Topotactic transition between perovskite and brownmillerite phases for epitaxial LaCoO3−δ films and effects thus resulted

Abstract

Oxygen vacancy distribution has a direct effect on the crystal structure and physical properties of complex oxides, resulting in versatile applications. Here, we report on a reversible topotactic phase transition between the perovskite and brownmillerite structures for the LaCoO3−δ (δ = 0–0.5) epitaxial film by annealing the sample under different conditions. In the atmosphere of 2 × 10−4 Pa, LaCoO3 film is transformed from the perovskite structure to the brownmillerite structure when annealing temperature exceeds 500 °C. Meanwhile, the magnetic order transits from ferromagnetic to anti-ferromagnetic. Variable-range hopping demonstrates the electronic transport process for both phases. The incorporation of oxygen vacancies results in an upward shift of the lnρ-T−1/4 curve, without affecting the lnρ-T−1/4 slope. We found signatures for preferential distribution for oxygen vacancies; the latter prefer to appear near high spin Co3+ ions in the initial stage when they are introduced into the lattice, resulting in abnormal magnetic and transport behaviors.