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Crystal structure and fluorescence of compounds Ln2Me4+Me6+O8
J. inorg, nucl. Chem., 1968, Vol. 30. pp. 2091 to 2099.
CRYSTAL
Pergamon Press.
Printed in Great Britain
STRUCTURE AND FLUORESCENCE COMPOUNDS LnzMe4+Me6+Os
OF
G. BLASSE Philips Research Laboratories, N. V. Philips' Gleoilampenfabrieken, Eindhoven, Netherlands (Received 15 February 1968) A b s t r a c t - T h e crystal structures of compounds Ln2Me4+Me6+Os are reported (Ln = La, Gd, Y or Lu; Me 4÷ = Si, Ge or Ti; Me 6÷ = Mo or W). Nearly all of these compounds have the scheelite structure or the fergusonite (distorted scheelite) structure. A remarkable exception is YzSiMoOs, which has zircon structure. In this compound the Si4÷ and Mo 6+ ions are long-range ordered. The fluorescence of the Eu 3+ ion in these host lattices was also investigated. The super structure of Y2SiMoO8 was confirmed by the spectral energy distribution of the Eu 3+ ion in this lattice. The emission spectra of the scheelites point to a considerable degree of order in the case of Me 4+ = Si. The low efficiency of the fluorescence is discussed. The fluorescent of the unactivated host lattices and of Bi3+ and Tb 3÷ ions in these lattices is dealt with. INTRODUCTION
SOME years ago Finch e t al.[ 1] described a series of compounds with composition Ln2GeMoOs (Ln = Pr, Nd, Gd, Tb, D y , Ho, Er, Yb). These compounds are isomorphous with scheelite (CaWO4). The Ge 4÷ and Mo n÷ ions occupy the tetrahedral W 6+ sites of the scheelite structure in a disordered way. In the course of our study of the fluorescence of rare-earth ions it seemed interesting to investiagte these compounds and compounds with a related composition. This paper reports on the crystal structure and fluorescence of the compounds Ln2Me4+Me6+O8 (Ln = La, Gd Y or Lu; M e 4+ = Si 4+, G e 4+ or Ti 4+ and Me 6+ = W 6+ or M06+). In some materials Eu 3+, Tb 3+ or Bi 3÷ ions were introduced and their fluorescence investigated. EXPERIMENTAL Samples were prepared by firing intimate mixtures of high purity oxides at appropriate temperatures in air. Samples with composition LnzTiWOs were fired at 1400°C, those with composition Ln2TiMoOs at 1250°C, those with compositions LrhSiWOs and LnzSiMoO8 at 1150°C and those with composition Ln2GeWO8 and Ln2GeMoOs at 1000°C. All samples were cooled slowly. Compounds containing silicon or germanium could only be obtained by adding some percentage of LnF3 to the starting materials. X-ray diagrams were obtained on a Philips diffractometer using Cu Ka radiation. The optical measurements were performed as described before[2].
RESULTS
The results of the X-ray analysis of our samples are shown in Table 1. Literature data on compounds LnMeS+O4 have been included. Nearly all compounds 1. C. B. Finch, L. A. Harris, G. Wayne-Clark, Rare Earth Research 111 (Edited by LeRoy-Eyring), p. 107. Gordon and Breach, New York. 2. A. Bril and W. L. Wanmaker,J. electrochem. Soc. 111, 1363 (1964). 2091
2092
G. BLASSE T a b l e 1. C r y s t a l l o g r a p h i c d a t a o n c o m p o u n d s L r ~ M e 4 + M e ~ + O s ( t h i s w o r k ) a n d L n M e S + O 4 ( f r o m W y c k o t V s c o m p i l a t i o n [ 16]). Cell d i m e n s i o n s a r e g i v e n in ,~
Me4+ M e 6+ L a 2 M e 4 + M e 6 + O a G d 2 M e 4 + M e 6 + O s
Y2Me4+Me6+Os
Si
W
fergusonite a=b=5-15 c = 11-90 T = 90"5°
fergusonite a = 5.12b = 4.92 c = 11.28 T = 92 .5o
scheelite a = 5.00 c = 11.09
Si
Mo
fergusonite
fergusonite
a=b=5.16 c = 11.89 T = 90"5°
a = 5.10;b = 4.99 c = 11.32 y = 91.5 °
zircon with superstructure a = 7.05 c = 6.34
Lu~Me4+Mee+Os
Ge
W
scheelite a = 5.25 c = 11.81
scheelite a = 5.11 c = 11.20
scheelite a = 5.06 c = 11.06
scheelite a = 5.02 c = 10.90
Ge
Mo
scheelite a = 5.24 c = 11.80
scheelite a = 5.10 c = 11.22
scheelite a = 5.05 c = 11.02
scheelite a = 5-02 c = 10.96
scheelite a = 5.16 c = 11.17
scneelite a ---- 5-12 c= 11.00
scheelite a = 5.06 c = 10.85
scheelite a = 5.15 c = 11-15
scheelite a = 5.11 c = 11.02
scheelite a = 5.07 c = 10.88
huttonite zircon fergusonite
zircon zircon fergusonite
zircon zircon fergusonite
Ti
Ti
P V Nb
W
Mo
huttonite huttonite fergusonite
investigated have the scheelite structure (CaWO4) or fergusonite structure (YNbO4). The latter structure can be considered as a monoclinically distorted scheelite structure. However, there are some exceptions: "Lu2SiWOs" and "Lu~SiMoOs" were not formed under our experimental conditions. The X-ray diagrams show the reflections of Lu2WO6 and Lu2 M006 which were reported earlier by us [3]. The compound YzSiMoOs has the zircon structure, i.e. it can be derived from YPO4 (or YVO4) by replacing two P~+ (or V 5÷) ions by a Si 4÷ and a Mo 6÷ ion. The X-ray powder pattern of Y2SiMoOs shows the reflections expected for a zircon lattice. For all of these the sum of the indices should be even (the zircon structure with space group D ~ is based on body-centered crystallographic lattices). In addition nearly all reflections for which the sum of the indices is odd are present with a weak intensity. This points to long-range order between the Si4+ and Mo 6+ ions without change of the unit-cell dimensions. 3. G . B l a s s e , J . inorg, nucL Chem. 28, 1 4 8 8 ( 1 9 6 6 ) .
