Divalent europium compounds in the systems EuMoO and EuWO

Mat. Res. Bull. Vol. 6, pp. 31-40, 1971. the United States.

DIVALENT

EUROPIUM

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

COMPOUNDS

Printed in

IN T H E S Y S T E M S

Eu-Mo-O AND EuoW-O G r e g o r y J. McCarthy M a t e r i a l s R e s e a r c h Laboratory The Pennsylvania State University University Park, Pennsylvania 16802 (Received November 20, 1970; Communicated by R. Roy) ABSTRACT A study was made of compound formation in the systems E u - M o - O and E u - W - O with the objective of preparing the Eu 2+ analogues of the known Sr 2+ compounds EuMoO4, E u W O 4 and EuMoO3. The scheeIRe-type compounds E u M o O 4 and E u W O 4 were easily prepared. No E u M o O 3 compound could be prepared over the temperature range 1200 ° - 1400°C. In the course of the study three new compounds containing trivalent rare earths were encountered, Eu2Mo207, Srn2Mo207 and Eu6WO12. The first two have cubic pyrochlore-type structures while E u 6 W O 1 2 has a defect cubic fluorite structure. The necessity of considering thermodynamic p a r a m e t e r s as well as size, charge and polarizability in s y s t e m a t i c c r y s t a l c h e m i s t r y is discussed. Introduction This work is part of a continuing study of the stability of oxide c o m pounds in which both of the cations can have variable valences.

We have r e -

cently completed a review of the stabilities of oxides in the s y s t e m Eu-O (1) and chose this as one of the binary oxide s y s t e m s , with Mo-O and W-O as the others.

These latter two s y s t e m s have been thoroughly studied by Phillips

et al. (2-4). The t e r n a r y s y s t e m s Eu-Mo-O and Eu-W-O could contain oxides with one or m o r e combinations of Eu 2+ or Eu 3+ with (Mo, W) 6+ or (Mo, W)4+. We e x a m i n e d ' t h e s e s y s t e m s for compounds in which the europium was di31

32 valent.

EUROPIUM COMPOUNDS

Vol. 6, No. 1

Any study of Eu 2+ compound formation is aided frequently by the ex-

istence of a previously studied i s o s t r u c t u r a l Sr 2+ compound. A recent compilation of effective ionic radii in oxides (5) lists rEu2+ = r s r 2 + = 1.25.~ (in 8-fold coordination).

Known t e r n a r y oxides in Sr-Mo-O and Sr-W-O include

the scheelite s t r u c t u r e compounds SrMoO 4 (6) and SrWO 4 (7), the perovskite SrMoO 3 (8) and a d i s t o r t e d (NH4)3FeF6-type structure compound Sr3WO 6 (9). By the usual c r y s t a l c h e m i c a l c r i t e r i a of size, charge and polarizability we would expect to find analogous Eu(Mo, W)O4 and EuMoO 3 compounds.

The ex-

istence of Eu3WO 6 was not examined at this time. Experimental Starting m a t e r i a l s were reagent grade WO3, MOO3, Mo, W and high purity Eu203 (99.99%).

The d e s i r e d bulk compositions for a particular prep-

aration could be obtained by mixing appropriate amounts of Eu203, (Mo, W)O3 and (Mo, W) metal or EuO and (Mo, W)O2 or (Mo, W)O3. cording to a method d e s c r i b e d by Shafer (10).

EuO was p r e p a r e d a c

No appreciable difference in

r e a c t i o n time or crystallinity of reaction products was noted for e i t h e r s t a r t ing mixture.

The m i x t u r e s w e r e p r e s s e d into pellets and placed in molybde-

num crucibles.

All reactions were c a r r i e d out at 1400°C for 18-36 hours in

a purified argon a t m o s p h e r e .

A very thin reaction zone at the surface of the

pellet in contact with the molybdenum crucible was easily r e m o v e d before examination of the reaction products.

Phase identification was by reflected and

t r a n s m i t t e d light m i c r o s c o p y and by x - r a y diffraction.

