LaMnO3+δRevisited

Abstract

Samples of LaMnO3+δwith 0≤δ≤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−εMn1−εO3withε=δ/(3+δ). Room temperature X-ray diffraction reveals a two-phase region separating an O′-orthorhombic phase stable over 0≤δ≤0.06 and a rhombohedral phase, stable in the range 0.10≤δ≤0.18, that transforms below room temperature to an O′-orthorhombic phase. Transport and magnetic studies indicate the following evolution of electronic properties with increasingδ. 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 superparamagnetic 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 increasingδ, the perovskite tolerance factor increases, and at a critical valuetc≈0.97, a transition from small-polaron to a peculiar nondispersive delocalized state of the conduction electrons occurs belowTc. At highestδ, these conduction electrons introduce a double-exchange spin–spin coupling in the matrix that varies as cos(θij/2); this ferromagnetic coupling competes with the antiferromagnetic Mn:t3–O: 2pπ–Mn:t3superexchange, which varies as cosθij. Consequently an equilibriumθijincreases withδto give a cant angle 0°≤θ≤180°, which introduces metamagnetic behavior of the matrix between clusters and a ferromagnetic magnetization that decreases relatively sharply with increasingδin the range 0.13≤δ≤0.18.