Non-stoichiometry, structural defects and properties of LaMnO3+delta with high delta values (0.1<=delta<=0.29)

Strongly oxygenated LaMnO3+delta perovskites, with nominal Mn-IV contents up to 58%, have been prepared by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (200 bar). A high-resolution neutron powder diffraction study of four representative samples with 0.11 less than or equal to delta less than or equal to 0.29 reveals the presence of both La and Mn vacancies. Contrary to previous studies, it is found that there are a substantially higher proportion of Mn vacancies, depending rather sensitively on the oxidation conditions. The oxidation state for Mn calculated for the refined stoichiometry La1-xMn1-yO3 is in good agreement with the delta values previously determined by thermal analysis. Further to this, it is also found that as delta increases the Mn-O bond lengths shorten, the Mn-O-Mn angles progressively increase and the perovskite structure becomes more regular, which is consistent with the incorporation of Mn-IV cations. The presence of Mn vacancies (as much as 13% in samples prepared under high oxygen pressure) perturbs the conduction paths for the transport of holes across Mn-O-Mn, weakening the double-exchange interaction. This structural disorder explains the observed decrease of the ferromagnetic Curie temperature (T-c) as delta increases.