Negative thermal expansion coefficient and isostructural transition in fcc cesium

The equation of state of solid cesium in the face-centered-cubic (fcc) structure is examined theoretically by means of ab initio calculations. The electronic energy is calculated within density-functional theory, whereas vibrational contributions to energy and entropy are calculated within the quasiharmonic approximation. The thermal expansion coefficient is predicted to be negative for pressures above 3.5 GPa and up to the end of the stability range of the fee phase for all T. The fee phase becomes unstable around 4.3 GPa, where a transverse phonon mode with a wave vector along (110) becomes soft. According to the calculations an isostructural transition does not occur.