Hyperfine Interactions 15/16 (1983) 755-758 9 J.C. Baltzer Scientific Publishing Company

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MOSSBAUER STUDIES ON Fe2MoO~ J. Ghose

Department of Chzmi6try, Indian Institute of Technology, K~gpur-721302, India

The M S s s b a u e r spectrum of Fe2MoOq at 4.2 K was fitted to two six llne h y p e r f i n e patterns with field values of 218• kOe, due to Fe 2+ on A-sltes, and 414• kOe due to the p r e s e n c e of Fe 2+, along with small amounts of Fe z+ on the B-sltes. The spectrum with an applied t r a n s v e r s e field of 50 k O e was fitted to three h y p e r f i n e patterns. The field values of these patterns suggest n o n - c o l l i n e a r spin arrangement in Fe2MoO4.

I.

Introduction

The magnetic studies on Fe2MoO~ have been carried out by several authors using the methods of magnetization, neutron diffraction and M~ssbauer spectroscopy. Although the studies by Abe et al. 111 and Ghose et al. 121 are quite extensive, yet the controversy regarding the valence state of Fe and Mo is not yet resolved as the M6ssbauer spectra obtained below the ordering temperature of the spinel were rather complex, and done of the authors were able to resolve them into the respective A-site and B-site patterns. However, the average isomer shift values obtained from the paramagnetic spectra, led Abe et al. 131 and Gupta et al. 141 to suggest a 2-3-3 valence arrangement rather than a 2-2-4 arrangement. Further4aore the latter authors have also suggested the following cation distribution. 2+ 5 Fe0. 3+ 5 Fe0.

-

2+ 3+ 5 Mo3+~ 04 _Fe0. 5 Fe0.

But the neutron diffraction studies have shown that the small magnetic moment of 0.2 ~B cannot be accounted for by a F e 2 + ( 4 ~ B ) - F e 3 + ( 5 ~ B ) -Mo3+(3~B ) arrangement. Moreover, the value of Fe2+/Fe 3+ = 1 on the octahedral site is also not in accord with the results of the thermal and electrical studies on Fe2MoO~ 15,61. Thus the present work was taken up in an attempt to resolve the 4.2 K M~ssbauer spectra of Fe2MoO~ recorded by Ghose et al. 121, into the respective A-site and B-site hyperfine patterns. 2.

Experimental

The p r e p a r a t i o n of the spinel, its c h a r a c t e r i z a t i o n and recording of the M 6 s s b a u e r spectra has been described earlier 121. 3.

Results

The 4.2 K M 6 s s b a u e r spectrum was fitted to two six line hyperfine patterns c o r r e s p o n d i n g to A-site and B-site Fe atoms. The lines in both the patterns were rather broad. The value of the hyperfine fields obtained from these patterns were 414• kOe and 218• kOe respectively. The spectrum obtained with an a p p l i e d transverse field of 50 kOe

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at 4.2 K c o u l d be f i t t e d to t h r e e six l i n e h y p e r f i n e b r o a d lines. The h y p e r f i n e f i e l d s for t h e s e p a t t e r n s 415• kOe a n d 218• kOe respectively. 4.

patterns having w e r e 518• kOe

Discussion

The h y p e r f i n e f i e l d v a l u e s of 218• kOe and 414• kOe for the 4.2 K M ~ s s b a u e r s p e c t r u m in zero a p p l i e d field, i n d i c a t e t h a t one of the p a t t e r n s is due to o n l y Fe 2+ ions s i t u a t e d on one site, w h i l e the o t h e r site is p r o b a b l y p o p u l a t e d by b o t h Fe =+ and Fe 3+ ions as in Fe304. A h y p e r f i n e f i e l d v a l u e of 218• kOe further indicates t h a t Fe 2+ is on the A - s i t e as the v a l u e r e c o r d e d for B - s i t e Fe 2+ is r e l a t i v e l y small 171. The b r o a d l i n e s of the h y p e r f i n e p a t t e r n s m a y be a t t r i b u t e d to the e f f e c t s of c a t i o n - d i s o r d e r p h e n o m e n a on the l o c a l s y m m e t r y of the Fe ions in the s p i n e l lattice. T h i s m a y be i l l u s t r a t e d by the f o l l o w ing figure, t a k i n g o n l y the o c t a h e d r a l sites i n t o c o n s i d e r a t i o n .

