Electronic state of Fe4+ ions in perovskite-type oxides

Our Moessbauer effect studies on the electronic state of Fe4+ ions in perovskite-type oxides, Casub(1-x)Srsub(x)FeO3(0<=x<=1), Srsub(1-y)Lasub(y)FeO3(0<=y<=0.6), and some others, are summarized and discussed. The Fe4+ ions take a high spin state, tsub(2g)3esub(g)1, and the esub(g) electrons are accommodated in a narrow σ* band. The quarter-filled σ* band state is stable down to the lowest t emperature in SrFeO3 However, when the Sr-content is enough decreased, the esub(g) electrons get localized at low temperatures in a special way to form a mixed valence state, that is, a charge disproportionation of the Fe4+ ions into Fe3+(tsub(2g)3esub(g)2) and Fe5+(tsub(2g)3) ions which can be seen typically in CaFeO3 and Srsub(0.7)Lasub(0.3)FeO3. In the intermediate composition ranges of 0<x<1 and 0<y<0.3 quite unusual intermediate states are realized and, hence, the chemical formulas of these oxides at low temperatures can be expressed conveniently as Casub(1-x)Srsub(x)Fesub(0.5) sup((3+Δ)+)Fesub(0.5) sup((5-Δ)+)O3 and Srsub(1-y)Lasub(y)Fesub((1+y)/2)sup((3+delta)+)Fesub((1-y)/2)sup((5-delta)+)O3. (author)