STRUCTURE AND PROPERTIES OF THE LACUO3-DELTA PEROVSKITES

The high-pressure synthesis and electric and magnetic properties of the LaCuO3-delta system have been studied. The copper valence can be varied in this oxygen-defect perovskite almost continuously from + 3 to + 2, and three distinct ordered phases are observed over its 0.0 less-than-or-equal-to delta less-than-or-equal-to 0. 5 stability field. A tetragonal phase exists over the composition range 0.0 < delta less-than-or-equal-to 0.2, and the structure of a sample with 8 = 0.05 was refined by high-resolution synchrotron powder x-ray diffraction in space group P4/m, with a = 3.818 97(4), c = 3.972 58(6), and R = 0.0812. The tetragonal phase and also a monoclinic form stable between 0.2 less-than-or-equal-to delta less-than-or-equal-to 0.4 are metallic with room-temperature resistivities of (1.0- 3.0) X 10(-3) OMEGA cm. The phase is orthorhombic for larger delta, becoming an insulator as delta-->0.5. Superconductivity was not observed in any of the metallic samples. Ferromagnetic behavior is observed in the tetragonal materials, but the moment disappears at the delta congruent-to 0.2 phase boundary. The small moment is attributed to the existence of a canted antiferromagnetic array in the tetragonal phase. The results are discussed within the contexts of schematic band models and rationalized in terms of one in which the Fermi level moves through partially overlapping copper 3d(x2-y2) and oxygen pi(2p(perpendicular-to)* bands as delta increases.