Crystal structure and fluorescence of compounds Ln~Me4+Mee+Os
2093
The X-ray diagrams of compositions "La~TiWOs" and "La~TiMoOa" were rather complex. In both cases the reflections of La2Ti207 were observed. The other reflections could be indexed on a scheelite unit cell in the case of "La~TiMoOs". In the case of "La~TiWO8", however, these could not be indexed. This scheelite phase may have a composition in between La2(MoO4)3 (with ordered scheelite structure [4]) and La2TiMoOa. Compositions in between Y2SiWOs (scheelite) and Y2SiMoOs (zircon), Y2SiMoOs (zircon) and Y2GeMoOs (scheelite) Y~SiMoO8 and YVO4 (both zircon) and Y2SiWO8 (scheelite) and YVO4 (zircon) were also studied to obtain a rough idea of the extension of the solid solution regions involved. Large amounts of Mo can be introduced into Y2SiWOs: the miscibility gap occurs between approximately 75 and 90 atomic per cent of molybdenum. Also in the system Y~(Si, Ge) MoO8 the zircon phase has no broad concentration width: the miscibility gap occurs roughly between 75 and 90 atomic per cent of silicon. The system YzSiMoOa-YVO4 shows no miscibility gap, but the intensity of the superstructure reflections in the powder pattern of Y2SiMoOa decreases with increasing YVO4 content. For 20 mole per cent of YVO4 they have disappeared. Only a small amount of YVO4 could be dissolved in Y2SiWOs. There is a miscibility gap between approximately 10 and 40 mole per cent of YVO4. Samples with composition LrhSiWOa show weak, blue-green, fluorescence at room temperature under 254 nm excitation. At liquid nitrogen temperature the intensity of this fluorescence has considerably increased. The compounds LnsSiMoOs show orange fluorescence at this temperature, but not at room temperature. The materials containing germanium and titanium show only a weak, near-white fluorescence, even at liquid nitrogen temperature. Several samples were activated with Eu 3+ by replacing Ln s÷ for a few atomic percent by Eu a÷. The intensity of the red Eu a÷ fluorescence in these materials varies from weak to nearly undetectable (ultraviolet as well as cathode-my excitation). Compounds LnzSiWOa were activated by Bi3+. Materials of this type show a green emission under ultraviolet excitation. At liquid N2 temperature they show a red emission however. Activation with Tb 3÷ was also performed. Materials of this type did not fluoresce under ultraviolet or cathode-ray excitation. The spectral energy distribution of the emission of YzSiWOs-Eu and Y2GeWOs-Eu is given in Fig. 1. Although both compounds have scheelite structure, the spectra are different: that of Y2GeWOs-Eu shows broad lines, that of Y2SiWOs-Eu narrow lines. This is very well illustrated by the SD0-TF 1 emission transitions in the 590-600 nm region. Sharp line emission was observed for the scheelites (or fegusonites) Ln~SiWOs-Eu and Ln2SiMoOs-Eu. Broad line emission was observed for the scheelites Ln2GeWOs-Eu~÷, Ln2GeMoOs-Eua+ and Ln2TiWOs-Eu3+. Samples with composition Ln~TiMoOs-Eu3+ did not fluoresce. The spectral energy distribution of the emission of YzSiMoOs-Eu is given in Fig. 2. Figure 3 shows the spectral energy distribution of the emission, the diffuse reflection spectra and the excitation spectra of the emission of YzSiWOs 4. K. Nassau, H. J. Levinstein and G. M. Loiacono,J. Phys. Chem. Solids 26, 1805 (1965).
2094
G. BLASSE
~
_
640
_
L
620
-
-
600
Y
-
580
640
620
Mnm) 600
580
• Fig. I. Spectral energy distribution of the emission of Y2GeWOs-Euz+(a) and Y2SiWOaEu3+ (b) under 254 nm excitation. Along the ordinate the radiant powder per constnat wavelength interval (I) is plotted in arbitrary units. and Y2SiWOs-Bi; Fig. 4 the spectral energy distribution of the emission at 77°K of Y2 SiMoOs. T h e fluorescence of Y2SiW0.sMoo.208 was also studied. This composition fluoresces blue-green at 77°K under 2 5 4 n m excitation. T h e spectral energy distribution of the f o r m e r emission resembles that of the emission of undoped Y2SiWO8. T h e spectral energy distribution of the orange emission resembles that of the emission of undoped Y2$iMoO8 although the m a x i m u m of the emission band is at s o m e w h a t longer wavelengths than in the case of Y2SiMoOa (See Fig. 4). T h e fluorescence efficiency of the present materials is low. T h e energy conversion factor for cathode-ray excitation is 0-2 per cent or less. T h e quantum efficiencies for u.v. excitation are 10 per cent or (in m a n y cases) less. In the case of Y2SiWO8 the t e m p e r a t u r e dependence of the fluorescence was measured. T h e quenching range of the fluorescence extends f r o m 200 to 330°K. DISCUSSION In view of the work of Finch et al.[1] it is not surprising to find that m o s t of the materials studied have the scheelite structure. Finch et al. described the c o m p o u n d s Ln2GeMoO8 (Ln = Pr, Nd, G d , Tb, D y , H o , Er or Yb) as disordered scheelites i.e. the G e 4+ and Mo 6+ ions o c c u p y the tetrahedral sites in the scheelite lattice in a disordered way. This follows also f r o m the present work in a different way. T h e emission spectra of samples L n 2 G e M o O s - E u 8+ consist of broad lines. Usually the Eu 3+ emission of EuZ+-activated phosphors consists of narrow peaks
Crystal structure and fluorescence of compounds Ln2Me4+Me6+Os
J I
I
640
I
I
620
2095
600
580
~(nm) Fig. 2. Spectral energy distribution of the emission of Y2SiMoOa-Eu 3+ under 254 nm excitation. Relative quantum output Reflection (%)
/
10C
100
I
,
;x,
,,,
t
/.,,
/
///
50 /
/
600
\
/
/
/
', \',
,,
i
\
,~/-... \" ..,,
~
\
I
~'"~../
/' ~
',,. \ ;--.../' 500
400 v
380
350
,
50
.. 300 ~(nm)
250
Fig. 3. Left-hand side: Spectral energy distribution of the emission of Y2SiWO8 (drawn line) and of Y~SiWOs-Bi (broken line) under 254 nm excitation. Right-hand side: relative excitation spectra of the fluorescence (E) and diffuse reflection spectra (R) of Y2SiWO8 (drawn line) and of Y2SiWOs-Bi (broken line) Note the change in the wavelength scale.
