Low-temperature structure of Ba0.6K0.4BiO3

Abstract

The newly discovered copper-free superconductor ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{K}}_{\mathrm{x}}$ ${\mathrm{BiO}}_3$ has the ${\mathrm{CaTiO}}_3$-type structure at room temperature, but its structure at or below $T_c$ is not known. We report Rietveld refinements of the structures fit to neutron-powder-diffraction data taken at 297 and 10 K for a sample with x=0.4. We find that the structure remains cubic at 10 K, the dopant level remains close to that expected from the starting stoichiometry, the oxygen sites are essentially fully occupied, and the anisotropic thermal ellipsoids remain oblate with the unique axis along the Bi-O-Bi bonds.