Longitudinal-acoustic-phonon softening in YS, LaS, and CeSe

The nonorthogonal tight-binding method of Varma and Weber is used to calculate the phonon spectra of YS, LaS, and CeSe, which exhibit strong anomalies in the longitudinal-acoustic branch, especially near the L point, where it crosses the transverse ones. The latter feature is found to arise from metal-metal interactions, which have their origin in the predominantly metal-derived electronic energy bands near the {Γ} point at the Fermi energy. With the addition of six Born--von K\'arm\'an force constants to model the short-range interactions, excellent agreement is obtained between theory and experiment in all three compounds.