Preparation and structure of EuII2EuIIITaO6

Mat. Res. Bull. Vol. 10, pp. 113-116, 1975. in the United States.

P e r g a m o n P r e s s , Inc.

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PREPARATION AND STRUCTURE OF Eu~IEuIIITaO6 K. Sato, G. Adachi and J. Shiokawa Department of Applied Chemistry, Faculty of Engineering, Osaka University Yamadakami, Suita-Shi, Osaka-Fu, Japan

(Received November 20, 1974; Communicated by B. T. Matthias) ABSTRACT Europium tantalate, Eu~IEuIIITaO6, has been prepared by the r e action of Eu203 and Ta205 with Ta metal at 1250-1350°C in argon or vacuum. The crystal structure of this tantalate is found to be the (NH4)3FeF 6 type with the space group, O~-Fm3m. Four m o l e cul~s c ~ s t i t u t e the unit cell with a lattice constant of 8. 309~ for Eu~Eu"~TaO6 . The pycnometric density of 8.45 g. cc -1 for this compound is in good a g r e e m e n t with the X-ray density of 8.48 g. cc -1. The magnetic susceptibility as m e a s u r e d for this compound is also in good a g r e e m e n t with a t h e o r e t i c a l value (78.7 x 10 -6 emu/g). This compound is oxidized to EuIIITaO 7 at 250600°C in air. Introduction The structure of t e r n a r y oxides of the type X3YO 6 has been d e s c r i b e d in comparison with the (NH4)3FeF 6 type structure. Steward and Rooksby (1) found a s e r i e s of tungstates, such as Ba2CaWO6, which belonged to the (NH4)3FeF 6 type. B r i x n e r (2) c h a r a c t e r i z e d Ba3TaO5.5 and Ba2LaTaO 6 as m e m b e r s of this type. Galasso et al. (3) also reported a s e r i e s of Ba2LnTaO 6 (La: r a r e earth e l e m e n t s ) having the same structure. Shafer (4) reported that EuIIWoa could not be prepared in sufficiently pure form, though EuIISrWOa wa~pre~pared as a pure phase. This fact suggests that t h r e e e u r o p ~ m (II)Vions (Eu z+) are not able to coexist i n a n e formula unit of this tungstate. T h e r e f o r e , the corresponding tan talate, Eu.~ TaO.~ .~, may not be expected and another tantalate containing Eu 2-~, such as~Eu~Eu~7:~TaO6, would be formed. The r e s u l t s of the presept e x p e r i m e n t a l work are in accord with this expectation. Experi mental As raw m a t e r i a l s were used high purity europium sesquioxide (99.9%, Shin-etsu Chemical Corp.), tantalum pentoxide and Ta metal (99.8% and 99.9% purity respectively, Wako Pure Chemical Industries, Ltd.). Mixtures of varying compositions (Table 1) were ground together in an agate m o r t a r and 113

