Preparation and luminescence properties of Y2(WO4)3:Eu3+

Journal of Luminescence 29 (1984) 163-176 North-Holland, Amsterdam

PREPARATION AND LUMINESCENCE

163

PROPERTIES

OF

Y2(WO4 )3 : EU3 + T s u y o s h i K A N O , Setsuko S E K I a n d Shing Z h u n g C H I O U * Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo. 185, Japan

Received 26 August 1983

A red-emitting Y2(WO4)3:Eu 3+ phosphor (orthorhombic high temperature phase, anhydride) is prepared by two different methods: the firing of mixtures of constituent oxides and that of precipitates from aqueous solutions. After optimizing preparation conditions, the cathodoluminescence brightness reaches 56% that of Y202S:Eu 3+, a commercial red phosphor for color TV. Formation of a high temperature phase below the reported transition temperature is noted in the fired precipitates. This phase occurrence is shown to depend on the treatment of the precipitates to be fired. Reflection difference measurement of Eu-doped and undoped samples assigns an excitation band of about 245 nm to the Eu-O charge transfer band. Different by-products in the two preparation methods are identified by measuring emission spectra under selective excitation. Reversible hydration-dehydration of the phosphor is demonstrated by successively measuring photoluminescence first in vacuum and then in air at various temperatures. No deterioration of luminescence efficiency is observed after repeating this reversible structural change.

1. Introduction T r i v a l e n t tungstates as hosts for Eu 3+ a c t i v a t i o n have been s u r v e y e d for p h o t o l u m i n e s c e n c e efficiency b y B o r c h a r d t [1,2]. T h e structural a n d p h a s e r e l a t i o n s h i p s a m o n g trivalent tungstates of the t y p e L 2 ( W O 4 ) 3 have b e e n s t u d i e d b y N a s s a u et al. [3-5]. In L 2 ( W O 4 ) 3, o r t h o r h o m b i c crystal structure (C) occurs, where L = Er, Tm, Yb, Lu, Sc, In or A1. In larger trivalent ions ( G d , Tb, D y , H o a n d Y), it occurs as a high t e m p e r a t u r e phase. T h e structure C is hygroscopic; it a b s o r b s w a t e r in a h u m i d a t m o s p h e r e to f o r m L 2 ( W O 4 ) 3 • 3 H 2 0 , except w h e n L is a small ion (A1). This h y d r a t i o n has b e e n r e p o r t e d to affect Eu 3+ l u m i n e s c e n c e in Sc2(WO4) 3 a n d L u 2 ( W O 4 ) 3, i.e., e x c i t a t i o n p e a k shift to longer wavelengths a n d b r o a d e n i n g of emission s p e c t r a are caused [6]. In m12(WO4) 3, Eu 3+ emission is c o n c e n t r a t e d in the red 5 D 0 - 7 F 2 t r a n s i t i o n [6]. This c h a r a c t e r i s t i c emission is f a v o r a b l e for such a p p l i c a t i o n s as red p h o s p h o r s for c o l o r T V a n d high c o l o r - r e n d e r i n g fluorescent lamps. In this

* On leave from Shanhei No. 2 Electron Tube Factory. 0 0 2 2 - 2 3 1 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B]V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)

164

T. Kano et al. / Preparation and luminescence properties of Y,(WO~)~ : Eu ~ +

respect, Eu 3+ emission in Y2(WO4)3 (C-structure, anhydride) has been shown to have a high lumen equivalent than any of the red phosphors used so far for color picture tubes [7]. In this report, two methods of preparing Y2(WO4)3 : Eu 3+, a dry method and a wet method, are compared for improving cathodoluminescence efficiency. Boundary conditions for forming the C-structure are examined using the fired precipitates. Photoluminescence properties of the phosphors are also investigated with special reference to a hydration-dehydration process.

