Synthesis, Crystal Structure, and Properties of Oxygen-Deficient Lanthanum Nickelate LaNiO3−x (0 ≤ x ≤ 0.5)

Two kinds of oxygen-deficient phases around LaNiO2.5 were obtained by a low-temperature H2 reduction of perovskite-type LaNiO3. An antiferromagnetic LaNiO2.5 with TN 140 K seems to be stoichiometric and a small amount of excess oxygen leads to a new ferromagnetic phases. A structural refinement has revealed that the former (space group C 2/c) comprised one-dimensionally linked NiO6 octahedra along the c-axis and NiO4 square-planars connecting the octahedra chains. A cooperative Jahn–Teller distortion was observed in both coordination polyhedra, which suggested that LaNiO2.5 has octahedral Ni3+ ions with the low-spin state and square-planar Ni+ ions. Its magnetic property and XPS spectrum support this model. The new ferromagnetism with Tc 230 K could be understood due to a slight canting in the antiferromagnetic interaction between the octahedra and the square-planes.