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p r e s s e d at 70 K g / c m 2 into p e l l e t s , and f i r e d on a m o l y b d e n u m d i s h at 1250°C o r h i g h e r in a r g o n o r v a c u u m (~ 10 -'~ m m H g ) . A f t e r r a p i d c o o l i n g , the c e n t r a l p a r t of e a c h p e l l e t w a s t a k e n out as a s a m p l e . C h e m i c a l a n a l y s i s f o r any s a m p l e w a s m a d e bv the m e t h o d d e s c r i b e d in Ref. (5). T h e a n a l y t i c a l r e s u l t s w e r e as f o l l o w s . Eu'~IEuIIITaOa, found ( s a m ple no. 1), 2 4 . 2 w t % T a , 6 2 . 7 w t % Eu, E u : T a = 3. 08:~[. 00; calc. v, 2 4 . 7 w t % T a , 6 2 . 2 wt% Eu, E u : T a = 3 . 0 0 : 1 . 0 0 . X - r a y d i f f r a c t i o n d a t a of the r e s u l t i n g m a t e r i a l s w e r e o b t a i n e d with two different diffractometers. Method I: X - r a y d i f f r a c t i o n d a t a w e r e o b t a i n e d with a s t a n d a r d R i g a k u - D e n k i " G e i g e r - f l e x " d i f f r a c t o m e t e r u s i n g N i - f i l t e r e d C u K ~ r a d i a t i o n ( ~ = 1. 5418~,). Silicon, 99. 999% p u r i t y (a = 5 . 4 3 0 1 ~ ) , w a s e m p l o y e d as an i n t e r n a l s t a n d a r d . M e t h o d II: in o r d e r to d e t e c t w e a k r e f l e c t i o n s f r o m EuI2IEuIIITaO6, a R i g a k u - D e n k i " R o t a - f l e x " (200 mA, 40 KV) d i f f r a c t o m e t e r e q u i p p e d with a g r a p h i t e m o n o c h r o m a t o r w a s s u p p l e m e n t a l l y used. T h e m a g n e t i c s u s c e p t i b i l i t y d a t a of the s a m p l e s w e r e o b t a i n e d with a "Shimadzu magnetic balance MB- 1I". T h e d i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) and t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) w e r e a l s o m a d e . T h e h e a t i n g t r a c e w a s r e c o r d e d on a R i g a k u Denki DTA " T h e r m o f l e x " , and t h e h e a t i n g r a t e u s e d w a s 1 0 ° C / r a i n . Results T h e c o m p o s i t i o n of the p r o d u c t w h i c h w a s o b t a i n e d as a p u r e p h a s e , as w a s d e s c r i b e d in the f o r e g o i n g s e c t i o n , w a s E u 3 T a O 6 s u g g e s t i n g the e x i s t e n c e of m i x e d v a l e n c e s f o r e u r o p i u m ion (Eu 2+ and Eu 3+ ), n a m e l y , E u l I E u I I I T a O 6. T a b l e 1 s h o w s the r e s u l t s of m e a s u r e m e n t s of m a g n e t i c s u s c e p t i b i l i t y for the samples obtained under several experimental conditions. The magnetic s u s c e p t i b i l i t y p e r g r a m of s a m p l e , × g, f o r s a m p l e no. I is v e r y c l o s e to a c a l c u l a t e d v a l u e f o r EuIIEuIIITaO~. T h e r e f o r e , s a m p l e no. l w o u l d be the o c o m p o u n d , EuiiEuiiiTaO6.~ ~ X - r a y e x a m i n a t i o n by M e t h o d I d e t e c t e d EuO in s a m p l e s no. 3, 5, 6, 7, 8 and 9 as an i m p u r i t y , and t h e v a l u e s of ~ f o r t h e s e s a m p l e s a r e , t h e r e f o r e , g r e a t e r t h a n t h a t e x p e c t e d f o r EuIIEuIII1VaO6.,~ In o t h e r s a m p l e s , no EuO w a s d e t e c t e d . H o w e v e r , X - r a y e x a m i n a t i o n by Method II d e t e c t e d a t r a c e of E u O e v e n in s a m p l e no. 1. In T a b l e 2 a r e s u m m a r i z e d the X - r a y d i f f r a c t i o n d a t a f o r EugIIEuIIITaO6,~ ( s a m p l e no. 1) as wg~l a ~ , t h o s e f o r B a 3 T a O 5 5 f o r c o m p a r i s o n . A s s u m i n g t h a t t h e t a n t a l a t e Eu~.LEuL~TaO6 h a s a (NH4)~'I~eF 6 t y p e s t r u c t u r e as w a s g i v e n in Fig. 3 of Ref. (1), i n t e n s i t y c a l c u l a t i o n w a s m a d e w i t h t h e r e f i n e m e n t by moving the oxygen parameters. In o u r m o d e l , Ba 2+ , Ca 2+ and W6+ in Ref. 2+ 3+ (1) w e r e r e p l a c e d by Eu , Eu and T a 5+ r e s p e c t i v e l y . U s i n g t h r e e v a l u e s of u = 0 . 2 0 , u = 0 . 2 5 and u = 0 . 3 0 in the c a l c u l a t i o n , t h e a g r e e m e n t f a c t o r R = Z [ I o - I c I / ~ I o , w h e r e I o is t h e o b s e r v e d r e l a t i v e i n t e n s i t y , I c is the c a l c u l a t e d r e l a t i v e t n t e n s i t y , w e r e found to be 2 . 7 , 2 . 4 and 2.87c r e s p e c t i v e l y . T a k i n g a n a p p r o x i m a t e value of u = 0. 25 s e e m s to be t h e f a v o r a b l e a s s e s s m e n t f o r c o n s i d e r a t i o n of ionic r a d i i and s t r u c t u r e - c e l l d i m e n s i o n s .

