Electronic Properties of Mixed-Valence Manganates: The Role of Mn Substitutional Defects

Abstract

Single-phase perovskites in the solid solution series La0.7+yA0.3-yMn1-xMxO3 (with 0.00 ≤ x ≤ 0.10; A = Sr2+, M = Cu2+, Zn2+, Sc3+, Cr3+, Co3+, and Ga3+; A = Ba2+, M = Cu2+, Zn2+, and Sc3+) have been prepared via the acetic acid solutions freeze-drying method. This soft procedure makes possible strict stoichiometric control, and the synthetic variables allow one to maintain a constant proportion of Mn4+ (ca. 32%) in the 47 compounds prepared. In this way, the concentration of cationic vacancies at A and B sites is practically negligible in all cases. X-ray powder diffraction patterns corresponding to the 47 compounds have been completely indexed with rhombohedral perovskite cells. The crystal structures have been refined in space group R̄3c, in the hexagonal setting, from room-temperature data. The variation with x of a set of structural parameters (rhombohedral cell volume, rhombohedral cell angle, and B−O and A−O bond lengths) has been considered as a function of the mean sizes of cations at A and B sites. Manganates in these series present colossal magnetoresistance. The values of the critical temperature, Tc, for the paramagnetic−ferromagnetic transition in La0.7+yA0.3-yMn1-xMxO3 exhibit three different patterns, which clearly appear related to the mean size of cations at B sites. This fact has been interpreted by considering the variation of the electronic contribution to Tc with the structural disorder introduced by the presence of cations with different sizes at B sites.