LUMINESCENCE OF THE TUNGSTATE GROUP IN SCHEELITE AND FERGUSONITE

Transcription

1 R732 Philips Res. Repts 25, ,1970 LUMINESCENCE OF THE TUNGSTATE GROUP IN SCHEELITE AND FERGUSONITE by G. BLASSE Abstract Luminescence properties of single-phased materials in the systems CaW04-YNb04 and CaW04-YTa04 are described. The quenching temperature of the W04 emission decreases with decreasing tungsten concentration in these systems. The emission of compositions within these systems is situated at longer wavelengths than the emission of CaW04 and YNb04 themselves. 1. Introduction The luminescence of ionic groups having a central cation with np6ndo configuration has been extensively studied. Well-known examples are CaW04, YNb04 and YV ). The absorption and emission transitions are of a charge-transfer type. Only a few solid-solution series of compounds of this type have been studied, e.g. Ca(W,Mo)04 1 ) and Y(Nb,Ta)04 3 ). In these series the positions of the absorption and emission bands of the two constituent ionic groups differ considerably, so that energy transfer from one to the other is observed (W04 -+ Mo04 and Ta04 -+ Nb04, respectively). At first sight it does not seem interesting to study solid-solution series of compounds such as YNb04 and CaW04 containing different groups with similar electron configuration and spectral properties. These two compounds have about the same absorption edge (~ 250 nm), emission band (~ 400 nm) and quenching temperature. It appeared, however, that the luminescence properties do not vary linearly with composition as was expected. This is the subject of the present paper. In addition we also studied the system CaW04- YTa04' The compound CaW04 has scheelite structure, the compounds YNb04 and YTa04 fergusonite structure. Fergusonite is a distorted modification of scheelite 4). The W, Nb and Ta ions occupy tetrahedral oxygen holes. Brixner 5) studied the CaW04-YNb04 system and found a miscibility gap. The scheelite phase exists in the region % CaW04, the fergusonite phase in the region % YNb Experimental Samples were prepared by firing intimate mixtures of the starting materials (CaC0 3, W0 3, Y20 3, Nb20s, Ta20s; quality 99'99% or better) at temperatures between 1200 and 1500 C. They were checked by X-ray analysis.

2 232 G.BLASSE Optical measurements were performed as described previously 6,7). The definition of quenching temperature proposed by Kröger 1) is used. 3. Results and discussion Our crystallographic results on the system Ca W04- YNb04 are in reasonable agreement with those of Brixner 5): the miscibility gap extends from 30 to 50% CaW04. For the system CaW04-YTa04 this gap is somewhat broader (30 to 65 % CaW04). Luminescence properties were only measured for the single-phase materials. We will first give the results for the YNb04-rich samples and then for the CaW04-rich samples YNb0 4 -rich samples The efficient luminescence of YNb04 at 300 "K is quenched by the introduetion of CaW04 into this lattice. The quantum efficiency of the niobate emission of YNb04 at room temperature (50 %) 8) decreases to 20 % and to about 8 % by the introduetion of 1 and 3 % CaW04, respectively. The shape of the emission band at room temperature does not change drastically. An extra absorption band appears, however. It extends from about 380 nm to the absorption edge of pure YNb04 (~ 250 nm) and has, therefore, a considerable overlap with the niobate emission band which peaks at about 400 nm and extends down to about 300 nm. In view of this it is not surprising that excitation energy absorbed by the Nb04 groups is transferred to the centre with the longer-wavelength absorption. The emission of this centre is quenched at room temperature, but appears at lower temperatures. Figure 1 shows the temperature dependence of the luminescence intensity of (YNb04)o. 97(CaW04)O.03 and (ynb04)o.7s(ca W04)O.2S. The curve for the latter composition relates to one emission only (Nb04 emission is completely quenched), the curve for the former to two emissions, viz. the Nb04 emission with quenching temperature far above room tempera- Int. loor-~~~.---~~.---,---,---~ O~r-~--L---~--~--~--~~~ ISO T(OK) Fig. 1. Temperature dependence of the luminescence intensity for 254-nm excitation of 1. (YNb0 4 )o.97(caw0 4 )O.03; 2. (YNb0 4 )o.7s(caw0 4 )O.25; 3. (CaW04)O'9(YNb04)o'1; 4. CaWû4'

