Preparation and crystal structure of the deficient perovskite LaNiO2.5, solved from neutron powder diffraction data

Abstract

The deficient perovskite LaNiO_2.5 has been prepared in powder form with excellent crystallinity by controlled reduction of LaNiO₃ with Zr metal in evacuated ampoules at 400 °C. The neutron powder diffraction pattern could be indexed in a monoclinic unit-cell corresponding to a superstructure of perovskite with dimensions 2a₀× 2a₀× 2a₀(a₀: lattice parameter of the ideal cubic perovskite), also observed by electron diffraction. The structure was solved from the neutron powder data. The oxygen vacancies are ordered in such a way that square-planar NiO₄ and NiO₆ octahedra alternate in the ab plane along the [1 1 0] direction. Both kinds of Ni polyhedra are fairly distorted and tilted in order to optimize the La–O distances, giving rise to a highly strained structure of metastable character. In fact, the compound readily takes up oxygen, above 175 °C in air, to give the much more stable LaNiO₃ perovskite.