Oxidative nonstoichiometry in perovskites, an experimental survey; the defect structure of an oxidized lanthanum manganite by powder neutron diffraction
Oxide perovskites showing oxidative nonstoichiometry (ABO 3+ x) have been investigated. The structure of LaMn 3+0.76Mn 4+0.24O 3.12 has been investigated by powder neutron diffraction and a composition (La 0.94±0.02□ 0.06±0.02)(Mn 3+0.745Mn 4+0.235□ 0.02)O 3 with partial elimination of La 2O 3 and vacancies on both the A and B metal sites determined. A much smaller degree of nonstoichiometry has been found for LaVO 3+ x( x ⩽ 0.05), and LaCrO 3, and EuTiO 3 did not show nonstoichiometry under the conditions used. A single phase region from Ba 0.8La 0.2Ti 4+0.8Ti 3+0.2O 3.0 to Ba 0.8La 0.2Ti 4+O 3.1 has been confirmed for lanthanum-doped BaTiO 3, but the solubility of La 3+ in SrTiO 3 is very small; consideration of the ionic radii indicates that the dopant ion of higher oxidation state must be significantly smaller than the normal ion to stabilize a wide nonstoichiometric region with B site vacancies. The extensive nonstoichiometry shown by LaMnO 3+ x, in contrast to the other lanthanum-transition-metal perovskites LaBO 3, may result from the much larger reduction in ionic radius from Mn 3+ to Mn 4+ than is found for other transition-metal ions.