Samples of LaMnO3+delta with 0 less than or equal to delta less than or equal to 0.18 have been prepared via a precipitation method followed by heating at different temperatures and oxygen partial pressures. Oxidation is accommodated by the formation of cation vacancies to give La1-epsilonMn1-epsilonO3 with epsilon = delta/(3+delta). Room temperature X-ray diffraction reveals a two-phase region separating an O'-orthorhombic phase stable over 0 less than or equal to delta less than or equal to 0.06 and a rhombohedral phase, stable in the range 0.10 less than or equal to delta less than or equal to 0.18, that transforms below room temperature to an O'-orthorhombic phase. Transport and magnetic studies indicate the following evolution of electronic properties with increasing delta. Oxidation creates small-polaron holes that become increasingly trapped at cation vacancies with decreasing temperature in the paramagnetic domain. Some of these trapped holes are released on cooling through the onset of long-range magnetic order. In the O'-orthorhombic structure, the trapped holes form super-paramagnetic clusters below room temperature that become magnetically coupled with the onset of antiferromagnetic order in the hole-poor matrix to form a magnetic glass. The O'-orthorhombic structure sustains the cooperative Jahn-Teller deformation. The rhombohedral phase suppresses the cooperative Jahn-Teller deformation, and the hole-poor matrix becomes ferromagnetic. With increasing delta, the perovskite tolerance factor increases, and at a critical value t(c) approximate to 0.97, a transition from small-polaron to a peculiar nondispersive delocalized state of the conduction electrons occurs below T-c. At highest delta, these conduction electrons introduce a double-exchange spin-spin coupling in the matrix that varies as cos (theta(y)/2); this ferromagnetic coupling competes with the antiferromagnetic Mn: t(3)-O: 2p(pi)-Mn: t(3) superexchange, which varies as cos theta(ij). Consequently an equilibrium theta(ij) increases with delta to give a cant angle 0 degrees less than or equal to theta less than or equal to 180 degrees, which introduces metamagnetic behavior of the matrix between clusters and a ferromagnetic magnetization that decreases relatively sharply with increasing delta in the range 0.13 less than or equal to delta less than or equal to 0.18. (C) 1997 Academic Press. [References: 27]
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