3 W04 LUMINESCENCE IN SCHEELfTE AND FERGUSONITE 233 ture and the longer-wavelength emission with quenching temperature below room temperature. \. f/j).. \. -, -. -." t 50'r-~~~~- ~~~~ ~ ~ O~~~~~--~--~~~ ~L- ~ À (nm) Fig. 2. Spectral-energy distribution of the emission for 245-nm excitation of I. (YNb04)o.99(CaW0 4 )O.Ol and YNb0 4 (300 OK); 2. (YNb04)o.99(CaW0 4 )O.Ol and 2'. YNb0 4 (77 OK); 3. (YNb04)o'97(CaW04)O.03 interval in arbitrary units. (77 gives the radiant power per constant wavelength ij /' V'" /1 \ ~ I,ï "t-, The emission spectra are given in figs 2-4. At 300 ~K the emission band of (YNb04)O.99(CaW04)O.Ol is identical to that of YNb0 4 ; at 77 "K an addiloor-~--~~~~-, i'- o À (nm) Fig. 3. Spectral-energy distribution of the emission for 254-nm excitation of (CaW04)O'9(yNb04)o.1 and (ynb04)o'9(caw0 4 )O'1 (drawn curves, both at 77 OK) and (CaW04)O.9(YNb0 4 )o.1 (dashed curve, 300 OK). f/j).. 100r-r----,---.""7"' ,.--,-, I50r-r--t-t-t--r-~ t -t-f À (nm) Fig. 4. Spectral-energy distribution of the emission for 254-nm excitation of (YNb04)o'7S (CaW04)O.2S (drawn curve) and (yta0 4 )o'7s(caw0 4 )O.2S (dashed curve). Both at 77 ok.

4 234 G.BLASSE tional emission band appears on the long-wavelength side of the 300 OK band. For (YNb0 4 )0.97(CaW04)0.03 the intensity of the low-temperature emission has increased considerably (fig. 2). In the case of (YNb04)0.9(CaWÛ4)0.1 and (YNbÛ 4 )0.7s(Ca WÛ4)0.2S(figs 3 and 4) it is the only emission present. Figure 4 shows also that the emission of (YTaÛ4)0.7s(CaWÛ4)0.2s is practically identical to that ofthe analogous composition with niobium. This strongly suggests that the emission band with a maximum at about 470 nm is due to the W04 group, since the position of the emission bands of YNb04 and YTaÛ4 is very different 3). Summarizing these results we see Ca) The absorption and emission band ofthe WÛ4 group in YNbÛ4 is situated at considerably lower energy than in CaWÛ4 (and lower than those of the Nb04 group in YNb04). (b) The quenching temperature of its emission is much lower than that of the CaW04 emission. (c) WÛ 4 acts at room temperature as a killer of the emission of YNbÛ4: the NbÛ 4 group transfers its excitation energy to the W04 group, which relaxes thermally. Completely analogous results were obtained for the system YNb1_ 2xSixWx0 4 so that the nature of the ion that compensates the extra charge of W6+ relative to Nb5+ is not very important. Elsewhere we have argued that low quenching temperatures are to be expected if an activator ion with charge-transfer transitions is introduced in too large a hole 9). This is the case for W6+ in YNb04. The W-O distance in scheelite is 1.79 A 10), the Nb-O distance in YNb04 for site I 1 97 (2x) and 1 81 A (2 x) and for site II 1 93 A (2 x) and 1 90 A (2 x). It seems not unlikely that this is at least partly the reason why the quenching temperature of WÛ 4 in YNbÛ 4 is low. It is interesting to note that the quenching temperature of the W0 4 emission in YPl_2xSixWx04 (zircon structure) is considerably higher than the analogous niobate compounds, viz. 370 "K and about 200 "K, respectively. The P-Û distance in YPÛ4 is 1 50 All). In ref. 3 we have also reported a yellow emission for YNbÛ4 (at 77 "K under 365-nm excitation). Such an emission is also observed for the composition (YNb0 4 )1_x(CaW04)x. In these compositions it is situated at somewhat longer wavelengths (fig. 5). This band, which might be due to a triplet-singlet transition, will not be discussed here CaW0 4 -rich samples Whereas the influence of small amounts of WÛ4 on the luminescence of YNb0 4 is drastic, the influence of small amounts of Nb04 on the luminescence of CaW0 4 is less marked. The quantum efficiency of the tungstate emission of CaW0 4 is 60%, that of (CaWÛ4)0.9(yNbÛ4)0.1 still 25%. At the same

