Synthesis and structural study of LaNi1-xMnxO3+δ perovskites

The synthesis of LaNi(1-x)MnxO(3+delta) samples with different oxygen contents has been performed. Structural characterization was carried out by X-ray and neutron powder diffraction. The crystallographic structure of stoichiometric samples, delta = 0, evolves from an orthorhombic (LaMnO3) to a rhombohedral (LaNiO3) unit cell. Oxygen excess, delta > 0, seems to stabilize the rhombohedral unit cell. For instance, the unit cells at room temperature are orthorhombic and rhombohedral for LaNi0.1Mn0.9O3.0 and LaNi0.1Mn0.9O3.13, respectively. The X-ray patterns show the coexistence of both phases for LaNi0.5Mn0.5O3+delta at room temperature. This coexistence is not ascribed to chemical inhomogeneities, but to a structural phase transition. Neutron patterns collected from 1.5 to 300 K show a continuous evolution except for LaNi0.5Mn0.5O3.08 and LaNi0.1Mn0.9O3.13, which show a phase transition at around 290 and 220 K, respectively. The neutron patterns suggest the presence of an ordered arrangement of Ni and Mn atoms in the crystallographic unit cell. Such arrangement indicates that LaNi0.5Mn0.5O3 could be considered as a double perovskite (nominal formula, La2NiMnO6). (C) 2002 Elsevier Science Ltd. All rights reserved.