2096
G. BLASSE
i'"
ii,//K"--. ",,",,, \
-
)0
I/
I
500
"-
I
I
600
700 },(nm)
Fig. 4. Spectral energy distribution of the emission at 77°K of Y~SiMoO8 (drawn line) and Y2SiW0.sMo0.2Os (broken line) under 365 nm excitation.
corresponding to transitions from the 5D0 (or higher) level down to the 7F manifold. If, however, the surroundings of the Eu 3+ ion vary from activator site to activator site, broad emission lines are found due to the fact that the crystal field at the Eu 3÷ ion varies[5, 6]. If the compounds Ln2GeMoOs-Eu3÷ are disordered, the surroundings of the Eu a÷ ions are different due to the disorder of Ge 4÷ and Mo e+ ions. Therefore, broad emission lines are expected, as found experimentally. For this reason we assume that Ln2GeWOs and Ln2TiWOs are also disordered scheelites. Unfortunately the samples Ln2TiMoOs-Eu do not fluoresce. It seems probable, that they are also disordered. The disordered scheelite phase Ca0.8 Eu0.2 Nb0.2 W0.8 04[7] shows the same emission spectrum as the present scheelites. A different behaviour is shown by the silicon-containing scheelites and fergusonites. The emission of the EuZ+-activated samples consists of sharp lines, indicating long-range order between SP + and W 6+ (or Mo 6÷) ions. The x-ray patterns do not show reflections in addition tO the scheelite (or fergusonite) reflections not even after the samples have been annealed. Perhaps the degree of order between the Si4÷ and Mo 6÷ ions is high enough to influence the emission spectrum of the Eu z+ ions, but not high enough to give supersturcture reflections in the X-ray powder diagram. That the scheelites with Ge 4÷ and Ti 4÷ ions are even less ordered is compatible with the fact that the difference between the ionic radii of Me 4+ and Me 6÷ ions is greatest in the case of Si4+, a greater difference between the radii resulting in a higher ordering energy. Due to the uncertainty about the degree of order between the SP + and the hexavalent ions it is not possible to give a reason for the distortion from scheelite to fergusonite structure in the case of La~SiWOs and La2SiMoOa and the analogous gadolinium compounds (See Table 1). The distortion may either be due to a certain degree of order between the smaller cations or to geometric reasons (as in YNbOa, where no superstructure is possible). Note that the distortion does not vary regularly in the sequency La, Gd, Y. 5. G. Blasse and A. Bill, Philips Res. Rep. 21,368 (1966). 6. G. Blasse and A. BriI,J. inorg, nucl. Chem. 29,2231 (1967). 7. L. H. Brixner, J. electrochem. Soc. 111,690 (1964).
Crystal structure and fluorescence of compounds Ln2 Me 4+Me6+OB
2097
Let us now consider Y2SiMoOs. In view of the results for scheelites longrange order between the Si4÷ and Mo 6÷ ions on the tetrahedral sites is possible. The X-ray diagram shows indeed a large number of superstructure reflections. These have h + k-t-l odd, and can be indexed on the same unit cell as that of zircon (e.g. YVO4). The only possible arrangement for the Si4+ and Mo 6÷ ions is therefore: Si4÷ on ½, ½, 0 and ½, 0, ¼ and Mo 6+ on 0, 0, ½and 0, ½, ]. The symmetry is no longer tetragonal, but orthorhombic, although the powder diagram can be indexed on a (pseudo) tetragonal unit cell. In the system Y2SiMoOs-YVO4 the superstructure reflections already disappear for low vanadium-content, the V s÷ ions hampering the long-range order between the Si4÷ and Mo 6+ ions. The emission spectrum of Y2SiMoOs-Eu a+ consists of sharp lines, as is to be expected. We will now compare the emission spectrum of YVO4-Eu a÷ and that of Y2SiWOs-Eu z+ to study the influence of long-range order at the tetrahedral sites on the emission transitions. In YVO4-Eu the Eu a÷ ion occupies a site with D2d symmetry. The numbers of emission lines of the 5D0-TF0, 5Do-TF~ and 5D0-rF2 transitions of the Eu a+ ion at a site with this symmetry are zero, two and two, respectively. This has been found experimentally: 593.5 nm and 595.0 nm for the single and twofold degenerate 5D0-rF~ emission line and 615-5 nm and 619.4 nm for the single and twofold degenerate 5Do-TF2emission line, respectively[8]. Due to the order of Si4÷ and Mo 6÷ ions in Y2SiMoOs-Eu the site-symmetry of the Eu 3÷ ion is lowered from D2a to C2v. For this symmetry the numbers of emission lines of the 5D0-TFo, 5D0-rF~ and 5D0-TF2 transitions of the Eu 3+ ion are one, three and four, respectively. This is in good agreement with the experimental results (Fig. 2): 5DoJFo: 581 nm; 5D0-TF~:590, 593 and 598 nm; 5D0-TF2:610, 613,617 and 626 nm. The long-range order of the smaller cations in the zircon structure influences the spectral energy distribution of the Eu a+ emission strongly. This is especially clear for the 5D0-TF1 emission (around 595 nm), for which all components are observed in both cases. It is obvious to correlate the 590 and 598 nm components in the case of Y2SiMoOs-Eu to the doubly degenerate 595.0 nm component of YVO4-Eu, and the 593 nm component of Y2SiWOs-Eu to the 593.5 nm component of YVO4-Eu. This means that the doubly degenerate crystal-field component of the 7F~ level of Eu 3+ in YVO4 is split by some 250 cm -~ by the long-range order of the Si4÷ and Mo 6÷ ions. It remains, however, to be explained why the emission spectrum of Y2SiMoOsEu 3÷ indicates only one type of Eua+-centre. Due to the long-range order of Si4÷ and Mo 6+ ions two types of y3+ sites are present in Y2SiMoOs. One of these sites has two Si4÷ at roughly 3.2 A ( Y - O - S i angle-90°), two Si4÷ and two Mo 6÷ at roughly 3.7 ~ ( Y - O - S i (Mo) angle-180 °) and four Mo 6+ at 4.5 .~ (no oxygen ion in common). The other has the same surroundings with Si4÷ and Mo 6÷ ions reversed. This makes no difference for the neighbours at roughly 3-7/~. We are forced, therefore, to conclude that these neighbours are responsible for the lower-symmetrical crystal-field components relative to the situation in YVO4-Eu. Note that only for these neighbours in the angle Y - O - S i (Mo) - 1 8 0 °, so that strong interaction via o--bonding may be expected. 8. C. Brecher, H. Samelson, A. Lempicki, R. Riley and T. Peters, Phys. Rev. 155, 178 (1967).
