0.9632(8) 0.74(7)

0(1) 2c 0 0.5 0.1685(21) 1.02(28)

0(2) 4e 0.3549(8) 0.8549(8) 0.2875(12) 1.04(17)

0(3) 8f 0.3447(6) 0.4130(6) 0.1901(9) 0.48(12)

R_wp=14.89%, R_p=IO.92%, χ²=2.14 A schematic drawing of the structure of the undoped (i.e. not containing Ce³⁺ or Tb³⁺) compound CaLnAl₃O₇ is shown in Figure 1 to demonstrate the structure of the compounds of the invention.

The compound has the melilite-type structure. As shown in figure 1, the structure consists of layers of edge-shared (Ca, Ln) O₈ polyhedra, which are separated by the layers of corner-linked AlO₄ tetrahedra. The geometry of the (Ca₁Ln)O₈ polyhedra is a deformed square antiprism. Such an anion arrangement is expected to create a strong crystal field around the large lanthanide cations. In addition, the shortest distance, in the case of aluminates, between cations (Ca/Ln) is about 4.1 A. Such a short distance between adjacent cations favors energy transfer between Ce³⁺ and Tb³⁺ ions when they are introduced into the structure.

The luminescent properties of Ce³⁺-doped CaYAl₃O₇ are shown in Fig. 2. Figure 2 shows the excitation and emission spectra of CaY₀.₈Ce₀.2Al₃O₇.

The emission due to the presence of the Ce3⁺-ions is blue with the emission maximum at about 435 nm. The excitation spectrum of Ce³⁺ consists of two broad bands with the strongest between approximately 310 nm and 400 nm (i.e. near UV light). The figure demonstrates that Ca (Y, Ce) Al₃O₇ compounds of the melilite-type can absorb near-UV light efficiently and are suitable for use as the blue prime phosphor in a UV-LED light.

The Tb³⁺ emission in Tb³⁺-doped CaYAl₃O₇ is green consisting of several sharp emission lines corresponding to the transitions between the ⁵D₄ and the multiple ⁷Fj (J=2-6) energy levels as shown in Figure 3 (right hand spectrum - CaY₀.₈Tb₀.₂Al₃O₇ spectrum shown) . However, if doped only with Tb³⁺ these compounds cannot be excited efficiently using near-UV light as the first excitation band corresponding to the Tb³⁺ 4f-5d transitions is centered at shorter wavelength, about 275 nm. Therefore such a compound has more limited utility in lighting applications.

However when the compounds of the invention are co-doped with Ce³⁺, e.g. Ca (Y, Ce, Tb) Al₃O₇ compounds, the excitation spectrum of Tb³⁺ has additional bands in the near UV similar to those observed in the excitation spectrum of Ca (Y, Ce)Al₃O₇ shown in figure 2.

In particular, there is a strong excitation band centered at about 356 nm as shown in the excitation spectrum for CaY₀.7Ce₀.15Tb_0.15Al₃O₇ of figure 3 (left hand spectrum) . This allows Tb³⁺ containing compounds, when co-doped with Ce³⁺, to emit green light efficiently when excited by near-UV light (at around 380 nm) .

The energy transfer from Ce³⁺ to Tb³⁺ in the Ca (Y, Ce, Tb) Al₃O₇ system was found to be incomplete, as a residual Ce³⁺- emission is observed in the emission spectrum upon excitation with light of λ=380 nm. The intensity of these residual Ce³⁺ emissions depends on the relative amounts of Ce and Tb. Phosphor materials with a low Ce and Tb concentration, e.g. CaY_0-7Ce₀.₁₅Tb₀.1₅Al₃O₇, show a bright whitish emission under a high-pressure mercury discharge lamp (λ=365 nm) . On the other hand the emission color of the phosphors in a compound with a relatively high Ce and Tb concentration, e.g. CaCe_0-75Tb₀.₂₅Al₃O₇, is essentially green.

The above experiments demonstrate that the Ce³⁺ and Tb³⁺ containing compounds contain two emission centers, thus emitting different colors (Fig. 2 and Fig. 3) . The amount of each color emitted can be adjusted as desired by altering the level of Ce³⁺ and Tb³⁺ doping applied.