Most of the phases

encountered were colored black, grey, yellow, orange or blue and could be readily distinguished.

X-ray powder data were obtained on a d i f f r a c t o m e t e r

calibrated with high purity silicon metal (a ° = 5.4301.~) using N i - f i l t e r e d CuK~ radiation ( 4 = 1. 54178,~).

Cell p a r a m e t e r s were d e r i v e d from a least

s q u a r e s c o m p u t e r r e f i n e m e n t of data from 9-20 unambiguously indexed r e flections. In previous studies (11, 12) we have shown the importance of oxygen analysis on run products when preparing reduced compounds in open s y s t e m s . This procedure was used routinely here.

To d e t e r m i n e a compound's oxygen

s t o i c h i o m e t r y we heated a weighed portion of it in a i r and m e a s u r e d the p e r -

Vol. 6, No. 1

EUROPIUM

centage increase in its weight.

COMPOUNDS

33

F o r EuWO4, the theoretical weight gain is

found from the reaction 2 EuWO 4 + 1 / 2 0 2 -~ Eu2W20 9. Stoichiometric EuWO4 should show a gain in weight of 1.82% on heating in air. The weighing and heating procedure is described in detail elsewhere (11). Results Eu (Mo, W ) O 4 Mixtures of starting materials corresponding to the bulk composition E u M o O 4 and E u W O 4 were reacted. coarse grained.

The E u M o O 4 product was deep blue and

The pellet had shrunk somewhat indicating either advanced

sintering or a small amount of melting.

The x-ray diffraction pattern was

that of a well crystallized scheelite structure compound whose peaks corresponded very closely in relative intensity and position to those of S r M o O 4 (6). The cell parameters of both compounds are included in Table 1, the powder pattern of E u M o O 4 is listed in Table 2. The average of several weight gain on oxidation runs on E u M o O 4 was 2.71% compared to a theoretical value of 2.70%.

The E u M o O 4 sample was thus stoichiometric to within the sensitivRy

of the experiment. Similar resuRs were obtained for E u W O 4. The product was an orange coarse grained compound with a well crystallized scheelite type x-ray pattern with very good correspondence of peak intensity and position to the S r W O 4 TABLE

1

Cell Parameters of the Scheelite Structure Corn )ounds.

ABO 4 EuWO 4

EuMoO 4 SrWO 4 SrMoO 4

ao(~k)*

Co (~)*

V(~ 3)

= +0. 024

11. 936 T 11.990 A = -0.054

349. 4 347. 9 T& = +I. 5

5.417 T A = +0.023 5. 394

11. 951 T 12. 020 A = -0.069

350. 7 349.8 "~A = +0. 9

5. 411 i 5. 387

*all + 0. 001 or better.

Sr(Mo, W)O4 data from r e f e r e n c e s (6, 7).

34

E U R O P I U M COMPOUNDS

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TABLE 2 X - R a y Data f o r E u M o O 4 and EuWO 4. EuMoO 4 (hkl)

EuWO4

dobs

dcalc

I/11

4.91 3.12 2. 997 2.692

4.91 3. 125 2. 997 2. 693

1 100 30 15

2. 356 2. 191 2.063 2. 003 1. 905 1. 815 1. 769

. 356 2. 191 2. 063 2. 003 1. 904 1. 815 1. 770

2 3 55 11 1 24

dob s

dcalc

I/11

4.93 3. 221 2. 984 2. 705 2. 372 2. 353

4.93 3. 221 2. 984 2. 705 2. 372 2. 353

5 100 18 20 3 2

2. 004 1.913 1.822 1.765 1. 699 {1. 645 1. 643 1. 6104 1. 4920 1. 3527 1. 3065 1. 2972 1. 2472 1.2320 1. 2099 1. 1765 1. 1395 1. 1212

40 12 1 25 2

101 112 004 200 211 114 105 213 204 220 222 116 215 312 303 224 OO8 4OO 2O8 316 332 4O4 420 228 1.1.1 424

1. 607 1. 4986 1. 3467 1.3100 1. 2964 1.2423 1. 2288 1. 2048 1. 1777 1. 1400 1. 1180

p a t t e r n (7).

