On the cubic perovskites La 0.2Sr 0.8Cu 0.4M0.6O 3− y ( M = Co, Fe)

Abstract

A new oxygen-deficient perovskite La 0.2Sr 0.8Cu 0.4 M 0.6O 3− y ( M = Co, Fe) has been synthesized that exhibits a large concentration of oxygen vacancies (0.3 ≤ y ≤ 0.58). Despite this, X-ray powder diffraction shows no superstructure of the primitive cell, indicating a random distribution of the vacancies on the oxygen sites. This result has been confirmed by electron diffraction and high-resolution microscopy, which show that most of the crystals exhibit the regular perovskite cell. In the cobalt phase, X-ray absorption spectroscopy at Co K-, Cu K- and Cu L 3-edges has shown that cobalt is in the Co(III) valence state even after oxygen annealings and that copper is in a mixed valence state, Cu(II)/Cu(III). In contrast, in the iron phase, which is less oxygen deficient than the cobalt phase, Mössbauer spectroscopy has shown that iron exhibits a mixed valence state, Fe(III)/Fe(IV). The conductivity of this new phase exhibits a transition from a semiconducting to a semimetallic behavior; it increases after oxygen annealings, in agreement with the increase in the hole density, i.e., the Cu(III) content.