SYNTHESIS, CRYSTAL-STRUCTURE, AND PROPERTIES OF OXYGEN-DEFICIENT LANTHANUM NICKELATE LANIO3-X (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.5)

Two kinds of oxygen-deficient phases around LaNiO2.5 were obtained by a low-temperature H-2 reduction of perovskite-type LaNiO3. An antiferromagnetic LaNiO2.5 with T(N) congruent-to 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 C2/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 T(c) congruent-to 230 K could be understood due to a slight canting in the antiferromagnetic interaction between the octahedra and the square-planes.