2. Experimental In the dry method, samples were prepared by firing intimate mixtures of YZO3(99.99%), Eu203 (99.9%) and WO 3 or HzWO 4 using alumina crucibles in air. In the wet method, an aqueous (0.9Y, 0.1Eu)(NO3) 3 solution and ammonia water were added in drops at constant p H to an aqueous 5 ( N H 4 ) 2 0 . 1 2 W O 3 solution at room temperature. The resulting suspension was boiled for 30 min. The precipitates were washed, separated and fired in air. The mixing ratio used corresponds to (0.9Y + 0 . 1 E u ) / W = 2/3. For the experiments in section 3.2, the precipitates were prepared from a boiling solution. The resulting suspension was boiled for 3 h, keeping the pH at 8 or 6. After being washed, the precipitates were treated with a dilute aqueous H N O 3 solution or a dilute aqueous ammonia solution for 2 h at room temperature. Qualitative analyses of Y(Eu) and W in solution were made by adding to a 10 ml test solution, separated from the precipitates, a 10 ml 1 N N a O H solution and a 10 ml 1 N HCI solution, respectively. White precipitates appearing in the alkaline solution and the acid solution prove the presence of Y(Eu) and W in solution, respectively. Cathodoluminescence was measured with a conventional demountable apparatus under 10 kV electron beam excitation. Emission and excitation spectra were obtained using a Hitachi MPF-4 fluorophotometer with a cryostat. The emission spectra for fig. 10 and reflection spectra were obtained using a specially designed apparatus [8]. For reflection spectra of anhydrides, samples were measured immediately after being dried at 140°C. High temperature X-ray diffraction patterns were measured by Rigaku Corporation.

3. Results and discussion

3.1. Preparation methods and cathodoluminescence brightness Preliminary experiments on the Eu concentration dependence of cathodoluminescence brightness in (Yi-,-, Eux)2 (WO4)3 showed that the maximum brightness was attained at an Eu concentration of about 10 mol%. Since

72 Kano et aL / Preparation and luminescence properties of Y_,(W04)3: Eu 3 +

165

chromaticity coordinates of E u 3 + emission have only slight dependence on Eu concentration around 10 tool%, all samples were prepared at that concentration in the following experiments. Figure 1 shows the mixing ratio dependence of the cathodoluminescence brightness for samples prepared by the dry method at firing temperatures from l l 0 0 ° C to 1400°C. The optimum mixing ratio is (0.9Y + 0.1Eu)/W = (2.1)/3 at a firing temperature of 1300°C. When excess WO 3 is mixed, fired products become greenish colored due to the precipitation of excess WO 3. This causes a decrease in cathodoluminescence intensity. To dissolve the excess WO 3, fired products were treated with an alkaline solution. However, this treatment drastically lowered brightness. Repeating the firing contributes a little to the improvement of the cathodoluminescence intensity, but the firing in oxygen is not effective, and that in argon drastically lowers brightness. As slow cooling was found to be better than rapid cooling, the fired samples were cooled overnight in the furnace after the electric power for it had been cut off. Various fluxes were tested without success. One problem in using flux is that alkali metal ions in flux are easily incorporated into the tungstates.

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T. Kano et al. / Preparation and luminescence properties o f Y: ( W 0 4 ) 3 : Eu "~+

Figure 2 shows the pH dependence of cathodoluminescence for Y2(WO4)3 : Eu 3+ prepared by the wet method. The low brightness of the fired precipitates prepared from acid solutions is due to an excess WO 3 as was observed in the dry method. Qualitative analyses of acid solutions indicated complete precipitation of W and incomplete precipitation of Y(Eu). In firing at 700°C, an anomalous occurrence of high temperature phase contributes to emission, as will be discussed in a later section. Figure 3 shows SEM photographs of precipitates before and after firing. The deviation from stoichiometry due to incomplete precipitation of Y(Eu) in acid solutions leads to a composition [4] with a low melting temperature, which would help crystal growth. This explains why the fired precipitated prepared from acid solutions have a larger particle size than those prepared from alkaline solutions. It should be noted that, in the fired precipitates prepared from alkaline solutions, such fine powders as those in the order of 1 p.m in diameter are fairly luminescent. By the way, it is very difficult to prepare fairly luminescent, fine powders of the order of 1 i~m in the phosphors of the conventional types. Figure 4 compares the firing temperature dependences of the cathodoluminescence brightness in samples prepared by the dry and wet methods. Optimum firing temperature is remarkably lower in the wet method ( l l 0 0 ° C ) than in the dry one (1300°C). Slightly higher brightness was obtained by the wet method than by the dry one. Thus, optimization of preparation methods have improved the brightness of Y2(WO4)3:Eu 3+ phosphor to 56% that of Y202S: Eu 3÷, a commercial red phosphor for color TV,

