PREPARATION AND CRYSTAL-STRUCTURE OF THE DEFICIENT PEROVSKITE LANIO2.5, SOLVED FROM NEUTRON POWDER DIFFRACTION DATA

The deficient perovskite LaNiO2.5 has been prepared in powder form with excellent crystallinity by controlled reduction of LaNiO3 with Zr metal in evacuated ampoules at 400 degrees C. The neutron powder diffraction pattern could be indexed in a monoclinic unit-cell corresponding to a superstructure of perovskite with dimensions 2a(0) x 2a(0) x 2a(0) (a(0): 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 [110] 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 degrees C in air, to give the much more stable LaNiO3 perovskite.