  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 [111]
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 to 200 K, 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 weak (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 the soft
phonon energies or linewidths.
