Structural instability in Ba1−xKxBiO3

We report results of the ultrasonic investigation of Ba1−xKxBiO3 single crystals for two potassium concentrations x≈0.35 and x≈0.47 in a wide temperature range. The softening of both the transverse c44 and the longitudinal c11 modes have been observed at temperatures between 200 K and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hysteresis was discovered. We proposed a model in which the softening of the elastic moduli, the hysteresis, and the maximum in the attenuation of sound can be explained by assuming a coupling of the acoustic modes with the anharmonic oscillations of O6 octahedra.