EVIDENCE FOR NICKEL-(I)-RICH MIXED-OXIDE WITH A DEFECT K2NIF4-TYPE STRUCTURE

Reduced phases obtained from lanthanum mixed nickel oxide, i.e., La2-xSrxNiO4-y, K2NiF4 type, have been studied. Reduction under controlled conditions led to the composition LaSrNiO3.1 containing formally more than 80% of Ni in the valence state (I). Structural calculations and local studies by X-ray adsorption spectroscopy provide evidence for a lowering of the Ni octahedral coordination configuration. V-square-pyramidal and IV-square configurations are obtained, depending on the reduction level. The oxygen vacancies are highly ordered along the b axis of the orthorhombic unit cell, in agreement with recent defect modeling of this structure. © 1992.