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T. K a n o et a L /

Preparation and luminescence properties o f Y,( 14/'04)3 " EU "~+

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3.2. Occurrence of a high temperature phase dependent on treatments of starting materials According to the phase diagram of L z ( W O 4 ) 3 [4], the temperature of the phase transition from the low temperature monoclinic phase (A) to the high temperature orthorhombic phase (C) in Y2(WO4)3 is around 790°C. As described above, the structure (C) is hygroscopic, but the structure (A) is not. This difference enables one to determine the mixing ratio of the two phases by measuring the weight increase due to the hydration of samples kept in air. To determine any additional factors affecting the formation of the two structures, boiled precipitates were treated with various pH solutions, and fired at 700°C, a little below the reported transition temperature. It was found that boiled precipitates exhibit characteristic luminescence even before firing, as reported elsewhere [10]. The fired precipitates were kept in air for more than 4 days and the subsequent weight increase due to hydration was measured. Cathodoluminescence brightness of the fired products was found to have a linear dependence on the measured weight increase (fig. 5). The observed linear dependence is extrapolated to the result on the precipitate fired at 1000°C. The hydrate samples are converted to the anhydride (C-structure) during evacuation for cathodoluminescence measurement. Therefore, the results shown in fig. 5 indicate that the structure (C) partially occurs even at 700°C, due to the

T. Kano et al. / Preparation and lumineseenee properties of Y,( WO 4)~ ."Eu "~÷

169

treatments of the starting materials, and that the cathodoluminescence efficiency of the phosphor with the C-structure, thus formed at 700°C, is equal to that prepared at 1000°C. The results in fig. 5 show that treatments of precipitates with acid or alkaline solutions favor the occurrence of the C-structure at a lower temperature. Qualitative analyses showed that Y(Eu) was partially dissolved by acid treatment and W by alkaline treatment. These observations indicate that the phase occurrence depends more on the states of starting materials than on temperature. However, the photoluminescence of the boiled precipitates before firing was not affected by such a treatment. The observed occurrence of the C-structure at low temperature is possibly explained by assuming that deviation of the treated precipitates from stoichiometry favors the occurrence of the C-structure. These experiments should provide a new aspect to the phase relationships among trivalent tungstates.

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T. Kano et at. / Preparation and luminescence properties of Ye(W04).~: Eu "¢+

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3.3. Excitation spectra f o r Eu 3 ÷ emission

Figure 6 shows the temperature dependence of excitation spectra for Eu 3+ emission in the C-structure observed in vacuum. The excitation peak becomes higher and shifts to longer wavelengths with increasing temperature. To assign excitation spectra, reflection spectra of an Eu-doped sample and those of an undoped sample were compared around room temperature (fig. 7(a)). The measurement of the reflection difference in the two samples (fig. 7(b)) reveals the existence of an absorption band peaking around 245 nm which closely corresponds to the excitation peak shown in fig. 6. Thus, the excitation band is assigned to a broad-band adsorption due to Eu-doping. It is known that E u - O charge transfer transition causes broad-band absorption

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T. Kano et al. / Preparation and luminescence properties of Y2( W04)3 ."Eu ~ +

171

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around 220-310 nm [11]. The photoluminescence efficiencies of various Eu 3+activated mixed metal oxides under charge transfer band excitation increase with decreasing wavelength of the absorption peak [11]. The observed quantum efficiency o f Y 2 ( W O 4 ) 3 : E u 3+ (about 30% as will be described in a later section) agrees approximately with that of Eu 3+-activated phosphors reported, which have charge transfer bands around 250 nm. Thus, the excitation band around 245 nm i n Y2(WO4)3: Eu 3+ is concluded to be due to E u - O charge transfer absorption. Figure 8 shows excitation and emission spectra at 150°C for Y2(WO4)3 : Eu 3+ prepared by the dry method with 2% excess Y203 mixing. Excitation at 240 nm UV light gives rise to the emission of Y2(WO4)3: Eu 3+ with the C-structure that was reported previously [7]. At 280 nm UV excitation, the emission spectrum observed is different from that by 240 nm UV excitation; for example, the emission band around 595 nm due to 5D0-TF t transition of Eu 3+ is split into three lines. This emission is similar to the reported Eu 3+ emission in Y2WO6 [12].

