Ultrasonic investigation of structural instability in superconducting Ba((1-x))K(x)BiO3 single crystals

We report results of the ultrasonic investigation of Ba1-xKxBiO3 single cry stals for two potassium concentrations x approximate to 0.35 and x approxim ate to 0.47 in a wide temperature range. The softening of both the transver se c(44) and the longitudinal cll modes have been observed at temperatures between 200 K and 50 K. In the case of Ba0.65K0.35BiO3 a pronounced hystere sis was discovered. We proposed a model in which the softening of the elast ic moduli, the hysteresis, and the maximum in the attenuation of sound can be explained by assuming a coupling of the acoustic modes with the anharmon ic oscillations of O-6 octahedra.