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

We have been able to synthesize a new bismuth-based oxide, SrBiO3, and to induce superconductivity by K doping performed under high pressure. T-c 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 P2(1)/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 degreesC, whereas the K-doped phases decompose around 300 degreesC with a progressive loss of oxygen until the (Sr,K)BiO2.5 stoichiometry is reached. The oxygen loss is accompanied by a decrease of T-c. An increase of T-c 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 T-cmax 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.