T h e s e x - r a y d a t a a r e a l s o l i s t e d in T a b l e s 1 and 2. H o w e v e r ,

1.638

1. 639 {1 638 1. 607 1. 4987 1. 3467 1. 3096 1. 2963 1. 2421 1. 2285 1. 2046 1. 1778 1. 1437 1. 1177

2. 1. 1. 1. 1.

004 913 822 765 699

30

1. 644

13 4 1 8 12 4 4 5 3 4 6

1. 6102 1. 4921 1. 3529 1. 3067 1. 2972 1. 2472 1. 2322 1. 2101 1. 1767 1. 1396 1. 1211

t h i s w a s not t h e f i r s t t i m e that EuWO 4 had b e e n p r e p a r e d .

20 14 5 4 13 16 6 8 7 11 9 6

S h a f e r (10) p r e -

p a r e d EuWO 4 f r o m a m i x t u r e of E u O and WO 3 in a s e a l e d s y s t e m and c o n f i r m e d that the e u r o p i u m w a s d i v a l e n t in the c o u r s e of m a g n e t i c m e a s u r e m e n t s . He m e a s u r e d c e l l p a r a m e t e r s of a ° = 5 . 4 1 ~ and c o = 11.95/~ and t h e s e w e r e used as trial parameters

in o u r l e a s t s q u a r e s r e f i n e m e n t .

The weight gain

a v e r a g e on o u r s a m p l e w a s 1.82% which is the s a m e a s the t h e o r e t i c a l value. It w a s i n t e r e s t i n g to note that w h e n goEng f r o m m o l y b d a t e to t u n g s t a t e in t h e Sr 2+ c o m p o u n d s , t h e c e l l p a r a m e t e r s

b e h a v e in o p p o s i n g m a n n e r .

The

Vol. 61 No. 1

E U R O P I U M COMPOUNDS

parameter a o increases while c o decreases.

35

The same behavior was shown

f o r E u M o O 4 and EuWO 4 a s is s e e n in T a b l e 1.

It is a l s o i n t e r e s t i n g to note

that a c c o r d i n g to t h e r a d i i l i s t e d in T a b l e 3, a l l f o u r s c h e e l i t e s t r u c t u r e c o m pounds should have exactly the same cell parameters.

TABLE 3

That they

E f f e c t i v e Ionic Radii in O x i d e s . *

d o n ' t s i m p l y p o i n t s to the l i m i t a -

Ion

Coordination

Radius ~)

Eu 2+

8

1.25

S r 2+

8

1.25

w a s n o t e d f o r E u M o O 4 on h e a t i n g

Eu 3+

8

1.07

in a i r .

S m 3+

8

1.09

t h e p r o d u c t w a s white and its

Mo 6+

4

0.42

weight gain indicated that EuMoO 4

W 6+

4

0.42

had b e e n c o m p l e t e l y o x i d i z e d .

Mo 4+

6

0. 650

h a d a b r o a d e n e d but r e l a t i v e l y

W4+

6

0. 650

strong scheelite-type x-ray pat-

T i 4+

6

0. 605

tern.

* a f t e r r e f e r e n c e (5).

t i o n s of u s i n g a p u r e h a r d s p h e r e ionic m o d e l . An ihteresting behavior

A f t e r 12 h o u r s at 600°C

The cell parameters

It

of t h i s

p h a s e (a o = 5 . 2 7 + 0.01,~, c o = 1 1 . 4 0 + 0. 03A a n d V = 3 1 7 . 0 + 0. 9A 3) w e r e c o n s i d e r a b l y s m a l l e r t h a n t h o s e of E u M o O 4.

