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CHARGE DISPROPORTIONATION IN Fe4+-OXIDES WITH PEROVSKITE-TYPE STRUCTURES
M. Takano, N. Nakanishi, Y. Takeda, S. Naka
To cite this version:
M. Takano, N. Nakanishi, Y. Takeda, S. Naka. CHARGE DISPROPORTIONATION IN Fe4+-
OXIDES WITH PEROVSKITE-TYPE STRUCTURES. Journal de Physique Colloques, 1979, 40
(C2), pp.C2-313-C2-315. �10.1051/jphyscol:19792110�. �jpa-00218480�
JOURNAL DE PHYSIQUE Colloque C2, supplPment au no 3, Tome 40, mars 1979, page C2-313
CHARGE DISPROPORTIONATION IN
F ~ ~ + - O X I D E SWITH
PEROVSKITE-TYPE S T R U C T U R E S M. Takano, N. Nakanishi, Y. ~akeda* and S. ~ a k a *Department o f Chemistry, Faculty o f Science, Konan University, Kobe, Japan
W Synthetic Crystal Research Laboratory, Faculty o f Engineering, Nagoya University, Nagoya, Japan
Rdsum6.- On a dtudib deux series de solutions solides de structure perovskite Cal-xSrxFeOa et
Srl- LaxFe03 pour tester notre modele de distribution de charges dans les oxydes Fe de type ~ e ~ + (2Fei(+ + Fe3+
+
FeS'). Le spectre MEssbauer de Sro ,sL.ao ,sFeO3 1 4 K confirme clairement ce modSle.Pour les deux sdries, le modele semble correct 2 la tempgrature TN.
Abstract.- For a further examination and elaboration of our simple charge disproportionation model for Fe4+-oxides, 2 ~ e " + Fe3+ + Fe5+, two series of solid solutions Cal-xSrxFe03 and Srl-xLaxFe03 with the perovskite structure have been studied. The Msssbauer spectrum of Sro.sLao.sFeO3 at 4 K clearly indicates the disproportionation. For both series of oxides, the disproportionation seems to set in at the TN1s.
I. Introduction.- As shown in our recent papers /1,2/, we have studied Fe4+-oxides with the perov- skite structure and proposed a simple and challen- ging model of a charge disproportionation, 2Fe4+ -+
Fe3+ + Fe5+. The sharp contrast between the ~ 5 ~ s - bauer spectra of CaFeOs /I/ and SrFe03 /3/ at 4 K has been regarded as indicating the coexistence of Fe3+ and Fe5+ ions in the former oxide and the coa- lescence into Fe4+ ions in the latter /2/.
This model is challenging particularly in the sense that Fe5+ is quite a rare valence state detected only as an impurity of ppm order in SrTiO3 by ESR /4/. And this is a simple model because a strong covalency may make it difficult to distin-
model to this new system also.
Sro.sLao.sFeO3 has been studied so far. Sample preparation will be described elsewhere. Magnetic and electrical measurements show that this oxide is a semiconductive weak-ferromagnet with a T
N Of
230 K. In the paramagnetic region a temperature- independent quadrupole splitting of 0.23 mm/s attri- buted to a rhombohedral crystalline distortion from cubic symnetry /6/ was observed. The Fe ions are in an average state, and the CS at 285 K is 0.20 mm/s relative to Fe metal.
The complex magnetic spectrum at 4 K is shown in Figure 1.
guish the electronic states of the Fe ions in terms of nominal valency.
For a further examination and elaboration
of our model two series of solid solutions Gal-,SrX Ir K
Fe03 and Srl-xLaxFe03 with the perovskite structure have been studied. The results on the first series of oxides are described by Takeda et al. in a com- panion paper.
2. Charge Disproportionation in Sro.sLao.sFe03.- Mbssbauer measurements on Sri-,LaXFeO3 (= (I-X) srFe4+03. x ~ a ~ e 3+03) were reported by some investi- gators /5,6/. They observed an average oxidation state due to a rapid electron exchange for X = 0.4
% 0.8 at room temperature. The magnetic hyperfine spectraat 78 K were, on the other hand, interpreted to be resolved into two components, each assigned to the individual oxidation states, Fe3& and "Fe4+"
/6/. However, we have repeated Gssbauer measure- ments and analysis expecting an application of our
4 : : : : : : : : : : : t
-12 -8 -4 0 4 8 12
VELOCITY (M/$ )
Fig. I : The Gssbauer spectrum and the computed fit of Sro. sLao. sFeO3 at 4 K.