2098
G . BLASSE
Such a situation does not occur for Y2SiWO8-Eu. The 5D0-TF, emission of this phosphor (See Fig. 1) contains at least five lines, indicating more than one type of Eu 3+ ions (for one type the maximum is three). The occurence of Y2SiMoO8 with zircon structure is clearly an exception. All other compounds Ln2Me4+Me6+O8 studied have the scheelite or the (clearly related) fergusonite structure. The energy difference between the zircon and scheelite phase of Y2SiMoOs is expected to be small in view of the fact that replacement of small amounts of Si4÷ by Ge 4+ or of Mo 6÷ by W 6+ results in compositions with scheelite structure. It is interesting to note that in the system Y2SiWOs--Y2SiMoO8 there is a narrow miscibility gap at the zircon side, whereas in the system Y2SiWO8- YVO4 there is a considerably broader miscibility gap at the scheelite side. The fluorescence of the unactivated host lattices Ln2SiWOa resembles that of other tungstates with scheelite structure like CaWO4[9] and Y2(WO~)3[10]. The emission of LnzSiWOs is situated in the same spectral region as in the case of CaWO4 and Y2(WO4)3. The quenching temperature of the fluorescence of Ln2SiWO8 is about equal to that of Y2(WO4)3 but lower than that of CaWO4. The absence of fluorescence in Tb3+-activated tungstates was reported before [11]: Y6WO,2-Tb and Y2WO6-Tb do not fluoresce. The host lattice absorption edge is situated in the neighbourhood of 300 nm, as in the case of Y2SiWOs. Only Y2(WO4)3-Tb was found to fluoresce. The absorption edge of this host lattice is situated at shorter wavelengths, viz. 245 nm. The fluorescence of Y2SiWOs-Bi resembles that of the Bi 3+ activated tungstates, for example Y2WO6-Bi[12]. The shift of the absorption edge to longer wavelengths has been ascribed to a charge transfer process in which an electron is transferred from the 6s orbital of Bi3+ to the empty 5d orbital of W 6÷. The emission consist of the same process and shifts therefore to longer wavelengths compared to the host lattice emission (Fig. 3). The red emission of Y2SiWOs-Bi at low temperatures was also observed for YzWOs-Bi and cannot be assigned at the moment. Finally we may ask why the efficiency of the phosphors LnzMe4+Me6+O8 is so low. Consider for example, Y2SiMoOs-Eu 3+. The isomorphous phosphor YVO4-Eu has a very high efficiency[13]. It can be shown that Y~SiMoOs-Eu does not satisfy the conditions for efficient Eu3+-activated phosphors given by us before[14]. The molydate emission of the host lattice is situated far in the red and by no means overlaps the absorption of Y2SiMoO8 (this compound is white has its absorption edge at 270 nm. Energy transfer through the lattice is therefore highly improbable. There seems to be no reason why the transfer from the molybdate group to an Eu 3+ ion should not occur. The molybdate emission overlaps especially the 7F0-SD0 absorption line of the Eu3+ion and the M o - O - E u angle is expected to be large[14]. If we assume that the probability ofmolybdate---~molyb9. I0. 11. 12. 13. 14.
F . A . Kr6ger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam (1948). H.J. Borchardt,J. chem. Phys. 39,504 (1963). G. Blasse and A. Bril, PhilipsRes. Rep. 22,481 (1967). G. Blasse and A. Bril,J. chem. Phys. 48, (1968). In press. A. Bril, W. L. Wanmaker and J. Broos,J. chem. Phys. 43, 311 (1965). G. Blasse, J. chem. Phys. 45, 2356 (1966).
Crystal structure and fluorescence of compounds Lr~ Me4+Mee+O8
2099
date transfer is zero and that of molybdate---~Eu3+ transfer is high, it is possible to make a quantitative estimation of the quantum efficiency for excitation in the molybdate absorption band. Nearly all exciting quanta are absorbed by the molybdate groups. Because of the absence of transfer through the host lattice this exciting energy can only be transferred to Eu 3÷, if the molybdate group has a nearest Eu 3÷ neighbour. For Eu 3+ concentrations of 1, 2 and 4 atom per cent the probability of such a situation and, therefore the maximum quantum efficiency of the Eu 3+ emission is 4, 8 and 15 per cent, respectively the number of y3+ sites which the Mo 6+ ion has under large M o - O - Y angles is four; the probability of Eu a÷ neighbours is therefore 1 - ( 1 - x ) 4, if x is the Eu3+-concentration). The experimental values of the quantum efficiency for excitation in the molybdate group for Eu3+-concentrations of 1,2 and 4 atom per cent are 3, 7 and 13 per cent, respectively. Although the accuracy of the experimental figures is not very high, the agreement between the two sets of values is satisfactory. For the phosphors Ln2SiWOs-Eu 3+ with scheelite structure the situation is different. From Fig. 4 it can be seen that the emission and absorption band of the host lattice overlap each other in this case, although the overlap is small. It may be, however, that transfer through the lattice is nevertheless impossible. This can be the case if the order between the Si4÷ and W 6+ ions is of such a kind that the tungstate groups are not near neighbours in the crystal lattice (in Y2SiMoO8 order is such that the molybdate tetrahedra still touch each other). The emission of Y2SiW0.sMo0.2Os (scheelite structure) points to inefficient transfer through the tungstate host lattice. Under long-wave u.v. excitation the emission is situated in the red, under shortwave ultraviolet excitation in the blue. In the former case the molybdate groups are excited and molybdate emission occurs; in the latter case the tungstate groups are excited and tungstate emission occurs. This is an unusual behaviour for tungstates containing a certain amount of molybdenum. Usually excitation energy absorbed by the tungstate groups is transferred efficiently to the molybdate groups, the energy overlap being very favourable [9, 15]. From this it can only be concluded that in YzSiW0.sMoo.208 there are geometrical reasons preventing energy transfer between the hexavalent cation polyhedra. This will then also be the case for the phosphors LneSiWOs-Eu and explains why the Eu 3+ efficiency for excitation into the host lattice is low. Similar phenomena probably play a role in the compounds containing germanium or titanium. In view of their weak fluorescence even at low temperatures, they were not studied further. The emission of the molybdate group in Y2SiMoO8 (zircon structure) is slightly different from that of the molybdate group in YzSiW0.8Oo.208 (scheelite structure). The emission of YzSiW0.sMoo.208 under 365 nm excitation shows also a weak band at about 450 nm (tungstate emission). The excitation spectrum of the tungstate group extends in fact up to this excitation wavelength. Acknowledgements-The author is greatly indebted to Dr. A. Bril for the performance of the optical measurements and to Mr. J. de Vries for the preparation of the samples. 15. G. Blasse and A. Bril,J. chem. Phys. 45, 2350 (1966). 16. R . W . G . Wyckoff, Crystal Structures, 2nd Edn. Interscience, New York (1965).
