THE SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF SR2CUO4-X,X-APPROXIMATELY-0.1

Sr2CuO3 (orthorhombic, Immm) is converted at about 400°C under oxygen at 160 bar to Sr2CuO3.9 [tetragonal, I4 mmm; a = 3.7907(7), c = 12.417(2) Å], which is structurally related to La2CuO4. Although the formal Cu oxidation state in this material appears to be higher than any previously reported for Cu in an oxide environment (around + 3.8), powder neutron diffraction data provide no evidence for O2-2 or O-2 ions, but are consistent with a highly defective structure containing Sr, Cu, and O vacancies. The cation vacancies are in a 2:1 ratio to maintain the stoichiometry, and the O vacancies are confined to the CuO2 layers. Additional peaks in the neutron diffraction profile are indicative of an ordering process to give an extended, possibly incommensurate, structure. CuK-edge EXAFS suggests that the resulting local Cu coordination contains two very short CuO bonds (1.755 Å). © 1990.