Elemental Metals under Pressure

Abstract

Parameter-free calculations based on the density-functional theory are used to examine high-pressure phases of solids. For the elemental semiconductors particular attention is paid to the orthorhombic (Cmca) structure (Si-VI). The same structure, even with very nearly the same relative atomic coordinates, is found for Cs in the high-pressure phase Cs-V. In the Cmca structures the atoms tend to form dimers. Ge and Rb also have high-pressure phases with the same Cmca structure. The thermodynamic properties of the low-pressure phases of cesium, Cs-I (b.c.c.) and Cs-II (f.c.c.), are examined, and the equation of state is calculated for P up to 4.5 GPa and temperatures from 0 to 300 K. The contributions to energy and entropy from the phonons are calculated within the quasi-harmonic approximation. The thermal expansion coefficient of f.c.c.-Cs is predicted to be negative for P above 3.5 GPa for all T. Cs-II becomes dynamically unstable when P exceeds 4.3 GPa, where a transverse phonon mode with wavevector along (110) becomes soft. As a consequence, a Van der Waals loop does not develop in the isotherms, and an isostructural (f.c.c. → f.c.c.) transition cannot occur. In that case Cs-III must have a structure that is not f.c.c.