Origin of bcc to fcc phase transition under pressure in alkali metals

The electronic, dynamical and elastic properties of body-centered cubic (bcc) alkali metals (Li, Na, K, Rb and Cs) at high pressure are extensively studied to reveal the origin of the phase transition from bcc to face-centered cubic (fcc) by using ab initio calculations. The calculated 3D Fermi surface (FS) shows an anisotropic deformation by touching the Brillion zone boundary at the N point with pressure for Li, K, Rb and Cs due to the s -> p and s -> d charge transfers, respectively. However, no clear charge transfer is found in Na, in favor of an isotropic FS even at very high pressure. The traditional charge transfer picture and the newly proposed Home-Rothery model in understanding the bcc -> fcc transition are thus questionable in view of their difficulties in Na. In this paper, a universal feature of pressure-induced instability of the tetragonal shear elastic constant C' and the softening of the transverse acoustic phonons along the [0 xi xi]-direction near the zone center for all the alkali metals is identified. Analysis of the total energy results suggests that C' instability associated with the soft mode is the driving force for the bcc -> fcc transition, which could be well characterized by the tetragonal Bain's path.