Oxygen excess LaMnO3+delta perovskites with a wide range of delta values (and Mn-IV contents) have been obtained by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (up to 200 bar). Three representative samples, with delta values of 0.11, 0.15 and 0.26 (Samples 1-3) have been studied by neutron diffraction in the 2-250 K temperature range. The magnetic structures, ordered magnetic moments and long-range ordering temperatures have been determined. Sample 1 is orthorhombic and exhibits a purely ferromagnetic structure below 165(10) K; Samples 2 and 3 are rhombohedral and show a canted ferromagnetic structure, below T-c = 140(10) K and 65(10) K, respectively. The observed decrease of T-c and the magnitude of the ordered magnetic moments as delta increases has been interpreted as a consequence of the presence in the crystal structure of Mn vacancies, which perturb the connecting paths for the transport of holes across Mn-O-Mn, progressively preventing the double-exchange interaction. Also the dramatic reduction of the observed magnetic moments for increasing Mn-IV contents can be explained in a system which in fact shows spin-glass features. (C) 1997 Elsevier Science Ltd.
