Structural effects of Co and Cr substitution in LaMnO3+delta

Series of perovskite oxides with the composition LaMn1 - xMxO3 + delta (M = Cr, Co; 0 less than or equal to x less than or equal to 1) have been synth esized by thermal treatment of precursors obtained by freeze-drying of acet ic acid solutions. The oxides have been characterized by X-ray diffraction, and the Mn4+ content and, thus, the oxygen excess, delta, has been determi ned by redox back-titration. LaMnO3.14 and LaCrO3 phases have the rhombohed ral-LaAlO3 and the orthorhombic-GdFeO3 structures, respectively. The LaMn1 - xCrxO3 + delta phases have the rhombohedral structure for x less than or equal to 0.3, and the orthorhombic structure for x greater than or equal to 0.5. LaCoO3 has, as LaMnO3.14, the rhombohedral structure. However, the La Mn1 - xCoxO3 + delta phases have orthorhombic structure for 0.05 less than or equal to x less than or equal to 0.55. The evolution of the crystal symm etry has been interpreted by considering the effect of the size of the subs tituting ion on the capability of the perovskite network to accommodate the oxygen excess (delta).