Structural instability and superconductivity in [FORMULA]

We report results of the ultrasonic investigation of Ba 1-x K x BiO 3 superconducting (SC) single crystals for two potassium concentrations [FORMULA] and [FORMULA] 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 K and 50 K. In the case of Ba 0.65 K 0.35 BiO 3 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 BiO 6 octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting phase transition.