172

7". Kano et al. / Preparation and luminescence properties of Y.,(W04).¢ ." E u 4

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T h e f o r m a t i o n of a trace a m o u n t of Y2WO6 as a b y - p r o d u c t is n a t u r a l l y expected, b e c a u s e 2% excess Y20 3 was used as a s t a r t i n g material. W h e n 10% excess W O 3 was used in the d r y m e t h o d , the emission of Y2WO6 : Eu 3+ was not o b s e r v e d at 280 nm excitation. Similar selective excitation e x p e r i m e n t s were carried out on Y2(WO4)3: Eu 3÷ p r e p a r e d by the wet m e t h o d to d i s c r i m i n a t e l u m i n e s c e n t b y - p r o d u c t s that m a y d e p e n d on p r e p a r a t i o n methods. T h e fired p r e c i p i t a t e s p r e p a r e d at P H 8 a n d 9 give rise to an emission with 280 n m UV excitation, that c a n n o t be identified as Eu 3+ emission in the related tungstates r e p o r t e d . N o l u m i n e s c e n t b y - p r o d u c t was detected on the fired p r e c i p i t a t e s p r e p a r e d at P H 7. In this way, different b y - p r o d u c t s that d e p e n d on p r e p a r a tion m e t h o d were d e t e c t e d by selective excitation.

72 Kano et al. / Preparation and luminescence properties of Y_,(W04).~. Eu +

173

3. 4. H y d r a t i o n a n d d e h y d r a t i o n

Figure 9 shows effects of temperature and air pressure on the emission intensity of Y2(WO4)3:Eu 3+. The emission intensity of the anhydride in vacuum decreases to 607o of that of the initial value with a decrease in temperature from 230°C to 25°C, as shown by the dotted line. With the introduction of air into the vacuum chamber at 25°C, the emission intensity rapidly decreased to 1/3, indicating the rapid hydrate formation at the surface of the phosphor. On the other hand, the total hydration process, as observed by the weight increase in air, requires about 4 days for completion. This suggests slow diffusion of water into the phosphor. Photoluminescence brightness of Eu 3+ in the hydrate under 254 nm UV excitation at room temperature was found to be 10% that of Y20~ : Eu 3+ , a red phosphor for high color-rendering fluorescent lamps. The quantum efficiency of (0.8Y, 0.2Eu)2(WO4) 3 was reported to be 10% previously [2]. As this reported value corresponds to that of the hydrate in our measurement, the sample in the previous report [2] might possibly have been Y2(WO4)3. 3 H 2 0 : E u 3+ (hydrate) but not Y2(WO4)3:Eu 3+ (anhydride). When the hydrate is heated in air as shown by the solid line in fig. 9, the emission intensity begins to increase at 60°C and reaches the level of the anhydride at 90°C. This change is ascribed to the dehydration of the hydrate. Above 90°C, the emission intensity increases with increasing temperature, as shown by the solid line. The correspondence of the solid line to the dotted line in fig. 9 indicates that no deterioration of the anhydride occurs after the hydration-dehydration cycle. This was confirmed by another experiment. The phosphor was set in an atmospheric oven for a month. The oven was kept at 140°C only during the day, thus allowing the phosphor to hydrate during the night and dehydrate during the day. After 25 cycles of this reversible process,

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174

T. Kano et al. / Preparation and luminescence properties of Y2(WO4)s : Eu 3 +

no deterioration in the cathodoluminescence of the anhydride phosphor was detected. Figure 10 shows the emissionspectra of the hydrate (a) and the anhydride (b). The sharp emission line at 612 nm for the anhydride due t o 5D0-TF2 electric dipole transition is broadened by the hydration. As is well known, non-uniformity in crystal fields surrounding Eu 3÷ gives rise to the broadening

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T Kano et aL / Preparation and luminescence properties o f Y.,(WO~) 3 : Eu "~+