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EUROPIUM

TANTALATE

115

TABLE 1 Experimental Sample No. 1 2 3 4 5 6 7 8 9

C o n d i t i o n and )(g f o r S a m p l e s .

Starting Material 15Eu203+6Ta+2Ta205 " " " " 15 E u 2 0 3 + 8 T a + T a 2 0 5 " 15 E u 2 0 3 + 1 0 T a "

calculated value; EuIIEuIIITaO Eu~ITaO5.5' Since X-rav diff r a c t i o n of EuI2IEuIIITaO6 and B a 3 T a O 5 5 h a v e d e veloped esser]tially ident i c a l p a t t e r n s a s s e e n in T a b l e 2, t h e f o r m e r m u s t have a (NH4)3FeF 6 type structure. The pycnometric density measured for t h i s c o m p o u n d is 8 . 4 5 g. c c -1, w h i c h c o n f i r m e d t h e p r e s e n c e of f o u r f o r m u l a u n i t s of E u ~ l E .u l i T i T a O in a u n i t cell (calculated density by u s i n g t h e X - r a y r e s u l t ; 8 . 4 8 g. c o - l ) . The magnetic susceptibility for EuIIEuIIITaO~ (sample no." 1) w a s m ~ a s u r e d in the r a n g e of 77 to 2 8 8 ° K . The temperature depend e n c e of that f o r E u I I E u I I I T a O g is e x p r g s s e d by th~ e q u a t i o n , )~M = 8 . 5 6 / ( T - 3 . 4 6 ) , and f r o m t h i s e q u a t i o n , the total effective m a g -

Atm.

Heating Temp.

Heating Time

vac. " " " Ar vac. Ar vac. Ar

1250°C 1.5hrs. " 3hrs. " 4hrs. 1350°C 3hrs. 1250°C " 1250°C " " " " " . . . .

:~g(15 °C) xl0-6emu/g 80.0 83.0 87.4 83.6 86.2 85.7 95.7 88.3 109.0

Impurity

EuO EuO EuO EuO EuO EuO

, 7 8 . 7 x 10 -6 e m u / g . 109 x 10 -6 e m u / g . TABLE 2

ao

Powder Diffraction Data for EuIIEuIIITaO~. = 8 . 3 0 9 A ; u = 0 . 2 5 . S p a c e gro~up; O 5 - F n ~ 3 m . II

EuIIEuIIITaO6 hkl 111 200 220 311 222 400 331 420 422 333 440 531 442 620 533 622 444 711 640 642

Ba3TaO5.5*

do

dc

Io

Ic

do

Io**

798 153 936 504 2. 076 1.907 1.858 1.696 1.599 1.469 1.404 1.314 1.266 1.199 1.164 1.110

4. 797 4. 155 2. 938 2. 505 2. 399 2. 077 1. 906 1.858 1.696 1.599 1.469 1.404 1. 385 1.314 1.267 1. 253 1.199 1.164 1. 152 1.110

0.4 1.3 100 1.0

0.3 1.2 100 0.3 0.5 28.0 0.15 0.5 43.4 0.03 17.7 0.15 0.2 18.5 0.05 0.03 6.0 0.04 0. 1 23.2

5.01 4.34 3.07 2. 618 2. 170 1.992 1. 772 1.671 1.534 1.468 1.373 1.323 1.253 1.217 1. 116

m w s+ w sm s m m+ w m+ w w wm

4. 4. 2. 2.

