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DISTRIBUTION OF IRON ATOMS IN THE SPINEL
FeInxCr2-xS4(0
≤ x ≤ 2)
L. Brossard, L. Goldstein, M. Guittard
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dkcembre 1976, page C6-493
DISTRIBUTION
OF IRON
ATOMS
IN THE SPINEL
FeInxCrz
-,
S4(0
<
x
<
2)
L. BROSSARD, L. GOLDSTEIN, M. GUITTARD Laboratoire de Magnetisme, C. N. R. S., Meudon-Bellevue, France
RBsumB. - Les spectres paramagnktiques Mossbauer du 57Fe dans FeInsCrz-zS4 & T = 300 K ont kt6 correctement interprktks suivant la distribution ionique (~e??~In:') ( ~ e p ~ n ~ ' , ~ r z ~ ) S4.
Le Fezf en site A s'est revele une sonde sensible a son environnement de cations et plus parti- culihement i l'indium en raison de la taille de ce dernier. Les variations de la scission quadru- polaire du Fez+ en site B ont 6t6 correlees avec le parambtre << u D caracterisant la distorsion de
I'octabdre d'anions induite par l'indium en site A. Le signe positif de Vzz a T = 4,2 K conduit un doublet fondamental du Fez+ en site B contrairement aux oxydes (singulet fondamental). L'absence de Fez+ en site B est confirm& dans Fe1+~Crz-~S4 et dans FeCrzS4 non stoechiom6trique a la temperature ambiante.
Abstract.
-
Mossbauer paramagnetic spectra of 57Fe in FelnzCrz-zS4 at T = 300 K have been correctly interpreted following the ionic formula (~e?-f,~n:+) (~e$+~n:T~crj?-iz) S4. A site Fez+ is very sensitive to his cationic environment and particularly to the size effect of In. The variations of quadrupole splitting of B site Fez+ have been related with the (( u u) parameter ofanionic octaedron distorted by indium in A site. The positive sign of Vzz at T = 4.2 K suggest a doublet as ground state of B site Fez+, in opposition with oxide's conclusions (singlet as ground state). The absence of room temperature B site Fez+ is confirmed in F ~ I + ~ C ~ Z - , S ~ and in non stoichiometric FeCr 2S4.
1. Introduction.
-
Indium thiospinels MInzS, (M = Fe, Co, Ni)have been reported to be inverse and paramagnetic for T > 4.2 K [I]. Goodenough [2] attributed the anomalous and negative Weiss constants 8 to the delocalisation of the e, electrons due to strong cova- lency. Eibschiitz et al. [3] studied FeInzS4 by Moss- bauer effect down to 77 K and attributed the large quadrupole splitting to the trigonal component of the crystal field on B site ; he assumed the ground state to be a singlet. On the other hand, FeCr2S4 [4] is a ferri- magnet (T, : 177 K) and normal spinel.
We studied the solid solution FeInxCrz-,S4 a t room tem~erature bv Mossbauer effect in order t o
of the intensities of the (400, 422, 444, 333, 531, 800) peaks shows that the rate of population of iron from A to B site increases continuously with x. Furthermore the anion shift parameter ct u )) varies slowly ( x = 0,
u = 0.385 ; x = 1.2, u = 0.388). The Mossbauer spectra were taken with a conventional spectrometer. The absorber powder (10 mg ~e/cm') was pressed bet- ween two mylar foils without any glue to avoid any strain in A site which is very sensitive to them.
The spectra were fitted using a least squares routine based on convolution of lorentzian lines [7]. The mean line-width of Mossbauer peaks varies from 0.27 to 0.40 mm/s (Table I).
describe the evolution of the inversion of Fez+ ions,
Variation with x at T = 300 K of mean linewidth which are distributed among A tetrahedral and B
re,,
inversion parameter y, quadrupole interaction
octahedral sites. Moreover it was suggested the pre-
I
E
IB
and isomer shifta6
of Fez + in B site.sence of Fez+ in B site of the system Fe,+xCr,-,S4 [ 5 ]
and in non stoichiometric FeCrzS4 compounds [6]. x 0 0.1 0.2 0.4 0.8 1.0 1.2 1.6 2.0
Solid solution FeInxCrz-,S4 provides a good tool to define the hyperfine characteristics of Fez+ in B site even at low x.
