Soft phonons and structural phase transition in superconducting Ba0.59K0.41BiO3

We have observed a softening of phonons and a structural phase transition in a superconducting Ba0.59K0.41BiO3 (Tc = 31 K) single crystal using elastic and inelastic neutron scattering measurements. The soft phonon occurs for the [1 1 1] transverse acoustic mode at the zone boundary. The phonon energies in this vicinity are found to continuously decrease with decreasing temperature from above room temperature. This softening stops at a temperature close to Ts, where a structural phase transition from cubic to tetragonal symmetry occurs. The overall results are consistent with previous data that reported phonon softening and a (0.5, 0.5, 0.5) type superstructure in several Ba1−xKxBiO3 systems. However, we also find weaker (0.5, 0.5, 0) type superstructure peaks that reveal an additional component to the modulation. No significant change related to the superconductivity was observed for these soft phonon energies or linewidths.

► Neutron results of the lattice dynamics and structural phase transition in BKBO.
► New (0.5, 0.5, 0) type structure is observed, in addition to (0.5, 0.5, 0.5) type.
► Soft phonon associated with the structural distortion at 153 K was investigated.
► Ground state structure is described by the lower symmetry of P4/mmm.
► A strong electron-phonon interaction in BKBO is suggested.