Properties of Undoped and Ca-Doped LaMnO3 − Non-Stoichiometry and Defect Structure

Oxygen-excessive LaMnO3+y samples with 0.02 ⩽ y ⩽ 0.26 have been prepared by thermal decomposition in air at 450 °C of a precursor obtained from an aqueous solution of lanthanum nitrate and manganese acetate (atomic ratio La/Mn = 1). The obtained solid is annealed at different temperatures from 700 to 1000 °C and under three oxygen pressures: 1.00, 0.21, and 10−4 atm. Samples with 0.14 ⩽ y ⩽ 0.26 are simple-phased rhombohedral perovskites, the distortion of which decreases as the oxygen content (and Mn4+ content) increases. Samples with 0.02 ⩽ y ⩽ 0.09, obtained under low oxygen pressure (10−4 atm), have orthorhombic symmetry. In this case, the order of lattice parameters c/√2 < a < b, characteristic of O′-perovskites, is a consequence of the Jahn−Teller distortion of the oxide octahedron around the d4 Mn4+ cation. The volume (Vf) per LaMnO3+y formula unit decreases regularly with increasing oxygen content (i.e., oxidation). Study has shown that the oxygen content is best described in terms of La and Mn vacancies, rather than of interstitial anions. Density measurements confirm the cation vacancy model involving equal amounts in La and Mn sub-lattices. The infrared spectra of LaMnO3+y samples have been interpreted in connection with the structures of these solids. Correlations have been established between the spectral evolution and the crystal distortion of these compounds. La1−xCaxMnO3±y oxides with 0 < x ⩽ 1 have been synthesised by the same route, at 700 °C and under an oxygen pressure of 0.21 atm. It is shown that a negative correlation exists between the calcium content x and the degree of non-stoichiometry: at low Ca content, the oxygen content is higher. The ability to form over-stoichiometric compounds in air decreases with increasing Ca content and mostly disappears at x ⩾ 0.7. Samples with 0.1 ⩽ x ⩽ 0.4 are simple-phased rhombohedral perovskites with constant Mn3+/Mn4+ ratios. Samples with 0.5 ⩽ x ⩽ 1 have orthorhombic symmetry. In this case, a linear variation of the Mn4+ content as a function of the Ca content is observed. The defect structure of these compounds is described in terms of La and Mn vacancies when 0 < x ⩽ 0.7 (0 ⩽ y ⩽ 0.19) and anion vacancies when 0.8 ⩽ x ⩽ 1 (−0.04 ⩽ y < 0). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)