5 wo. LUMINESCENCE IN SCHEELITE AND FERGUSONITE 235 f/j> ~ /\ ", ~\, \, 1/ \ ''I. I ''I. I ); I " o " ~ r-, À(nm) Fig. 5. Spectral-energy distribution of the emission for 365-nm excitation of YNb0 4 (drawn curve) and (YNb04)o.9(CaW04)O.1' (dashed curve). Both at 77 "K, time the quenching temperature decreases from 410 "K to 390 "K. The slope of the temperature-dependence curve, however, changes more markedly (see fig. I). Figure 3 shows the spectral-energy distribution of this composition at 77 and 300 "K. This emission band is the same as observed for the W0 4 emission on the YNb04-rich side of the system (fig. 3). It is shifted to longer wavelengths compared with pure CaW0 4 (410 nm). The same results are observed for (CaW0 4 )1_x(YTa0 4 )x so that the emission can be ascribed to W0 4 groups. Further we found that the size of the compensating y3+ ion is not of importance, since the results for (CaW0 4 )o.9(lnnb0 4 )o.1 (Ln = La, Gd, Y, Lu) are the same within the experimental error. That the niobate group should influence the W04 group is not very likely. This follows also from the fact that Cao.9NaO.OSYo.osW04 shows fluorescence properties similar to those of (CaW0 4 )o.9(ynb0 4 )o.1. In fig. 6 we have plotted all the quenching temperatures observed. Note that 00 I Mise. gap rv ~~ -.- ~ ~ o r ~. 300, "ö- 200 ~ [J 00 0 o BD 100 CaW04 YNb04 (x) YTa04 (0) Fig. 6. Quenching temperatures of the tungstate emission in materials investigated. x: system CaW04-YNb04; -o-: YP04-Si,W; 0: system CaW04-YTa04; +: CaW0 4 -Na,Y; 0: YNb04-Si,W;.: YNb0 4 (niobate emission). These temperatures were determined from the temperature-dependence curves by the procedure proposed by Kröger 1).

6 236 G. BLASSE the quenching temperature of the W0 4 emission depends linearlyon the tungsten concentration in the system CaW0 4 -YNb04 The results for this system show once again how very sensitive luminescence properties are to changes of the local environments, even within one and the same crystal structure. 4. Acknowledgement The author is indebted to Dr A. Bril for the performance of the optical measurements and to Miss A. D. M. de Pauw for the preparation of the materials. Eindhoven, May 1970 REFERENCES 1) F. A. Kröger, Some aspects of the luminescence of solids, Elsevier Pub!. Comp. Inc., Amsterdam - New York, ) G. Blasse and A. Bril, Z. phys. Chem. N.F. 57, 187, ) G. Blasse and A. Bril, J. of Luminescence, in press. 4) A. L. Komkov, Kristallografiya 4, 836, ) L. H. Brixner, J. electrochem. Soc. 111, 690, ) A. Bril and W. L. Wanmaker, J. electrochem. Soc. 111, 1363, ) G. Blasse and J. de Vries, J. electrochem. Soc. 114, 875, ) W. L. Wanmaker, A. Bril, J. W. ter Vrugt and J. Broos, Philips Res. Repts 21, 270, ) G. Blasse, J. chem. Phys. 51, 3529, ) M. I. Kay, B. C. Frazer and I. Almodovar, J. chem. Phys. 40, 504, ) I. Krstanovic, Z. Krist. 121, 315, 1965.