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MÖSSBAUER STUDIES OF Fe3-xMgxO4 : APPEARANCE OF VERWEY TRANSITION
J. Dormann, T. Merceron, P. Renaudin, V. Brabers
To cite this version:
J. Dormann, T. Merceron, P. Renaudin, V. Brabers. MÖSSBAUER STUDIES OF Fe3-xMgxO4 :
APPEARANCE OF VERWEY TRANSITION. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-
177-C1-178. �10.1051/jphyscol:1980150�. �jpa-00219719�
JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n o 1, Tome 41, janvier 1980, page C1-177
@SBAUER STUDIES OF Fe3,&gq : APPEARANCE OF TRANSITION J.L. Dormann, T. Merceron, P. Renaudin and V.A.M, Brabers
+
Laboratoire de MagnQtisme, C.N.R.S., 1, Place A? Briand, 92190 Meudon-BeZZevue, France.
+ Eindhoven University of TechnoZogy, Eindhouen, The NetherZands.
Introduction
-
A t the Verwey temperature (Tv = 12010 magnetite e x h i b i t s distorsion from cubic t o orthor- hombic ( o r monoclinic) symmetry. Above Tv, magne- t i t e i s a conductor. This was f i r s t explained a s due t o a rapid electron hopping between the Fe2+and the ~ e ions a t the B-site, the c r y s t a l sym- ~ + metry distorsion preventing the e l e c t r o n i c exchange below Tv. Then, two models were suggested : pair- wise localized hopping / I / and band description of the i t i n e r a n t electron /2/. On the other hand, below Tv, t46ssbauer spectra of magnetite i s compli- cated and one ( o r two) s i t e s with typical parame- t e r s appear /3/. This sexted has been a t t r i b u t e d t o a fr'action of the Fe2+ (B) ions which orders below Tv due t o the crystal phase t r a n s i t i o n . In order t o elucidate these problems, substituted magnetites have been extensively studied : non magnetic impurities in A s i t e (Zn, Cd) /4/, magnetic impu- r i t i e s (Ni, Co) o r vacancies i n B s i t e /1, 5, 6/.
No pertinent answer was given so f a r f o r impurities i n B s i t e , t h u s substituted magnetite with non magnetic impurities i n B s i t e was studied.
Experiment
-
The general composition was : Fe3,xMgx04 f o r 0 < x 6 0.4The sample preparation (sing1 e crystal except f o r x = 0.3) has been described elsewhere /7/ : Miissbauer spectra (4,2 t o 300 K) were made using a conventional spectrometer operated in the cons- t a n t acceleration mode,
Results and discussion
-
A t room temperature, three main features caracterized the spectra : ( I )Table 11'- 6 value: (with r e i p e c t t o metallic iron) H values ( f o r x = 0.05 and 0.4) and mean charge f o r B s i t e s a t 300 K.
The inte,nsity of the Fe3+ s i t e increases s i g n i f i - cantly with x ( t a b l e 1) ; (11) t o f i t c o r r e c t l y t h e spectra one Fe3+ s i t e and three (x = 0.05) t o four B s i t e s have t o be taken i n t o consideration, (111) the isomer s h i f t 6 i s independent of x, a l l t h e B s i t e s (except B4) have the same value of 6 (6 (B4) i s s l i g h t l y lower). ( t a b l e 2 ) . The i n t e r p r e t a t i o n of (1) i s d i f f i c u l t , i t i s not possible t o separate exactly ~ e ( A ) ~ from Fe3+(B), + t h e i r hyperfine parameters a r e very close together.
Furthermore the zecoi l e s s f r a c t i o n f A could d i f f e r from f B . Recent data from Hlggstrom
et
/8/ haveshown t h a t f A / f B = 1.25 in the range 120-500 K. The amount of Mg i n A s i t e i s not known exactly. Never- t h e l e s s , the following statement can be made ; s t a r 7 t i n g from the temperature dependence of 6 , t h e 300 K values of 6 ( ~ e ~ + (B)) = 0.38 2 0.03 mm/s and 6 (Fe2+(6)) = 0.96 2 0.04 mm/s with respect t o the metal1 i c iron are deduced. Then, i f i t i s supposed t h a t the mean charge of each B s i t e i s proportional t o 6 , we can recalculate t h e r a t i o f A / f B and t h e a w u n t of Fe3+(B), (Table 1 ) . I t was found f A / f B = 1.20
'
0.03 whatever i s x and f o r 77 < T < 300 K.The r e s u l t s do not depend upon the inversion f a c t o r O.10 OS2O 0'30 Oa40 of Mg which was not consequently measured. I t was 0.39 0.385 0.415 0.43 0.487 0.505 estimated t o 7/8. This good agreement within a l l Fe +(B) 0.085 0. 14 0.24 0.28 compositions and temperatures j u s t i f i e s t h i s method
of calculation. From the hyperfine f i e l d values ( H ) , Table I
-
Relative i n t e n s i t y of Fe3+ s i t e , occupa- i t i s easy t o r e a l i z e t h a t B1 and B2 s i t e s corres- tion r a t i o of ~ e (R) ~ assuming 7/8 + B s i t e pond t o the two s i t e s of magneti t e /8/. Furthermore, a t 300 K.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980150
C1-178 JOURNAL DE PHYSIQUE
the B4 s i t e has the highest H values and a mean value of the charge greater than 2.5'. This s i t e could r e s u l t from a r i c h Mg 2+ region, rich in Fe 3+
too b u t the amount of ~ e i s lower than ~ + M~'+, thus the mean charge could be greater than 2.5' and averaged spin values higher.
The temperature variation ( f i g . 1) shows t h a t above a given temperature Te (= 220 K, f o r x = 0.2) an extra s i t e appears with a 6 value corresponding t o a mean charge s l i g h t l y lower than 3'. The 6
values of the B s i t e s , corrected from the second
Fig. 1 Mtssbauer spectra f o r x = 0.2 f o r d i f f e r e n t temperatures.
order Doppler e f f e c t correspond t o mean charge di- verging from 2.5'. Around 120 K, the system i s s t a - bilized and f o r x = 0.2 the following d i s t r i b u t i o n of cations on B s i t e s is deduced :
(estimated value) which corresponds t o a total charge 7.96. A t a temperature Tp, a new modificatior occurs and the spectra i s similar t o pure magnetite, p a r t i c u l a r l y the F ~ ~ + ( B ) s i t e appear with typical parameters. I t i s possible t o define a temperature which corresponds t o the crystallographic phase transformation T and a temperature Te corresponding t o the s t a r t i n g point of modification of exchange. P For pure magnetite T Te = 120 K. The as deduced values T a r e 105 2 P 5 K (x = 0.05), 75 2 5 K (x = 0.1) 65
*
P 5 K ( x = 0 . 2 ) , 55'
5 K (x = 0.3),20
'
10 K (x = 0.4) a r e i n good agreement with ma- gnetic data /7,9/. The temperature Te increases w i t h x, but i s more and more smooth.-for x = 0.4, Te may be greater than 300 K because 6 (B4) < 0.67 (mean charge > 2.5+). Furthermore the existence of a small amount of blg2+ in A s i t e complicates the exchange process, in the case of a impurity i n A s i - t e , 6 increases w i t h x, and no F ~ ~ + ( B ) i s observed /4/.In conclusion we have shown t h a t i n Mg substi-, tuted magnetite, the r e s u l t s are more consistent assuming a localized hopping model and up t o x = 0.4 a crystal 1 ographic modification occurs a t a tempera- t u r e T
P '
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