NONSTOICHIOMETRY, STRUCTURAL DEFECTS AND PROPERTIES OF LAMNO3-VALUES R-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.29)(DELTA WITH HIGH DELTA)

Strongly oxygenated LaMnO3+delta perovskites, with nominal Mn-IV conte nts up to 58%, have been prepared by thermal decomposition of metal ci trates followed by annealings either in air or under high oxygen press ure (200 bar). A high-resolution neutron powder diffraction study of f our 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 substanti ally higher proportion of Mn vacancies, depending rather sensitively o n the oxidation conditions. The oxidation state for Mn calculated for the refined stoichiometry La1-xMn1-yO3 is in good agreement with the d elta values previously determined by thermal analysis. Further to this , it is also found that as delta increases the Mn-O bond lengths short en, the Mn-O-Mn angles progressively increase and the perovskite struc ture becomes more regular, which is consistent with the incorporation of Mn-IV cations. The presence of Mn vacancies (as much as 13% in samp les prepared under high oxygen pressure) perturbs the conduction paths for the transport of holes across Mn-O-Mn, weakening the double-excha nge interaction. This structural disorder explains the observed decrea se of the ferromagnetic Curie temperature (T-c) as delta increases.