2. Experimental.
-
Samples of FeInxCrz-,S4were prepared by heat treatment under HzS atmos- phere of mixed oxides issued from cod&composition of nitrates. The <<
a
)) parameter of the spinel determin-ed by room temperature X-rays analysis shows that the compounds follow the Vegard's law ( a = 9.998
A
for x = 0 ; a = 10.610A
for x = 2.0). The analysisTex 0.27 0.35 0.40 0.35 0.33 mm/s y 0 0 0.04 0.08 0.38 x-y 0 0.1 0.16 0.32 0.42 1 E
I B
1.50 1.50 1.56 mmls BB 0.77 0.78 0.80 mm/s(Fel
-,
In,), (Fey Inx-, Crz-6/Fe metal S, Tamb
C6-494 L. BROSSARD, L. GOLDSTEIN, M. GUITTARD x.0
---
fl--
N s = 198 loL ..
. . Pmax ,19.2*/..
. rex = o 27 rnm/s.
. , ' . s'..,. . . . Pmax, 4.5% . . . , . . . rex, 0.33rnm/s % . ..
. . n ,.
, , !. . , . X=l.O '", ,; , .. .;...:c- ': ; . I , . . . '. "' ,I.. ( ,, ..' , : N r r , 1 9 7 1 0 ~ , , ,:; ,, :. . . .
,: .1_ . . ....
Prnax, 4.5% 9 , , ., . #C' i i .I rex, 0.30mm/~ ..
x, 1.2 , I ' ,,( ..I... ...*' :,,..
* N w ,197 loL '..,.
, . " ..--I, ,,:: ' \, 3.- , , , , . , . Pmax, 4.8%FIG. 1. - Experimental and calculated room temperature
Mossbauer spectra of FeInzCrz-zS4 for different values of x ;
zero velocity is the barycenter of iron metal spectrum.
Figure 1 shows the room temperature paramagnetic Mossbauer spectra for different values of x :
- for x = 0, Fe2+ in A site is characterised by a singlet (8, = 0, 6, = 0.58 mmls) ;
- for x = 2, ~ e in ~ B site gives a doublet +
(eB = 1.62 and 6, = 0.855 mm/s) in agreement with previous measurements [3] ;
- in the intermediate ranges, Fez+ exists simultane- ously in A and B site..~and. presents the following features.
- Fez+ in B site.
-
The corresponding doublet does not appear for.x
= 0,05 and x = 0.1. Inversionbegins to exist for x
-
0.2. The linewidth is quite small and does not vary with x and is not sensitiv to the different cation distribution among A and B site.The concentration y of Fe2+ in B site has been deter- mined in a first step by the minimisation of mean quadratic error, without any assumption concerning A site distribution cf. figure 2.
FIG. 2. - Variation with x at T = 300 K of 6 ~ ,
1
EIB,
y and( x - Y ) .
-
Fe2+ in A site. - The central part of the spectra due to Fe2+ in A site is the superposition of several contributions corresponding to different environ- ments. We assume the following ionic distributionwhere x and y are respectively the substitution and inversion rate.
Fe2+ in A has 12 B first neighbours. So the proba- bility P(p, n ; x, y ) of finding
p iron
(n
-
p) indium and (12-
n) chronium ions as nearest B neighbours of an A site isIn order to reproduce the experimental spectra with a minimum number of 'doublets, the different probabi- lities P have been summed following two kinds of configurations : the first one gives O,1,2,
....
N(Fe) and12; 11,
...
(In+
Cr) in B ; the second one considers...
....
DISTRIBUTION OF IRON ATOMS I N THE SPINEL C6-495
For all x, the best fitting of the spectra is obtained with the second assumption. Next nearest neighbours environment has been neglected. So, at least at room temperature, in the paramagnetic region, Fez + in
A site is more influenced by its B In3+ nearest neigh- bours than by B Fez+.
3. Discussion of results. - Concerning A site
Fez+, it is pointed out that the isomer shift 6, corres- ponding to each configuration (0, 1 , 2 , 3 and 4 In on B site) increases by 0.01 mm/s per indium near neigh- bour : the presence of In3+ on B site reduces the num- ber of superexchange interactions Fe2+(A)-S-M(B)
(M = Cr3+ or Fez +). This reduction of covalency effects and of the delocalisation tendency of the 6th 3d electron contributes to an increase of 6,. Furthermore, for each configuration, 6, increases with x as a result of increase of the unit cell parameter leading to a decrease of s electron charge density at iron nucleus. Moreover, the quadrupole splitting E, attributed to each configuration of In distribution increases by 0.17 mm/s per Indium : the splitting of the 'E, ground level of Fez+ in A is due to statistical distribution of In, Fe and Cr ions among B sites. According to the nume- rical analysis of spectra it is emphasized that the size difference between In3+ and Cr3+ is more efficient than the charge difference between Fez+ and Cr3+.
