Phonon dispersion curves of fcc La

Large single crystals of fcc La were grown in situ and were used to study the lattice dynamics of this phase of La by coherent inelastic neutron scattering. The phonon dispersion curves have been measured along the [ξ00], [ξξ0], [ξξξ], and [0ξ1] symmetry directions at 660 and 1100 K. The T[ξξξ] branch exhibits anomalous dispersion for ξ>0.25 and, in addition, close to the zone boundary, the phonon frequencies of this branch decrease with decreasing temperature. This soft-mode behavior may be related to the β→α transformation in La, an assumption supported by recent band-theoretical calculations of the generalized susceptibility of fcc La. At X the frequencies of the L[ξ00] branch are considerably lower than those of the corresponding branch of γ-Ce; a similar but not as pronounced effect is observed for the frequencies of the L[ξξξ] branch close to the point L. Since the calculated generalized susceptibility of fcc La exhibits strong peaks at X and L, these anomalies may be due to the renormalization of the phonon frequencies by virtual f↔d transitions to the unoccupied $4f$ level in La. The data were used to evaluate the elastic constants, the phonon density of states, and the lattice specific heat at constant pressure CP⁽ˡ⁾.