First-principles phonon spectrum in bcc Ba: Three-ion forces and transition-metal behavior.

Abstract

The influence of d electrons on the structural and vibrational properties of the heavy alkaline-earth metals increases with atomic number. The occurrence of the bcc structure in Ba (versus fcc in Ca and Sr) suggests the onset of transition-metal behavior, where the bottom of the 5d band has crossed below the Fermi level. This behavior also appears to be revealed in the recently measured phonon spectrum, which is found to be somewhat anomalous. We have made a first-principles analysis of the phonon spectrum of Ba in the context of the new transition-metal generalized pseudopotential theory. We find that partial filling of the 5d band is essential both to stabilize the bcc structure and to lower the calculated phonon frequencies into the measured range. Furthermore, the inclusion of three-ion angular forces (in addition to normal two-ion pair forces) is necessary to explain the observed anomalous lowering of the longitudinal branch below the transverse branch in the [100] direction.