Abstract

Series of perovskite oxides with the composition LaMn_{1 − x}M_xO_{3 + δ} (M = Cr, Co; 0 ≤ x ≤ 1) have been synthesized by thermal treatment of precursors obtained by freeze-drying of acetic acid solutions. The oxides have been characterized by X-ray diffraction, and the Mn⁴⁺ content and, thus, the oxygen excess, δ, has been determined by redox back-titration. LaMnO_3.14 and LaCrO₃ phases have the rhombohedral-LaAlO₃ and the orthorhombic-GdFeO₃ structures, respectively. The LaMn_{1 − x}Cr_xO_{3 + δ} phases have the rhombohedral structure for x ≤ 0.3, and the orthorhombic structure for x ≥ 0.5. LaCoO₃ has, as LaMnO_3.14, the rhombohedral structure. However, the LaMn_{1 − x}Co_xO_{3 + δ} phases have orthorhombic structure for 0.05 ≤ x ≤ 0.55. The evolution of the crystal symmetry has been interpreted by considering the effect of the size of the substituting ion on the capability of the perovskite network to accommodate the oxygen excess (δ).