Structural instability and superconductivity in Ba1-xKxBiO3

We report results of the ultrasonic investigation of Ba1-xKxBiO3 superconducting (SC) single crystals for two potassium concentrations x congruent to 0.35 and x congruent to 0.47 in a wide temperature range including the normal and the SC states. An instability of the crystal lattice that develops above the superconducting phase transition leads to a softening of both the transverse c(44) and the longitudinal c(11) modes at temperatures between 200 Ii and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hysteresis was discovered. Low temperature X-ray powder diffraction analysis does not reveal any change of the cubic structure in the samples within a resolution of our X-ray technique. The softening of the elastic moduli, the hysteresis, the maximum in the attenuation of sound along with the possible short- (or long-) range structural distortion can be explained qualitatively in a simple model by assuming a coupling of the acoustic modes with the anharmonic oscillations: of BiO6 octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting phase transition.