Claims

1. A compound, according to general formula 1:

M(Il)Lπ(JIl),__p__gCe(JIJ)_pTb(JII)_gM'(JJJ)₃θ₇ (1) wherein M(II) represents a divalent metal selected from the group, constituting of Ca, Sr and mixtures thereof ;

Ln(III) is a trivalent metal selected from the group consisting of Y, Gd, La, Lu and mixtures thereof; M' (III) is another trivalent metal selected from the group consisting of Al, Ga and mixtures thereof; and p and q represent fractional numbers varying between 0 and 1, and l≥p+q>0; and, optionally, wherein: the trivalent metal M' (III) is substituted by a tetravalent component M''' (IV) selected from the group consisting of Si(IV), Ge(IV) and mixtures thereof, the ratio of the tetravalent M''' (IV) component to the trivalent metal M' (III) being at up to 1:1; and, to maintain the charge balance : the trivalent metal M' (III) is further substituted with a divalent metal M'' (II); or the divalent metal M(II) is substituted with a monovalent metal M'''' (I); or both the trivalent metal M' (III) and the divalent metal M(II) are substituted with selected amounts of divalent metal M'' (II) and monovalent metal M'''' (I) respectively, sufficient to maintain charge balance of the compound.

2. A compound according to claim 1 wherein p + q >0.1.

3. A compound according to claim 1 or claim 2 containing both Ce and Tb.

4. A compound according to any one of claims 1 to 3 wherein the divalent metal M'' (II) is present and is selected from the group consisting of Zn(II), Mg(II) and mixtures thereof .

5. A compound according to any one of claims 1 to 3 wherein the monovalent metal M'''' (I) is present and is selected from the group consisting of K, Na and mixtures thereof .

6. A compound according to any one of claims 1 to 4 wherein the compound is of formula 2 : M(II)Ln(III)₁.^Ce(III)_pTb(III)_qM'(III)₃__2rM"(II)_rM-(IV)_rO₇ (2)

wherein M(II), Ln(III), M' (III), M'" (IV), M" (II), p and q have the same meaning as before; and r is a fractional number having the value l≥r>0.

7. A compound according to any one of claims 1 to 3 and 5 wherein the compound is of formula 3 : Af(ll)_l-_rM""(J)_rMJI-O₁__p__gCe(JIΪ)_pTHlIϊ)_qM'(IIJ)₃__rM'"(JV)_rO₇ (3 )

wherein M(II), Ln(III), M' (III), M''' (IV), M'''' (I), P and q have the same meaning as before; and r is a fractional number having the value l≥r>0.

8. A compound according to any one of the preceding claims wherein the compound has the melilite structure.

9. A compound according to claim 1 selected from the group consisting of:

CaYi_-pCepAl₃θ7, CaYi_-qTb_qAl₃0₇ and CaYi-p.qCepTbqAlaOv .

10. A compound according to claim 9 selected from the group consisting of:

CaY₀.₈Ce₀.2Al₃O₇ , CaY₀.₈Tb₀.2Al₃O₇ , CaY₀.7Ce₀.1₅Tb₀.1₅Al₃O₇ , CaCe_0.-₇₅Tb₀.2₅A1₃O₇ and CaCeAl₃O₇.

11. The use of a compound according to any one of claims 1 to 10 as a phosphor.

12. A light comprising a compound according to any one of claims 1 to 10 as a phosphor.

13. A light according to claim 12 further comprising a UV LED as exciter for the phosphor.

14. A method for the preparation of a compound according to any one of claims 1 to 10 the method comprising: intimately mixing together metal oxides, or metal compounds decomposable under the reaction conditions to give metal oxide in calculated proportions to give a mixture having the desired relative compositions of the metal ions, ; and sintering the mixture under a reducing atmosphere or under vacuum conditions to produce the compound.

15. A method according to claim 14 wherein a hydrogen- containing reducing atmosphere, or a CO-containing reducing atmosphere, is employed and the sintering is carried out at a temperature in excess of 1000⁰C.