Growth and structural characterization of superconducting Ba_1-xK_xBiO₃ single crystals

Superconductivity near 30 K was discovered recently in a copper-free phase, Ba_1-xK_xBiO₃ (refs 1, 2). This material has structural and electronic properties in common with both the cuprate high-transition-temperature (high-T_c) superconductors and with BaPb_1-xBi_xO₃, which has a T_c of ∼12 K (ref. 4). Here we report the growth and structural characterization of single crystals of Ba_1-xK_xBiO₃. A maximum T_c of 30.5 K is found from d.c. susceptibility measurements for crystals of approximate composition Ba_0.6K_0.4BiO₃. The structure for a crystal with x = 0.37 is determined by X-ray diffraction and is found to be a simple cubic perovskite with a = 4.2869 (6) Å. A commensurate lattice distortion occurs for x = 0.04, which is identified as a distortion of the BiO₆ octahedra. Effects of structural distortions and concomitant changes in electronic properties in this non-magnetic system may contain clues to the role of phonons and electronic interactions in the microscopic pairing mechanism of high-T_c superconductors.