H e a t i n g at 800°C f o r 20 m o r e h o u r s y i e l d e d a p r o d u c t w i t h

sharper

p e a k s in w h i c h the s c h e e l i t e r e f l e c t i o n s w e r e s p l i t into two o r

three.

A new and e v e n m o r e c o m p l e x p a t t e r n w a s d e r i v e d w h e n the s a m e

s a m p l e w a s h e a t e d at 1050°C.

Finally,

a f t e r h e a t i n g to 1200°C t h e s a m -

ple m e l t e d . A m o d e l f o r the m e t a s t a b l e s c h e e l i t e - t y p e c o m p o u n d m i g h t be 3+ 6+ (Eu0. 8 9 ~ ] 0 . l l ) ( M o 0 . 8 9 [ - ] 0 . 11)O4. T h i s c o m p o u n d would have a m u c h smail e r unit c e l l t h a n E u 2 + M o 6 + O 4 due to t h e c o m p l e t e s u b s t i t u t i o n of the s m a l l e r Eu 3+ f o r Eu 2+ and the c a t i o n d e f e c t s . Eu2Mo20 7 S e v e r a l E u M o O 4 p r e p a r a t i o n r u n s g a v e d i f f r a c t i o n p a t t e r n s with a few extra peaks.

Weight gain experiments indicated that these run products had a

s l i g h t l y r e d u c e d bulk c o m p o s i t i o n .

This implied that there was a more re-

d u c e d t e r n a r y c o m p o u n d in t h e E u - M o - O s y s t e m .

The extra lines were read-

36

EUROPIUM COMPOUNDS

Vol. 6, No. 1

ily indexed on a p y r o c h l o r e - t y p e unit c e l l and the p u r e c o m p o u n d E u 2 M o 2 0 7 was easily prepared.

Weight gain r u n s i n d i c a t e d a s t o i c h i o m e t r i c compound.

A l e s h i n and Roy (13) have found that Sr 2+ is g e n e r a l l y too l a r g e for the A - s i t e in p y r o c h l o r e . T h i s i m p l i e s that the c a t i o n v a l e n c e s a r e r~u23 + .lvlo42 + _u 7 not 2+ 4+ 6+ Eu 2 [Mo , M o ]2 ° 7" To m a k e s u r e , we a t t e m p t e d to p r e p a r e the a n a l o gous S r 22+ [Mo 4+, Mo 6+ ] znO_'~ but without s u c c e s s " H o w e v e r , an i s o s t r u c t u r a l S m 3 + M o ~ + O 7 was e a s i l y p r e p a r e d . in T a b l e 4.

X - r a y d a t a f o r t h e s e c o m p o u n d s is g i v e n

Both c o m p o u n d s w e r e i s o s t r u c t u r a l with the c o r r e s p o n d i n g t i t a -

n a t e s p r e p a r e d by B r i x n e r (14), E u 2 T i 2 0 7 (a ° = 10. 192A) and S m 2 T i 2 0 7 (a ° = 10. 211.~).

The v a r i a t i o n in the c e l l d i m e n s i o n s f o r the f o u r c o m p o u n d s c o r -

r e s p o n d s to the d i f f e r e n t v a l u e s of the cation r a d i i listed in Table 3. TABLE 4 X - R a y Data for E u 2 M o 2 0 7 and S m 2 M o 2 0 7. Eu2M°20 7

(hkl) 111 311 222 400 331 333 440 622 444 800 662 840

1

dob s

dcalc

6. 01 3.13 2. 998 2. 595 2. 382 1. 997 1. 836 1. 565 1. 498 1. 298 1. 1909 1. 1605

6.00 3.13 2. 996 2. 595 2. 381 1. 998 1. 835 1. 565 1. 498 1. 298 1. 1907 1. 1605

Sm2Mo20 7 I/I 1 3 2 100 35 10 3 70 60 12 10 30 25

2

dob s

dcalc

I/I l

3.16 3.02 2. 601 2. 386 2. 002 1. 839 1. 568 1. 502 1. 301 1. 194 1. 164

3.14 3.00 2. 601 2. 387 2. 003 1. 839 1. 568 1.502 1. 301 1. 194 1. 164

3 100 40 8 1 65 55 15 8 28 22

1 P y r o c h l o r e - t y p e a = 10.380 + 0. 001/~. 2 o P y r o c h l o r e - t y p e a ° = 10.406 + 0.004/~. The q u e s t i o n that the Sm and thus the Eu in t h e s e c o m p o u n d s is d i v a lent could be r a i s e d .

