PREPARATION AND CHARACTERIZATION OF CaFeO3 AND ITS SOLID SOLUTION WITH SrFeO3

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PREPARATION AND CHARACTERIZATION OF CaFeO3 AND ITS SOLID SOLUTION WITH SrFeO3

Y. Takeda, S. Naka, M. Takano

To cite this version:

Y. Takeda, S. Naka, M. Takano. PREPARATION AND CHARACTERIZATION OF CaFeO3 AND

ITS SOLID SOLUTION WITH SrFeO3. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-331-

C2-332. �10.1051/jphyscol:19792117�. �jpa-00218487�

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JOURNAL DE PHYSIQUE Colloque C2, suppldment au n o 3, Tome 40, mars 1979, page C2-331

PREPARATION

AND

CHARACTERIZATION OF CaFe03

AND ITS

SOLID SOLUTION

WITH

SrFe03 Y. Takeda, S. Naka and M. ~akano*

Synthetic Crystal Research Laboratory, Faculty of Engineering, Nagoya University, A7agoya, Japan ' ~ e ~ a r t r n e n t of Chemistry, Faculty of S d a c e , Konm University, Kobe, Japan

Resume.- Une etude par effet Gssbauer du systlme Cal-xSrxFe03 (0 ,< X 4 0,75) montre une impor- tante dissociation m e 4 + + 17e3+ + ~e'+ en dessous de la temperature de transition ferromagnetique.

A la temperature ambiante, chaque oxyde a un spectre d'absorption 1BgSrement elargi; 1 4 ' ~ au con- traire, il apparart deux composantes magnetiques d'intensitgs pratiquement Bgales mais de champs internes et d6placements isomeriques differents. Les diffgrences s'attenuent lorsque la concentra- tion en Sn augmente.

Abstract.- A ~Gssbauer effect study on the Cal-xSrxFe03 (0

5

^X

5

0.75) system shows a charge dis- proportionation 2Fe" -c " ~ e ~ + " + "~e"" below the antiferromagnetic transition temperatures. At room temperature, each oxide shows a slightly broad syunnetric absorption, but the spectrum at 4 K consists of two kinds of magnetic patterns with nearly equal intensities but with different CS's and Hi's. Both the differences, or the difference between the electronic states of the two kinds of Fe ions, decrease with increasing Sr-content.

Some perovskite related structures containing alkaline earth and iron ions stabilize an unusual oxidation state of iron, ~e'*, SrFe03 /l/ and CaFe03 /2/ are known as stoichiometric oxides containing not Fe3+ but only ~ e " ions. Recently, we have re- ported a preparation (p02 > 20 kb, 1000'~) and a characterization of stoichiometric CaFe03 /3,4/.

Some of the results are summarized in Table I and are compared with the corresponding properties of SrFeO3. These two oxides make a sharp contrast in electrical conductivity and the MGssbauer spectra at low temperatures. The spectrum of CaFeO3 shows a single kind of absorption at 285 K, while, at 4 K, two kinds of magnetic hyperfine patterns I and I1 with nearly equal intensities but with greatly different parameter-values are observed. The spectrum is shown in figure 1.

On the other hand, SrFe03 shows a single magnetic pattern and the CS and Hi are close to the averaged values of the corresponding values of CaFe03 as in- dicated in Table I.

We have explained these results as follows taking the magnetic and electrical change of CaFeO3 at about 115 K into consideration. Below avout 115K, the d electrons in the narrow U' band become loca- lized disproportionately, i.e. 2 ~ e " + ~ e+ ~Fe5+. +

The sharp contrast between CaFeOs and SrFe03, especially in the Mzssbauer spectra at low temperatures, is very interesting and causes us to study the composition dependence on the solid solutions.

A series of samples corresponding to Ca1-,Srx Fe03 (0 2 X 2 0.75) have been prepared as follows.

Stoichiometric mixtures of Fe203, CaC03 and SrC03 were fired at 1250°C repeatedly and the oxygen defi- cient products were annealed under the same condi- tion as for CaFe03 (p02 > 20 kb. 1000°C). Powder X-ray diffraction for each oxide except for CaFe03 shows the same cubic perovskite-type pattern as of SrFe03. All these oxides are antiferrornagnets having T 'S between 115 K and 140 K. For any composition,

N

the %ssbauer spectrum at room temperature shows a slightly broad symmetric absorption with a CS of about 0.05 mm/s relative to Fe metal. However, at 4 K, a very interesting composition dependence is observed. Each spectrum can be divided into two kinds of magnetic hyperfine patterns I and 11 with Fig.] : Gssbauer spectrum of CaFeO3 at 4K. The dots

represent the experimental data and the solid lines nearly equal intensities as in the case of Cape&.

the two kind of magnetic patterns and thecr sum as In figure 2 the spectrum of Cao.s7sSro.125Fe03 is obtained by a computer fit. shown as an example.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792117

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JOURNAL D E PHYSIQUE

Table I : Comparison of CaFeOs and SrFe03

The Fe*+ ions in SrFeOn are in the "average"

state due to a rapid electron exchange. By replacing sr2+ by ca2+, however, the exchange is interrupted and a disproportionate localization results. And the electronic states are not determined locally by the numbers of ca2+ and sr2+ ions on the neighboring di- valent cation sites but uniformly by the total composition in a crystal. These experimental results seem to indicate the band-like nature of the electronic states.

SrFe03

CaFe03

Fig. 2 : ~Essbauer spectrum of Ca0.375Sr0.62~~e03 at 4K.

Ref erences Crys. structure

Cubic perovskite

Slightly distorted perovskite

Changes in parameter-values determined by

assuming two kinds of magnetic patterns having Lore= / I / Gallagher, P . K . , MacChesney, J.B. and ~uchanan, D.N.E., J. Chem. Phys.

41

(1964) 2429.

tzian line shape are shown in figure 3. The diffe-

/2/ Kanamaru, F., Miyamoto, H., Mimura, Y., Koizumi, rences between the parameters for the two kinds of M., Shimada, S., Kume, S., and Shin, S., Mat.

Fe ions decrease almost linearly with increasing Res. Bull.

5

(1970) 257.

I

I . . . . . . . . .

0 0.5 l1. 0

m / s

,

I

/3/ Takeda, Y., Naka, S., Takano, M., Shinjo, T., Takada, T., and Shinada, M., ibid. U ⁽¹⁹⁷⁸⁾

61.

electrical conductivity

metallic metallic metallic semiconductive Mag. structure

Mag.moment

I 1

/ 4 / Takano, M., Nakanishi, N., Takeda, Y., Naka, S.,

and Takada, T., ibid.

11

(1977) 923.

134K

'lSK

I + I I

T

^{I 1}

Fig. 3 : Composition dependence of the two kinds of Hi- and CS- values (I and 11) and the average values

I+II

) of Gal-,Sr,FeO,.

temp.

. - - RT 4K RT 4K screw -

3.1 uB of 4K under inves

-

tigation

CS(mm/s )

0.054 0.146 0.056

I + I I

2

1

Hi (T)

- - .- - - .

33.1

-

0.32 0.00 0.16 41.9 28.4' 35.2