Eur. Phys. J. B 16, 59-66 (2000)
https://doi.org/10.1007/s100510070249

Structural instability and superconductivity in Ba_1-xK_xBiO₃

¹ Physikalisches Institut, Universität Frankfurt, 60054 Frankfurt, Germany
² Institute for Low Temperature Physics and Engineering, 310164 Kharkov, Ukraine
³ Institute of physics of Solids and Semiconductors, 220072 Minsk, Belarus
⁴ Laboratoire d'Études des Propriétés Électroniques des Solides, CNRS, 38042 Grenoble, France

Corresponding author: ^a luethi@physik.uni-frankfurt.de

Received: 25 June 1999
Revised: 14 February 2000
Published online: 15 July 2000

Abstract

We report results of the ultrasonic investigation of Ba_1-xK_xBiO₃ 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 Ba_0.65K_0.35BiO₃ 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.