CRYSTAL
Pergamon Press.
Printed in Great Britain
STRUCTURE AND FLUORESCENCE COMPOUNDS LnzMe4+Me6+Os
OF
G. BLASSE Philips Research Laboratories, N. V. Philips' Gleoilampenfabrieken, Eindhoven, Netherlands (Received 15 February 1968) A b s t r a c t - T h e crystal structures of compounds Ln2Me4+Me6+Os are reported (Ln = La, Gd, Y or Lu; Me 4÷ = Si, Ge or Ti; Me 6÷ = Mo or W). Nearly all of these compounds have the scheelite structure or the fergusonite (distorted scheelite) structure. A remarkable exception is YzSiMoOs, which has zircon structure. In this compound the Si4÷ and Mo 6+ ions are long-range ordered. The fluorescence of the Eu 3+ ion in these host lattices was also investigated. The super structure of Y2SiMoO8 was confirmed by the spectral energy distribution of the Eu 3+ ion in this lattice. The emission spectra of the scheelites point to a considerable degree of order in the case of Me 4+ = Si. The low efficiency of the fluorescence is discussed. The fluorescent of the unactivated host lattices and of Bi3+ and Tb 3÷ ions in these lattices is dealt with. INTRODUCTION
SOME years ago Finch e t al.[ 1] described a series of compounds with composition Ln2GeMoOs (Ln = Pr, Nd, Gd, Tb, D y , Ho, Er, Yb). These compounds are isomorphous with scheelite (CaWO4). The Ge 4÷ and Mo n÷ ions occupy the tetrahedral W 6+ sites of the scheelite structure in a disordered way. In the course of our study of the fluorescence of rare-earth ions it seemed interesting to investiagte these compounds and compounds with a related composition. This paper reports on the crystal structure and fluorescence of the compounds Ln2Me4+Me6+O8 (Ln = La, Gd Y or Lu; M e 4+ = Si 4+, G e 4+ or Ti 4+ and Me 6+ = W 6+ or M06+). In some materials Eu 3+, Tb 3+ or Bi 3÷ ions were introduced and their fluorescence investigated. EXPERIMENTAL Samples were prepared by firing intimate mixtures of high purity oxides at appropriate temperatures in air. Samples with composition LnzTiWOs were fired at 1400°C, those with composition Ln2TiMoOs at 1250°C, those with compositions LrhSiWOs and LnzSiMoO8 at 1150°C and those with composition Ln2GeWO8 and Ln2GeMoOs at 1000°C. All samples were cooled slowly. Compounds containing silicon or germanium could only be obtained by adding some percentage of LnF3 to the starting materials. X-ray diagrams were obtained on a Philips diffractometer using Cu Ka radiation. The optical measurements were performed as described before[2].
RESULTS
The results of the X-ray analysis of our samples are shown in Table 1. Literature data on compounds LnMeS+O4 have been included. Nearly all compounds 1. C. B. Finch, L. A. Harris, G. Wayne-Clark, Rare Earth Research 111 (Edited by LeRoy-Eyring), p. 107. Gordon and Breach, New York. 2. A. Bril and W. L. Wanmaker,J. electrochem. Soc. 111, 1363 (1964). 2091
2092
G. BLASSE T a b l e 1. C r y s t a l l o g r a p h i c d a t a o n c o m p o u n d s L r ~ M e 4 + M e ~ + O s ( t h i s w o r k ) a n d L n M e S + O 4 ( f r o m W y c k o t V s c o m p i l a t i o n [ 16]). Cell d i m e n s i o n s a r e g i v e n in ,~
Me4+ M e 6+ L a 2 M e 4 + M e 6 + O a G d 2 M e 4 + M e 6 + O s
Y2Me4+Me6+Os
Si
W
fergusonite a=b=5-15 c = 11-90 T = 90"5°
fergusonite a = 5.12b = 4.92 c = 11.28 T = 92 .5o
scheelite a = 5.00 c = 11.09
Si
Mo
fergusonite
fergusonite
a=b=5.16 c = 11.89 T = 90"5°
a = 5.10;b = 4.99 c = 11.32 y = 91.5 °
zircon with superstructure a = 7.05 c = 6.34
Lu~Me4+Mee+Os
Ge
W
scheelite a = 5.25 c = 11.81
scheelite a = 5.11 c = 11.20
scheelite a = 5.06 c = 11.06
scheelite a = 5.02 c = 10.90
Ge
Mo
scheelite a = 5.24 c = 11.80
scheelite a = 5.10 c = 11.22
scheelite a = 5.05 c = 11.02
scheelite a = 5-02 c = 10.96
scheelite a = 5.16 c = 11.17
scneelite a ---- 5-12 c= 11.00
scheelite a = 5.06 c = 10.85
scheelite a = 5.15 c = 11-15
scheelite a = 5.11 c = 11.02
scheelite a = 5.07 c = 10.88
huttonite zircon fergusonite
zircon zircon fergusonite
zircon zircon fergusonite
Ti
Ti
P V Nb
W
Mo
huttonite huttonite fergusonite
investigated have the scheelite structure (CaWO4) or fergusonite structure (YNbO4). The latter structure can be considered as a monoclinically distorted scheelite structure. However, there are some exceptions: "Lu2SiWOs" and "Lu~SiMoOs" were not formed under our experimental conditions. The X-ray diagrams show the reflections of Lu2WO6 and Lu2 M006 which were reported earlier by us [3]. The compound YzSiMoOs has the zircon structure, i.e. it can be derived from YPO4 (or YVO4) by replacing two P~+ (or V 5÷) ions by a Si 4÷ and a Mo 6÷ ion. The X-ray powder pattern of Y2SiMoOs shows the reflections expected for a zircon lattice. For all of these the sum of the indices should be even (the zircon structure with space group D ~ is based on body-centered crystallographic lattices). In addition nearly all reflections for which the sum of the indices is odd are present with a weak intensity. This points to long-range order between the Si4+ and Mo 6+ ions without change of the unit-cell dimensions. 3. G . B l a s s e , J . inorg, nucL Chem. 28, 1 4 8 8 ( 1 9 6 6 ) .
