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 ∼= 0.35 and x ∼= 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 c44 and the longitudinal c11 modes at temperatures between 200 K 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. PACS. 62.65.+k Acoustical properties of solids – 74.72.-h High-Tc compounds