Oxygen distribution and the structure of La_2-xBa_xSrCu₂O_6±δ (0<x<0.25)

Compounds in the system La_2-xBa_xSrCu₂O_6±δ have been synthesised by conventional solid state methods and characterised in terms of oxygen stoichiometry and distribution. Thermogravimetric analysis of these materials has shown copper oxidation states as high as Cu^2.35+ for materials annealed in oxygen; a.c. susceptibility measurements, however, have shown that superconductivity is not present. The structures of four compounds in this system have been determined by powder neutron diffraction profile refinement. These have indicated a significant occupancy of the oxygen site situated between the copper-oxygen planes. The occupancy of this site disrupts the square pyramidal coordination of the copper ion and may be at least partially responsible for the absence of superconductivity in these materials which are structurally very similar to the superconducting compounds YBa₂Cu₃O_7-δ and La_2-xSr_xCuO_4-δ. The high anisotropic temperature factors (B₁₁ and B₂₂) observed for the interlayer oxygen site may indicate a static disordering in the unannealed materials.