Crystal structure and fluorescence of compounds Ln~Me4+Mee+Os
2093
The X-ray diagrams of compositions "La~TiWOs" and "La~TiMoOa" were rather complex. In both cases the reflections of La2Ti207 were observed. The other reflections could be indexed on a scheelite unit cell in the case of "La~TiMoOs". In the case of "La~TiWO8", however, these could not be indexed. This scheelite phase may have a composition in between La2(MoO4)3 (with ordered scheelite structure [4]) and La2TiMoOa. Compositions in between Y2SiWOs (scheelite) and Y2SiMoOs (zircon), Y2SiMoOs (zircon) and Y2GeMoOs (scheelite) Y~SiMoO8 and YVO4 (both zircon) and Y2SiWO8 (scheelite) and YVO4 (zircon) were also studied to obtain a rough idea of the extension of the solid solution regions involved. Large amounts of Mo can be introduced into Y2SiWOs: the miscibility gap occurs between approximately 75 and 90 atomic per cent of molybdenum. Also in the system Y~(Si, Ge) MoO8 the zircon phase has no broad concentration width: the miscibility gap occurs roughly between 75 and 90 atomic per cent of silicon. The system YzSiMoOa-YVO4 shows no miscibility gap, but the intensity of the superstructure reflections in the powder pattern of Y2SiMoOa decreases with increasing YVO4 content. For 20 mole per cent of YVO4 they have disappeared. Only a small amount of YVO4 could be dissolved in Y2SiWOs. There is a miscibility gap between approximately 10 and 40 mole per cent of YVO4. Samples with composition LrhSiWOa show weak, blue-green, fluorescence at room temperature under 254 nm excitation. At liquid nitrogen temperature the intensity of this fluorescence has considerably increased. The compounds LnsSiMoOs show orange fluorescence at this temperature, but not at room temperature. The materials containing germanium and titanium show only a weak, near-white fluorescence, even at liquid nitrogen temperature. Several samples were activated with Eu 3+ by replacing Ln s÷ for a few atomic percent by Eu a÷. The intensity of the red Eu a÷ fluorescence in these materials varies from weak to nearly undetectable (ultraviolet as well as cathode-my excitation). Compounds LnzSiWOa were activated by Bi3+. Materials of this type show a green emission under ultraviolet excitation. At liquid N2 temperature they show a red emission however. Activation with Tb 3÷ was also performed. Materials of this type did not fluoresce under ultraviolet or cathode-ray excitation. The spectral energy distribution of the emission of YzSiWOs-Eu and Y2GeWOs-Eu is given in Fig. 1. Although both compounds have scheelite structure, the spectra are different: that of Y2GeWOs-Eu shows broad lines, that of Y2SiWOs-Eu narrow lines. This is very well illustrated by the SD0-TF 1 emission transitions in the 590-600 nm region. Sharp line emission was observed for the scheelites (or fegusonites) Ln~SiWOs-Eu and Ln2SiMoOs-Eu. Broad line emission was observed for the scheelites Ln2GeWOs-Eu~÷, Ln2GeMoOs-Eua+ and Ln2TiWOs-Eu3+. Samples with composition Ln~TiMoOs-Eu3+ did not fluoresce. The spectral energy distribution of the emission of YzSiMoOs-Eu is given in Fig. 2. Figure 3 shows the spectral energy distribution of the emission, the diffuse reflection spectra and the excitation spectra of the emission of YzSiWOs 4. K. Nassau, H. J. Levinstein and G. M. Loiacono,J. Phys. Chem. Solids 26, 1805 (1965).
2094
G. BLASSE
~
_
640
_
L
620
-
-
600
Y
-
580
640
620
Mnm) 600
580
• Fig. I. Spectral energy distribution of the emission of Y2GeWOs-Euz+(a) and Y2SiWOaEu3+ (b) under 254 nm excitation. Along the ordinate the radiant powder per constnat wavelength interval (I) is plotted in arbitrary units. and Y2SiWOs-Bi; Fig. 4 the spectral energy distribution of the emission at 77°K of Y2 SiMoOs. T h e fluorescence of Y2SiW0.sMoo.208 was also studied. This composition fluoresces blue-green at 77°K under 2 5 4 n m excitation. T h e spectral energy distribution of the f o r m e r emission resembles that of the emission of undoped Y2SiWO8. T h e spectral energy distribution of the orange emission resembles that of the emission of undoped Y2$iMoO8 although the m a x i m u m of the emission band is at s o m e w h a t longer wavelengths than in the case of Y2SiMoOa (See Fig. 4). T h e fluorescence efficiency of the present materials is low. T h e energy conversion factor for cathode-ray excitation is 0-2 per cent or less. T h e quantum efficiencies for u.v. excitation are 10 per cent or (in m a n y cases) less. In the case of Y2SiWO8 the t e m p e r a t u r e dependence of the fluorescence was measured. T h e quenching range of the fluorescence extends f r o m 200 to 330°K. DISCUSSION In view of the work of Finch et al.[1] it is not surprising to find that m o s t of the materials studied have the scheelite structure. Finch et al. described the c o m p o u n d s Ln2GeMoO8 (Ln = Pr, Nd, G d , Tb, D y , H o , Er or Yb) as disordered scheelites i.e. the G e 4+ and Mo 6+ ions o c c u p y the tetrahedral sites in the scheelite lattice in a disordered way. This follows also f r o m the present work in a different way. T h e emission spectra of samples L n 2 G e M o O s - E u 8+ consist of broad lines. Usually the Eu 3+ emission of EuZ+-activated phosphors consists of narrow peaks
Crystal structure and fluorescence of compounds Ln2Me4+Me6+Os
J I
I
640
I
I
620
2095
600
580
~(nm) Fig. 2. Spectral energy distribution of the emission of Y2SiMoOa-Eu 3+ under 254 nm excitation. Relative quantum output Reflection (%)
/
10C
100
I
,
;x,
,,,
t
/.,,
/
///
50 /
/
600
\
/
/
/
', \',
,,
i
\
,~/-... \" ..,,
~
\
I
~'"~../
/' ~
',,. \ ;--.../' 500
400 v
380
350
,
50
.. 300 ~(nm)
250
Fig. 3. Left-hand side: Spectral energy distribution of the emission of Y2SiWO8 (drawn line) and of Y~SiWOs-Bi (broken line) under 254 nm excitation. Right-hand side: relative excitation spectra of the fluorescence (E) and diffuse reflection spectra (R) of Y2SiWO8 (drawn line) and of Y2SiWOs-Bi (broken line) Note the change in the wavelength scale.
