Polymorphic forms and defect structure formation within homogeneity domain of LaMnO 3 phase

Abstract

A detailed crystallochemical characterization of air annealed LaMnO 3+ δ samples of different La/Mn ratios is presented. This characterization comprises the temperature dependencies of the lattice parameters, the valence of manganese ( V Mn), and sample densities. The latter are compared with densities calculated for various models of possible distribution of the structure components. The data obtained have allowed: (a) to determine temperature stabilities of previously known individual structure forms appearing within the homogeneity domain of perovskite-related LaMnO 3 phase, (b) to confirm the presence of cationic deficiency, created on La and Mn sites in quantities which depend on both, the nominal La/Mn ratio and the temperature-dependent oxidation state of Mn ions greater than 3+, (c) to provide strong evidence for the presence of some excess oxygen ions. Their presumable locations may be either at vacant positions of the metal-sublattices mentioned above or at some interstitial positions available. Similarly to the cation deficiency, quantities of the excess oxygen have occurred highly dependent on the nominal La/Mn ratio and on the oxidation state of Mn ions, the latter variable versus temperature. For La/Mn=0.91:1 stoichiometry samples, a distribution characterized by a certain fraction of Mn ions on the La site has also been determined within some temperature range. Analytical formulae describing the appropriate distribution models found within the homogeneity domain of LaMnO 3+ δ versus temperature are given.