Structural studies of new superconducting bismuthates (Sr,K)BiO3

Abstract We have been able to synthesize a new bismuth-based oxide, SrBiO3, and to induce superconductivity by K doping performed under high pressure. Tc max is about 12K (for 45%–60% of K). These new compounds have a distorted perovskite-based structure. Powder synchrotron and neutron diffraction studies have shown that, as a function of K doping, the symmetry changes from monoclinic (space group P21/n) to tetragonal (space group I 4/mcm) and remains the same down to 4K. Above room temperature, SrBiO3 keeps the same structure until decomposition around 700°C, whereas the K-doped phases decompose around 300°C with a progressive loss of oxygen until the (Sr,K)BiO2.5 stoichiometry is reached. The oxygen loss is accompanied by a decrease of Tc. An increase of Tc was observed when resistivity was measured as a function of pressure. Powder synchrotron studies indicate that, for K-doped samples, pressure induces an increase of the tetragonal distortion, as well as a change of the tilt angle of the octahedra until the pressure corresponding to Tcmax is reached. The (Sr,K) site is more affected by the pressure effect than the Bi one. In the case of SrBiO3, no noticeable change was observed under pressure.