2096
G. BLASSE
i'"
ii,//K"--. ",,",,, \
-
)0
I/
I
500
"-
I
I
600
700 },(nm)
Fig. 4. Spectral energy distribution of the emission at 77°K of Y~SiMoO8 (drawn line) and Y2SiW0.sMo0.2Os (broken line) under 365 nm excitation.
corresponding to transitions from the 5D0 (or higher) level down to the 7F manifold. If, however, the surroundings of the Eu 3+ ion vary from activator site to activator site, broad emission lines are found due to the fact that the crystal field at the Eu 3÷ ion varies[5, 6]. If the compounds Ln2GeMoOs-Eu3÷ are disordered, the surroundings of the Eu a÷ ions are different due to the disorder of Ge 4÷ and Mo e+ ions. Therefore, broad emission lines are expected, as found experimentally. For this reason we assume that Ln2GeWOs and Ln2TiWOs are also disordered scheelites. Unfortunately the samples Ln2TiMoOs-Eu do not fluoresce. It seems probable, that they are also disordered. The disordered scheelite phase Ca0.8 Eu0.2 Nb0.2 W0.8 04[7] shows the same emission spectrum as the present scheelites. A different behaviour is shown by the silicon-containing scheelites and fergusonites. The emission of the EuZ+-activated samples consists of sharp lines, indicating long-range order between SP + and W 6+ (or Mo 6÷) ions. The x-ray patterns do not show reflections in addition tO the scheelite (or fergusonite) reflections not even after the samples have been annealed. Perhaps the degree of order between the Si4÷ and Mo 6÷ ions is high enough to influence the emission spectrum of the Eu z+ ions, but not high enough to give supersturcture reflections in the X-ray powder diagram. That the scheelites with Ge 4÷ and Ti 4÷ ions are even less ordered is compatible with the fact that the difference between the ionic radii of Me 4+ and Me 6÷ ions is greatest in the case of Si4+, a greater difference between the radii resulting in a higher ordering energy. Due to the uncertainty about the degree of order between the SP + and the hexavalent ions it is not possible to give a reason for the distortion from scheelite to fergusonite structure in the case of La~SiWOs and La2SiMoOa and the analogous gadolinium compounds (See Table 1). The distortion may either be due to a certain degree of order between the smaller cations or to geometric reasons (as in YNbOa, where no superstructure is possible). Note that the distortion does not vary regularly in the sequency La, Gd, Y. 5. G. Blasse and A. Bill, Philips Res. Rep. 21,368 (1966). 6. G. Blasse and A. BriI,J. inorg, nucl. Chem. 29,2231 (1967). 7. L. H. Brixner, J. electrochem. Soc. 111,690 (1964).
Crystal structure and fluorescence of compounds Ln2 Me 4+Me6+OB
2097
Let us now consider Y2SiMoOs. In view of the results for scheelites longrange order between the Si4÷ and Mo 6÷ ions on the tetrahedral sites is possible. The X-ray diagram shows indeed a large number of superstructure reflections. These have h + k-t-l odd, and can be indexed on the same unit cell as that of zircon (e.g. YVO4). The only possible arrangement for the Si4+ and Mo 6÷ ions is therefore: Si4÷ on ½, ½, 0 and ½, 0, ¼ and Mo 6+ on 0, 0, ½and 0, ½, ]. The symmetry is no longer tetragonal, but orthorhombic, although the powder diagram can be indexed on a (pseudo) tetragonal unit cell. In the system Y2SiMoOs-YVO4 the superstructure reflections already disappear for low vanadium-content, the V s÷ ions hampering the long-range order between the Si4÷ and Mo 6+ ions. The emission spectrum of Y2SiMoOs-Eu a+ consists of sharp lines, as is to be expected. We will now compare the emission spectrum of YVO4-Eu a÷ and that of Y2SiWOs-Eu z+ to study the influence of long-range order at the tetrahedral sites on the emission transitions. In YVO4-Eu the Eu a÷ ion occupies a site with D2d symmetry. The numbers of emission lines of the 5D0-TF0, 5Do-TF~ and 5D0-rF2 transitions of the Eu a+ ion at a site with this symmetry are zero, two and two, respectively. This has been found experimentally: 593.5 nm and 595.0 nm for the single and twofold degenerate 5D0-rF~ emission line and 615-5 nm and 619.4 nm for the single and twofold degenerate 5Do-TF2emission line, respectively[8]. Due to the order of Si4÷ and Mo 6÷ ions in Y2SiMoOs-Eu the site-symmetry of the Eu 3÷ ion is lowered from D2a to C2v. For this symmetry the numbers of emission lines of the 5D0-TFo, 5D0-rF~ and 5D0-TF2 transitions of the Eu 3+ ion are one, three and four, respectively. This is in good agreement with the experimental results (Fig. 2): 5DoJFo: 581 nm; 5D0-TF~:590, 593 and 598 nm; 5D0-TF2:610, 613,617 and 626 nm. The long-range order of the smaller cations in the zircon structure influences the spectral energy distribution of the Eu a+ emission strongly. This is especially clear for the 5D0-TF1 emission (around 595 nm), for which all components are observed in both cases. It is obvious to correlate the 590 and 598 nm components in the case of Y2SiMoOs-Eu to the doubly degenerate 595.0 nm component of YVO4-Eu, and the 593 nm component of Y2SiWOs-Eu to the 593.5 nm component of YVO4-Eu. This means that the doubly degenerate crystal-field component of the 7F~ level of Eu 3+ in YVO4 is split by some 250 cm -~ by the long-range order of the Si4÷ and Mo 6÷ ions. It remains, however, to be explained why the emission spectrum of Y2SiMoOsEu 3÷ indicates only one type of Eua+-centre. Due to the long-range order of Si4÷ and Mo 6+ ions two types of y3+ sites are present in Y2SiMoOs. One of these sites has two Si4÷ at roughly 3.2 A ( Y - O - S i angle-90°), two Si4÷ and two Mo 6÷ at roughly 3.7 ~ ( Y - O - S i (Mo) angle-180 °) and four Mo 6+ at 4.5 .~ (no oxygen ion in common). The other has the same surroundings with Si4÷ and Mo 6÷ ions reversed. This makes no difference for the neighbours at roughly 3-7/~. We are forced, therefore, to conclude that these neighbours are responsible for the lower-symmetrical crystal-field components relative to the situation in YVO4-Eu. Note that only for these neighbours in the angle Y - O - S i (Mo) - 1 8 0 °, so that strong interaction via o--bonding may be expected. 8. C. Brecher, H. Samelson, A. Lempicki, R. Riley and T. Peters, Phys. Rev. 155, 178 (1967).
