SYNTHESIS, STRUCTURE AND SUPERCONDUCTIVITY IN THE BA1-XKXBIO3-Y SYSTEM

The recent discovery of superconductivity near 30 K in Ba//1// minus //xK//xBiO//3// minus //y (x approximately equals 0. 4) is remarkable in that it is the first copper-free oxide superconductor that has a transition temperature (T//c) above that for the best intermetallic superconductor. The authors describe a two-step technique for synthesizing this compound starting with a stoichiometric oxide composition, which yields single-phase samples suitable for transport measurements. Neutron powder diffraction studies of samples wih varying potassium concentration show that superconductivity in Ba//1// minus //xK//xBiO//3// minus //y occurs only in a cubic perovskite phase which is stable at less than equivalent to 600 degree C and which forms only for x greater than 0. 25. Within this cubic phase, T//c, is highest for compositions near the structural phase transition (x approximately equals 0. 25) and decreases with increasing x.