H o w e v e r , we have shown (1, 15) that Sm 2+ is not a s t a -

ble v a l e n c e in a s e r i e s of oxides (halides, a sulfate and c a r b o n a t e a r e known) and its p r e s e n c e in t h i s c o m p o u n d is v e r y doubtful.

Vol. 6, No. 1

EUROPIUM COMPOUNDS

37

It is i n t e r e s t i n g to note that after slowly heating to 600°C in air, both Eu2Mo207 and Sm2Mo207 oxidized to give the s c h e e l i t e - l i k e x - r a y patterns. Attempts to p r e p a r e the analogous W4+ compounds, Eu2W207 and Sm2W207, at 1400°C w e r e unsuccessful.

The result for Sm2W207 a g r e e d

with that of Chang et al. (16) who did not find this compound in t h e i r study of the s y s t e m Sm-W-O from 1400 ° - 1700°C. EuMoO 3 We w e r e unable to prepare the Eu 2+ analogue to SrMoO 3.

P e l l e t s of

the appropriate bulk compositions showed partial melting after heating at 1400°C and gave poorly c r y s t a l l i z e d unknown x - r a y patterns. A r u n a t 1200°C showed no signs of melting but yielded the s a m e x - r a y pattern. _~6WO12 Chang et al. (9) did not find the compound Sr2+W4+O 3 of the s y s t e m Sr-W-O. Eu2+W'~+O3 . -

in t h e i r study

However, we made a single attempt at synthesizing

After heating at 1400°C, the product was readily identified as

a three phase mixture by reflection microscopy.

Its x - r a y pattern consisted

of EuWO4 + W metal + extra peaks which were indexed on a fluorite (or pyrochlore) unit cell.

The compound was identified as Eu6WO12 by c o m p a r i s o n

with the data for Sm6WO12 p r e p a r e d by Chang et al. (16).

It was prepared as

a pure phase by heating a 3:1 m o l a r mixture of Eu203 and WO 3 in a i r at 1400°C for 24 hours.

X - r a y data of this new compound are given in Table 5.

Chang and Phillips (17) found that Sm6WO12 and an i s o s t r u c t u r a l La6WO12 w e r e d i s o r d e r e d pyrochlore (or o r d e r e d defect fluorite) type r a t h e r than f. c . c . fluorite type due to the p r e s e n c e of weak (200) and (422) reflections. We did not o b s e r v e these two reflections in the Eu6WO12 patterns and thus indexed them on an f. c. c. fluorite cell.

Data for Sm6WO12 a r e included in

Table 5 for comparison. Discussion In the application of s y s t e m a t i c crystal c h e m i s t r y it is g e n e r a l l y held that the three c r i t e r i a of size, charge and polarizability a r e e s s e n t i a l l y all that is needed to predict whether a compound will form in a particular s t r u c -

38

EUROPIUM COMPOUNDS

Vol. 6, No. 1

TABLE 5 X-Ray Data for Eu6WO12 and Sm6WO12.* Eu6WO121 (hkl)fluor ite

dobs

Sm6WO122 dcalc

I/I 1

111 200

3.097 2. 683

3. 098 2.683

100 40

220 311 222 400 331 420 422

1. 897 1. 618 1.549 1. 342 1. 231 1.20O 1. 095

1. 897 1. 618 1. 549 1. 342 1.231 1.2OO 1. 095

55 45 14 6 16 12 11

(hkl)pyrochlore

dob s

I/11

200 222 400 422 440 622 444 800 662 840 844

(32

3. 123 2. 709 2. 206 1. 910 1. 629 1. 559 1. 352 1. 239 1. 208 1. 103

I 100

.