-
For the B site, figure 2 shows the variation with xof
a,,
I
EIB
and also of y and ( x - y) :the increase of isomer shift 6, with x is more pronounc'ed than in A site case and results of the same explanations (increasing of the unit cell and reduction of covalency) ;
the variation of the quadrupole splitting E, requir- ed detailed analysis.
-
It is well known that the trigonal component V, split the ground state 'T,, level of B site Fez+ in a singlet and a doublet ; V , is the sum of two contribu- tions, a positive one'which comes from B nearest neighbours and a negative one which results from thedistorsion of the anion octahedron (u # 318). In oxides [8] the crystal field due to the first cation neigh- bours predominates, the singlet being the ground state and in such a case, the principal component V,, of the electric field gradient is positive as in GeFez04 191,
where u = 318. In the present situation, the increase of the cc u D parameter with x leads to an increase of
( E
IB
and thus of V,,. It is believed that the importantdeformation of the sulfur octahedron gives rise to V,, along
<
11 1>
; furthermore, the effect of cations is lower than in oxides, the sulfur atoms pushing them faraway. Moreover, preliminary Miissbauer spectrum for x = 2 at 4.2 K where the compound exhibits short range magnetic order suggest a positive Vzz, resulting a doublet ground state. It should be emphasized that the discontinuity of1
E1,
between x = 0.6 and x = 1.0 is closely related to the respective variations of Fe concentration y and In concentration ( x-
y) in Bsites ; the same discontinuity is observed in the varia- tion of the anion shift parameter cc u >> with x and is explained by the fact in this range of composition, In is going into A site only.
In conclusion, the solid solution FeIn,Cr,-,S4 provides a good tool to study Fez+ in both A and B
sites. In particular, for low values of x , the hyperfine characteristics of B dilute ~ e " in FeCrzS4 matrix are
I
E-
1.50 mm/s6
-
0.76 mm/s towards iron metal.
The absence of such a doublet in the room temperature Mossbauer spectra of the systemFe, +,Crz-,S, 0
<
x<
0.5 [51and of non stoichiometric FeCrzS4 compounds [6] led
us to the conclusion that Fez+ does not exist in the B
site of these compounds contrarily to previous assump- tions. Further results concerning magnetic measure- ments and their interpretation with respective occupa- tion of B sites by Fez
+,
In3' and Cr3 + will be publishedelsewhere [lo].
References
[I] SCHLEIN, W. and WOLD, A. J., Solid State Chem. 4 (1972) 286.
[2] GOODENOUGH, 3. A., J. Solid State Chem. 4 (1972) 292. [3] EIBSCHUTZ, M., HERMON, E. and SHTRIKMAN, S., Solid
State Commun. 5 (1967) 529.
[4] GOLDSTEIN, L., GIBARD, P., C. R. Hebd. Sean. Acad. Sci.
269 (1969) 471.
[5] ROBBINS, M., WOLFF, R., KUNTZING, A. J., SHER- WOOD, R. C. and MIKSORSKY, M. A., J. Appl. Phys. 41 (1970) 1086.
[6] LOTGISRING, F. K., VAN DIEPEN, A. M. and OLIGHOEK, J. F.,
Solid State Commun. 17 (1975) 1149.
GIBART, P., GOLDSTEIN, L. and BROSSARD, L., J. Magn and Magiz. Materials 3 (1976) 109.
[7] DORMANN, J. L., BROSSARD, L., FATSEAS, G. A., Intern. rapport du laboratoire de MagnCtisme (1972) available in request.
[8] SLONCZEWSKI, J. C., J. Appl. Phys. 32 (1961) 253 S. 191 EIBSCHUTZ, M., GANIEL, U. and SHTRIKMAN, S., Phys.
Rev. 151 (1966) 245.
HARTMANN-BOUTRON, F. and IMBERT, P., J. Appl. Phys.
39 (1968) 775.