0

OXYGEN IONS CAT IONS

3

2

4

0

FIG.1. LABELLING OF THE B SITES

E a c h of the B - i o n s m a y n o t h a v e the same n e a r n e i g h b o u r ions, a n d as w a s s u g g e s t e d by F r a n k e et al. 181 for a s p i n e l h a v i n g Fe ~+, Fe 3+ a n d a n o t h e r ion M, the B - s i t e h y p e r f i n e p a t t e r n m a y h a v e at l e a s t 4 s u b s p e c t r a e.g. Bl B2 Bs B~

Fe Fe Fe Fe

2+ 2+ 2+ 2+

(0,2,4), (0,2,4), (0,2,4), (0,2,4),

a n d Fe 3+ (1,3,5) Fe 3+ (1,3), a n d M (5) Fe 3+ (I) a n d M (3,5) a n d M (1,3,5)

Thus, BI w i l l c o r r e s p o n d to the h y p e r f i n e f i e l d of Fe30~ a n d B4 to a "pure" Fe 2+ h y p e r f i n e f i e l d v a l u e . S i m i l a r l y , the A - s i t e p a t t e r n m a y be r e s o l v e d into two s u b s p e c t r a . T h e p r e s e n c e of Fe 2+ a n d Fe 3+ on the same s i t e is u s u a l l y a c c o m p a n i e d b y e l e c t r o n i c t r a n s f e r , w h i c h leads to f u r t h e r b r o a d e n i n g of the peaks.

J. Ghose, MOssbauer Studies on Fe2Mo04

757

The value of 414• kOe is much smaller than the B-site hyperfine field value of Fe304, and a comparison with the four values obtained by Franke et al. for the B-site subspectra of Fe3_xCrxO 4 [8 I shows that it is nearer to B, value. This indicates that in Fe2MoO~ iron is present p r e d o m i n a n t l y as Fe 2+. Thus the 2-3-3 cation arrangement (Fe2+/Fe3+=l) suggested by Gupta et al. 141 is rather improbable, as the Fe 3+ present in Fe2MoO, appears to be in very small amounts i.e. Fe2+/Fe~+>3. This is also c o r r o b o r a t e d by the small change in c o n d u c t i v i t y of Fe2MoO~ b e l o w 400 K 161. Thermal studies have also indicated the presence of only very small amounts of Fe 3§ (~3.5%) in Fe2MoO~ 191. Unlike the zero field spectrum, the 4.2 K spectrum in an applied transverse field could be fitted to three hyperfine patterns. The values of the h y p e r f i n e fields of these three patterns, i.e. 518• kOe, 415• kOe and 238• kOe are rather interesting. Only the B-site hyperfine field remains unaltered, but the A-site h y p e r f i n e field value increases with the a p p l i c a t i o n of a field. In addition a pattern having a large field value is also present. This suggests that some of the spins on the B-site are parallel to the applied transverse field, which is able to align them along its own direction, thereby increasing the h y p e r f i n e field value by 50 kOe. Thus, these results indicate that the spins in Fe2MoO~ are not collinears and as suggested by Ghose et al. 121 the n o n - c o l l i n e a r i t y is on the B-site. From the above results, it may be c o n c l u d e d that in Fe2MoO~ Fe is present p r e d o m i n a n t l y in the +2 state and only traces of Fe 3+ are present on the B-sites, which cause electronic transfer between Fe 2+ and Fe 3+ leading to a b r o a d e n i n g of the peaks and a B - h y p e r f i n e field value of 414• kOe. The spectrum with an applied field also suggests n o n - c o l l i n e a r spin a r r a n g e m e n t in Fe~MoO~. Further work is in progress on low t e m p e r a t u r e M ~ s s b a u e r spectra of Fe~MoO~. Acknowledgements The author wishes to thank Prof.Dr. G. Ritter and Dr. D.L. Nagy helping w i t h the c o m p u t a t i o n and also for helpful discussions.

for

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

M. Abe, M. K a w a c h i and S. Nomura, J. Phys. Soc. Japan, 33 (1972) 1296 J. Ghose, N.N. Greenwood, G.C. Hallam and D.A. Read, J. Sol. State Chem. 11 (1974) 239 M. Abe, M. K a w a c h i and S. Nomura, J. Phys. Soc. Japan, 34 (1973) 565 M.P. Gupta, S.M. Kanetkar, S.K. Date, A.S. N i g a v e k a r and A.P.B. Sinha, J. Phys. C: Solid State Phys., 12 (1979) 2401 J. Ghose, J. Therm. Anal., 24 (1982) 171 J. Ghose, Proc. Seventh Int. Conf. on Thermal Analysis, 1982, ed. B. M i l l e r (John Wiley & Sons, 1982), p. 706 H.B. Mathur, A.P.B. Sinha and C.M. Yagnik, Indian J. Appl. Phys., 5 (1967) 155 H. Franke and M. Rosenberg, J. Mag. Mag. Mater., 9 (1979) 74 J. Ghose, u n p u b l i s h e d results