2098
G . BLASSE
Such a situation does not occur for Y2SiWO8-Eu. The 5D0-TF, emission of this phosphor (See Fig. 1) contains at least five lines, indicating more than one type of Eu 3+ ions (for one type the maximum is three). The occurence of Y2SiMoO8 with zircon structure is clearly an exception. All other compounds Ln2Me4+Me6+O8 studied have the scheelite or the (clearly related) fergusonite structure. The energy difference between the zircon and scheelite phase of Y2SiMoOs is expected to be small in view of the fact that replacement of small amounts of Si4÷ by Ge 4+ or of Mo 6÷ by W 6+ results in compositions with scheelite structure. It is interesting to note that in the system Y2SiWOs--Y2SiMoO8 there is a narrow miscibility gap at the zircon side, whereas in the system Y2SiWO8- YVO4 there is a considerably broader miscibility gap at the scheelite side. The fluorescence of the unactivated host lattices Ln2SiWOa resembles that of other tungstates with scheelite structure like CaWO4[9] and Y2(WO~)3[10]. The emission of LnzSiWOs is situated in the same spectral region as in the case of CaWO4 and Y2(WO4)3. The quenching temperature of the fluorescence of Ln2SiWO8 is about equal to that of Y2(WO4)3 but lower than that of CaWO4. The absence of fluorescence in Tb3+-activated tungstates was reported before [11]: Y6WO,2-Tb and Y2WO6-Tb do not fluoresce. The host lattice absorption edge is situated in the neighbourhood of 300 nm, as in the case of Y2SiWOs. Only Y2(WO4)3-Tb was found to fluoresce. The absorption edge of this host lattice is situated at shorter wavelengths, viz. 245 nm. The fluorescence of Y2SiWOs-Bi resembles that of the Bi 3+ activated tungstates, for example Y2WO6-Bi[12]. The shift of the absorption edge to longer wavelengths has been ascribed to a charge transfer process in which an electron is transferred from the 6s orbital of Bi3+ to the empty 5d orbital of W 6÷. The emission consist of the same process and shifts therefore to longer wavelengths compared to the host lattice emission (Fig. 3). The red emission of Y2SiWOs-Bi at low temperatures was also observed for YzWOs-Bi and cannot be assigned at the moment. Finally we may ask why the efficiency of the phosphors LnzMe4+Me6+O8 is so low. Consider for example, Y2SiMoOs-Eu 3+. The isomorphous phosphor YVO4-Eu has a very high efficiency[13]. It can be shown that Y~SiMoOs-Eu does not satisfy the conditions for efficient Eu3+-activated phosphors given by us before[14]. The molydate emission of the host lattice is situated far in the red and by no means overlaps the absorption of Y2SiMoO8 (this compound is white has its absorption edge at 270 nm. Energy transfer through the lattice is therefore highly improbable. There seems to be no reason why the transfer from the molybdate group to an Eu 3+ ion should not occur. The molybdate emission overlaps especially the 7F0-SD0 absorption line of the Eu3+ion and the M o - O - E u angle is expected to be large[14]. If we assume that the probability ofmolybdate---~molyb9. I0. 11. 12. 13. 14.
F . A . Kr6ger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam (1948). H.J. Borchardt,J. chem. Phys. 39,504 (1963). G. Blasse and A. Bril, PhilipsRes. Rep. 22,481 (1967). G. Blasse and A. Bril,J. chem. Phys. 48, (1968). In press. A. Bril, W. L. Wanmaker and J. Broos,J. chem. Phys. 43, 311 (1965). G. Blasse, J. chem. Phys. 45, 2356 (1966).
Crystal structure and fluorescence of compounds Lr~ Me4+Mee+O8
2099
date transfer is zero and that of molybdate---~Eu3+ transfer is high, it is possible to make a quantitative estimation of the quantum efficiency for excitation in the molybdate absorption band. Nearly all exciting quanta are absorbed by the molybdate groups. Because of the absence of transfer through the host lattice this exciting energy can only be transferred to Eu 3÷, if the molybdate group has a nearest Eu 3÷ neighbour. For Eu 3+ concentrations of 1, 2 and 4 atom per cent the probability of such a situation and, therefore the maximum quantum efficiency of the Eu 3+ emission is 4, 8 and 15 per cent, respectively the number of y3+ sites which the Mo 6+ ion has under large M o - O - Y angles is four; the probability of Eu a÷ neighbours is therefore 1 - ( 1 - x ) 4, if x is the Eu3+-concentration). The experimental values of the quantum efficiency for excitation in the molybdate group for Eu3+-concentrations of 1,2 and 4 atom per cent are 3, 7 and 13 per cent, respectively. Although the accuracy of the experimental figures is not very high, the agreement between the two sets of values is satisfactory. For the phosphors Ln2SiWOs-Eu 3+ with scheelite structure the situation is different. From Fig. 4 it can be seen that the emission and absorption band of the host lattice overlap each other in this case, although the overlap is small. It may be, however, that transfer through the lattice is nevertheless impossible. This can be the case if the order between the Si4÷ and W 6+ ions is of such a kind that the tungstate groups are not near neighbours in the crystal lattice (in Y2SiMoO8 order is such that the molybdate tetrahedra still touch each other). The emission of Y2SiW0.sMo0.2Os (scheelite structure) points to inefficient transfer through the tungstate host lattice. Under long-wave u.v. excitation the emission is situated in the red, under shortwave ultraviolet excitation in the blue. In the former case the molybdate groups are excited and molybdate emission occurs; in the latter case the tungstate groups are excited and tungstate emission occurs. This is an unusual behaviour for tungstates containing a certain amount of molybdenum. Usually excitation energy absorbed by the tungstate groups is transferred efficiently to the molybdate groups, the energy overlap being very favourable [9, 15]. From this it can only be concluded that in YzSiW0.sMoo.208 there are geometrical reasons preventing energy transfer between the hexavalent cation polyhedra. This will then also be the case for the phosphors LneSiWOs-Eu and explains why the Eu 3+ efficiency for excitation into the host lattice is low. Similar phenomena probably play a role in the compounds containing germanium or titanium. In view of their weak fluorescence even at low temperatures, they were not studied further. The emission of the molybdate group in Y2SiMoO8 (zircon structure) is slightly different from that of the molybdate group in YzSiW0.8Oo.208 (scheelite structure). The emission of YzSiW0.sMoo.208 under 365 nm excitation shows also a weak band at about 450 nm (tungstate emission). The excitation spectrum of the tungstate group extends in fact up to this excitation wavelength. Acknowledgements-The author is greatly indebted to Dr. A. Bril for the performance of the optical measurements and to Mr. J. de Vries for the preparation of the samples. 15. G. Blasse and A. Bril,J. chem. Phys. 45, 2350 (1966). 16. R . W . G . Wyckoff, Crystal Structures, 2nd Edn. Interscience, New York (1965).