  The dispersion of phonons in Ba1-xKxBiO3 along the (3+q, 0, 0) direction in
reciprocal space was determined for x=0, 0.30, 0.37, 0.52 using high-resolution
inelastic X-ray scattering. The observed phonon energies near \Gamma were in
good agreement with published optical data. It was found that two high-energy
modes strongly soften near q=0.25 when the system becomes metallic (x >0.35).
There was no softening in the insulating phase, even at x=0.30, very near the
metal-insulator transition. This suggests that the phonon softening is linked
to the appearance of the Fermi-surface.