40 2 65 55 16 10 21 16 15

from r e f e r e n c e 17). ~Data Cubic defect fluorite-type a o = 5. 366 +

0. 003. 2 D i s o r d e r e d p y r o c h l o r e - t y p e a ° = lO. 8o .. ture type.

This is the basis of the well known structure field maps.

We have

studied s e v e r a l s y s t e m s which illustrate that where variable valences are involved a fourth, t h e r m o d y n a m i c , p a r a m e t e r must be considered.

Since Sr 2+

and Eu 2+ have virtually identical radii and Eu 2+ is known to form a number of stable oxide compounds, the f i r s t t h r e e c r i t e r i a would predict the existence of Eu 2+ analogous to known Sr 2+ compound.

This is valid for SrMoO4/

EuMoO4, SrWO4/EuWO 4 and others such as SrTiO3/EuWiO 3 (10) and SrNbO3/ EuNbO 3 (18).

However, why is there no EuMoO 3 analogue to SrMoO 3 and why

since r w 4 + = rMo4+ is t h e r e no Eu2W20 7 analogue to Eu2Mo20 7 ? Clearly we also need to consider o x i d a t i o n / r e d u c t i o n t h e r m o d y n a m i c s of the variable valence oxides and the ability of an available (by the first three c r i t e r i a ) s t r u c ture type to supply the energy needed to stabilize the particular set of valences in question. Acknowledgment This work was supported by the Advanced R e s e a r c h P r o j e c t s Agency with additional support for x - r a y powder data collection by the Joint C o m m i t -

Vol. 6, No. 1

EUROPIUM COMPOUNDS

tee on Powder Diffraction Standards.

39

We also acknowledge the helpful d i s -

cussions with Dr. Olaf Muller. References 1. G. J. McCarthy and W. B. White, J. L e s s - C o m m o n Metals 2..22, 409

(1970).

2. B. Phillips and L. L. Y. Chang, Trans. AIME 230, 1203 (1964). 3. B. Phillips and L. L. Y. Chang, Trans. AIME 23___33, 1433 (1965). 4. L. L. Y. Chang and B. Phillips, J. Am. Ceram. Soc. 5_22, 527 (1969). 5. R. D. Shannon and C. T. Prewitt, Acta. Cryst. B25, 925 (1969). 6. H. E. Swanson, N. T. Gilfrich and M. I. Cook, NBS C i r c u l a r 539, 50 (1957).

7. H. E. Swanson, N. T. Gilfrich and M. I. Cook, NBS Circular 539, 53 (1957). 8. L. H. Brixner, J. Inorg. Nucl. Chem. I_44, 225 (1960). 9. L. L. Y. Chang, M. G. Scroger and B. Phillips, J. Am. Ceram. Soc. 4__99, 385 (1966). 10. M. W. Shafer, J. Appl. Phys. 3_6_6, 1145 (1965). II. G. J. McCarthy, W. B. White and R. Roy, J. Inorg. Nucl. Chem. 3_~1, 329 (1969). 12. G. J. McCarthy, W. B. White and R. Roy, J. Am. Ceram. Soc. 5_~2, 463 (1969). 13. E. Aleshin and R. Roy, J. Am. Ceram. Soc. 4_.55, 18 (1962). 14. L. H. Brixner, Inorg. Chem. 3, 1065 (1964). 15. G. J. McCarthy, W. B. White and R. Roy, Inorg. Chem. 8, 1236 (1969). 16. L. L. Y. Chang, M. G. Scroger and B. Phillips, J. Inorg. Nucl. Chem. 2__88, 1179 (1966). 17. L. L. Y. Chang and B. Phillips, Inorg. Chem. 3, 1792 (1964). 18. J. Sanzgiri, G. J. McCarthy and G. G. Johnson, Jr. (to be published).