27.4 0.8 1.5 43.0 0.6 16.8 0.2 18.9 0.1 6.1 0.1 23.1

* D a t a f r o m R e f e r e n c e (2). **s = strong, m = moderate,

w = weak.

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netic moment of europium ions (Eu 2+ + 0.5 Eu 3+) in this compound is calculated to be a value of 8.31 B. M., which is excellently close to a t h e o r e t i c a l one (8.30 B. M.). The t h e r m a l behavior of the product in air was examined by means of DTA and TGA. The weight gain s t a r t s at about 260°C, and the maximum of the e x o t h e r m i c DTA peak is at about 400°C where a total gain of weight amounts to 2.1% of the starting material, Eu I~ EuIIITaO 6" This value a g r e e s with the total weight g~ain of 2.2% which is caYculated assuming the oxidation r e a c t i o n towards Eu~l~TaO7. Discussion Charge distribution among t h r e e europium ions, namely, coexisting of two Eu2+Twith one Eu 3+ in this compou~l, is interesting. Europium (II) tungstate, Eu~WO6, does not exist while Eu~SiO5 can be prepared (4). The free energy of formation of Ta205 and SiO 2 f r o m the corresponding pure metals at 1350°C, acting as the reduction power of each metal, are -166.3 and -145.4 K c a l / m o l e respectively. F r o m this point, it s e e m s that the reduction power of Ta m~tal would be good enough to p r e p a r e hypothet~calTEuIITaO~ 5" The Eu,IIEuIhTaO~ can be r e p r e s e n t e d by the formula Eu~'(Eu~}L'5~a~ 5~[J3 with a per~ovskite structure. If l a r g e r and m o r e basic ions than ~ 3 + , "such as Ba 2+ or Eu , are introduced in ABO 3 s t r u c t u r e together with Eu , Eu ion is forced to occupy half of the B - s i t e s (six-fold sites). Tantalum ion, Ta 5+, must occupy another half of the B - s i t e s . This situation r e s u l t s in an o r d e r e d a r r a n g e m e n t of the B - s i t e s because of the big difference in ionic radii between Eu 3+ and Ta 5+. On the other hand, in the case of Ba3TaO5.5, Ba2+ tends to take the A - s i t e s (twelve-fold sites) in this structure. However, o n e - t h i r d of Ba 2+ must occupy the B - s i t e s since the number of the A - s i t e s available for Ba 2+ is not enough. Consequently, since Eu 3+ can occupy the B - s i t e s more easily than Eu 2+, EuIIEuII1TaO 6 a r e m o r e stable than hypothetical EuIITaO5 .5 which would be i s o s t r u c t u r a l to Ba3TaO5.5. Conclusions A new divalent europium compound, Eu~EuIIITaO~, has been prepared. The s t r u c t u r e for this compound was detected l~y X - r a y pSwder diffraction method. The r e s u l t s of the magnetic susceptibility m e a s u r e m e n t s and DTA and T GA e x p e r i m e n t s confirmed that this compound was r e p r e s e n t e d by the formula Eu~IEuIIITaO 6 . References 1.

H. P. Rooksby and E. G. Steward, Acta Cryst. 4, 503 (1951).

2.

L.H.

3.

F . S . Galasso, G. K. Layden and D. E. Flinchbaugh, J. Chem. Phys. 44, 2703 (1966).

4.

M . W . Shafer, J. Appl. Phys. 366, 1145 (1965).

5.

T. Kato, Teiryo Bunsekiho, p. 642.

Brixner, J. Am. Chem. Soc. 80, 3214 (1958).

Maruzen, Tokyo (1938).