Synthesis and structural study of LaNi 1− xMn xO 3+ δ perovskites

Abstract

The synthesis of LaNi 1− x Mn x O 3+ δ 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, δ=0, evolves from an orthorhombic (LaMnO 3) to a rhombohedral (LaNiO 3) unit cell. Oxygen excess, δ>0, seems to stabilize the rhombohedral unit cell. For instance, the unit cells at room temperature are orthorhombic and rhombohedral for LaNi 0.1Mn 0.9O 3.0 and LaNi 0.1Mn 0.9O 3.13, respectively. The X-ray patterns show the coexistence of both phases for LaNi 0.5Mn 0.5O 3+ δ 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 LaNi 0.5Mn 0.5O 3.08 and LaNi 0.1Mn 0.9O 3.13, 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 LaNi 0.5Mn 0.5O 3 could be considered as a double perovskite (nominal formula, La 2NiMnO 6).