Combined X-ray and neutron powder diffraction study of the structure of Ba 4CaCu 3O x: Oxygen stoichiometry and cation order

Abstract

Cation ordering and oxygen stoichiometry of the nonsuperconducting compound, Ba 4CaCu 3O x , have been studied by a joint X-ray and neutron powder diffraction experiment. In contrast to the related phase Ba 4YCu 3O 8.5+δ, which was refined in the space group Pm3, we found that the higher symmetry space group Im3 m provides a better fit to the diffraction data for Ba 4CaCu 3O x . The cubic unit cell of Ba 4CaCu 3O x has a perovskite-like configuration, but possesses lattice lengths double those of a usual perovskite cell. X-ray scattering data clearly show substantial exchange between the Ca and Ba sites. Refinements of neutron data indicate two oxygen sites: an ( x, 0, 0) site that is essentially fully occupied and a ( 1 4 , 0, 1 2 ) site that is approximately 47% occupied. The joint X-ray and neutron refinement yields an average stoichiometry of Ba 3.92Ca 1.08Cu 3O 8.85 for the compound ( Z = 2) and an effective CuO hole concentration of +0.29. Ba 3.92Ca 1.08Cu 3O 8.85 is a dielectric; the absence of superconductivity in this material is discussed in terms of its structure and oxygen stoichiometry.