PHONON-DISPERSION OF BCC LA

The phonon-dispersion curves of the high-temperature bcc phase of La (gamma-La) have been measured at 1163 K. A characteristic valley of transverse and low-energy phonons extends along the [xixi0] and [xixi2xi] propagation directions. The eigenvectors of these modes are in the direction of the displacements needed for phase transitions to low-temperature close-packed structures. On the one hand they are indicative of the low potential barrier of the bcc phase for displacements toward these close-packed structures, whereas on the other hand the bcc structure is stabilized by the contribution of these low-energy modes to the lattice entropy. All these modes are strongly damped and have lifetimes of a few vibrational periods. Interference effects due to multiphonons can be the origin of the observed pronounced alterations of the one-phonon scattering law.