It is readily seen to consist of three kinds of magnetic hyperfine patterns, each showing some dis- tribution in the Mijssbauer parameters. However, the average values useful enough for our purpose were obtained by fitting broad Lorentzian lines. The CS, Hi and the relative intensity for each component are I : 0.43 mm/s, 50.7 T, 72
X;
I1 : 0.12 mm/s, 29.9 T, 14X;
and 111 : 0.02 =/S, 26.3 T, 14X.
It is characteristic that both the CS and Hi for
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792110
C2-3 14 JOURNAL DE PHYSIQUE component I are greatly different from those for the
other components. Moreover, the difference in inten- sity is also remarkable.
We explain these results by assuming again the charge disproportionation, neglecting the slight difference between components I1 and 111; component I comes from ~ e and the other two from " ~ e ~ + " . ~ + In comparison with CaFeOs, both the CS's for Fe3+ and Fe5+ in Sro.sLao.sFeO3 are more positive and the Hi's are larger. For a further discussion, however, measurements over a wide composition range seem to be required, for the published measurements 161
suggest these parameters being considerably compo- sition-dependent. The intensity ratio I : (I1 + 111) expected from our model is 3:1, which approximates the experimental value fo 72:28. Gallagher and Mac Chesney's assignment to Fe3+ and "Fe4+" has such difficulties as discussed in reference 121. For example, it is expected that the resolved magnetic spectrum indicates equal numbers of two kinds of Fe-species, which is not observed.
3. Charge Disproportionation Temperature.- We have observed that charge-disproportionate double Fe- states are rather common for perovskite ~e'+-oxides.
So, as a next problem, it is very interesting to determine at which temperature the disproportiona- tion sets in. As for CaFeO3, though the temperature- dependent spectra indicated very clearly the TN to be 116 K, the determination of the disproportion
temperature was not straightforward. As shown in Figure 2(A), a pair of peaks were observed in the paramagnetic region, and the peak positions (Fig.2 (B)) extrapolated to lower temperatures are compa- rable with the CS's at 4 K for the two kinds of Fe ions. On the other hand, an X-ray diffraction mea- surement at 293 K shows weak extra peaks due to a slight crystalline distortion from cubic spmetry.
Thus, the Mossbauer masurement on CaFeO3 only did not decide conclusively whether the double peaks are due to a temperature-dependent quadrupole inte- raction or the disproportionation proceeding gra- dually with decreasing temperature.
Then, a similar measurement was made on Ca0.875Sr0.125Fe03. Such a slight substitution of sr2+ for ca2+ modifies little the CS's and Hi's at 4 K as shown in our companion paper, while the crystalline distortion is reduced so that the extra diffraction peaks vanish. The data are also shown in Figure 2(B) for comparison. The saturated split- ting for this composition, say at 150 K, is about two third that of CaEe03 and this seems to indicate
a quadrupole interaction to be the origin.
Moreover, Cao.5Sro,5Fe03, whose crystalline symmetry is closer to cubic symmetry, and also Sro.sLao.sFeO3 showing clearly the disporportionation do not show any remarkable temperature-dependent increase in li- ne width or splitting.
Fig. 2 : (A) Mijssbauer spectra and computed fits of paramagnetic CaFe03. (B) Peak positions (open and closed circles) and the average positions (trian- gles) for CaFe03 and peak positions for Cao,875 Sro.12~Fe03 (half-closed circles).
On the other hand, for any oxide studied, the magnetic spectrum shows greater line widths as tem- perature approaches the TN indicating probably a more rapid electron exchange 121. Thus, we conclude at the present stage, that the Fe ions are divided into two at the T
N.
Acknowledgements.- The authors are indebted to Professor T. Takada and Professor T. Shinjo for providing them with facilities for MEssbauer measu- rement and to Professor M. Shimada for treating some of the samples under high oxygen pressures.
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