Phonon dispersion of bcc cerium

The phonon dispersion of the high temperature bce or delta -phase of cerium has been measured by inelastic neutron scattering, growing the single crys tals in situ on the spectrometer. The phonon dispersion shows a characteris tic valley of low energy and strongly damped phonons along T-1[xi xi0] and T-1[xi xi2 xi]. From that the physical origin of the bcc-to-fcc martensitic phase transition is deduced. A mechanism involving long wavelength phonons is proposed.