175

of emission spectra. Therefore, water molecules in the hydrate are assumed to be randomly distributed in the hydrate crystal. Figure 11 shows X-ray diffraction patterns of the hydrate (a) and the anhydride (b). As was described in the introduction, Y2(WO4)3.3H20 is known to belong to a group of hydrates of lanthanide tungstates, Lz(WO4) 3 • 3H20, whose X-ray diffraction patterns are reported on Ho2(WO4) 3 • 3 H 2 0 [3]. The X-ray diffraction patterns of Y 2 ( W O 4 ) 3 . 3 H 2 0 : E u 3+ shown in fig. 11(a) are in accord with those of Ho2(WO4) 3. 3H20, thus confirming the above structural relationship. The X-ray diffraction patterns of the C-structure have been reported on Sc2(WO4) 3 [13]. The X-ray diffraction patterns of Y2(WO4)3:Eu 3+ shown in fig. 11(b) are in accord with those of Sc2(WO4) 3 whose structure has been indexed as orthorhombic. The lattice constants for (0.9Y, 0.1Eu)2(WO4) 3 have been determined to be a = 1.006 nm, b = 1.395 mm, c = 0.998 nm. As shown, the hydrate is well crystallized, not amorphous.

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T. Kano et al. / Preparation and luminescence properties of Y2( W04)3 ."Eu'~ +

It is surprising to note that such a remarkable r e a r r a n g e m e n t of crystal structure is repeated a r o u n d room t e m p e r a t u r e without appreciable deterioration in luminescence efficiency.

4. Conclusion Methods of preparing a Y2(WO4)3: Eu 3+ (high t e m p e r a t u r e phase) phosphor have been refined. The brightness levels attained are in c a t h o d o l u m i n e s cence 56% that of Y202S: Eu 3+, in p h o t o l u m i n e s c e n c e with 254 n m UV excitation 30% that of Y2Os : Eu ~+ at 25°C a n d a b o u t 50% that of Y203 : Eu 3+ at 230°C. N o t a b l e characteristics of the p h o s p h o r are an increase of photol u m i n e s c e n c e intensity with increasing temperature, red emission with high l u m e n equivalent, a n d fine particle size with relatively high l u m i n e s c e n c e efficiency. The high temperature phase occurs at lower t e m p e r a t u r e after t r e a t m e n t of precipitates to be fired with acid or alkaline solutions. Qualitative analyses suggest that the phase occurrence d e p e n d s on the deviation from stoichiometry of the precipitates to be fired. The excitation b a n d a r o u n d 245 n m is ascribed to the E u - O charge transfer b a n d . A h y d r a t i o n - d e h y d r a t i o n process, which involves a change in crystal structures, is shown to be reversible with no appreciable l u m i n e s c e n c e deterioration.

References [1] [2] [3] [4] [5]

J. Hans, J. Borchardt, J. Chem. Phys. 38 (1963) 1251. J. Hans, J. Borchardt, J. Chem. Phys. 42 (1965) 3743. K. Nassau, H.J. Levinstein and G.M. Loiacono, J. Phys. Chem. Solids 26 (1965) 1805. K. Nassau, J.W. Shiever and E.T. Keve, J. solid State Chem. 3 (1971) 411. K. Nassau and J.W. Shiever, National Bureau of Standards Special Publication 364, Solid State Chemistry, Proc. 5th Materials Research Symposium (1972) 445. [6] G. Blasse and M. Ouwerkerk, J. Electrochem. Soc. 127 (1980) 429. [7] T. Kano, K. Kinameri and S. Seki, J. Electrochem. Soc. 129 (1982) 2296. [8] H. Yamamoto and K. Urabe, J. Electrochem. Soc. 129 (1982) 2069. [9] L. Hans, J. Borchardt, Inorganic Chem. 2 (1963) 170. [10] T. Kano, S. Seki and S. Chiou, Submitted to Yogyo-kyokai-shi(1983). [11] G. Blasse, J. Chem. Phys. 45 (1966) 2356. [12] G. Blasse and A. Bril, J. Chem. Phys. 45 (1966) 2350. [13] S.C. Abrahams and J.L. Bernstein, J. Chem. Phys. 45 (1966) 2745.