Phonon instabilities in high-pressure bcc-fcc and the isostructural fcc-fcc phase transitions of Cs

The phonon instabilities of cesium related to its pressure-induced bcc:to-fcc((1))-to-fcc((2)) phase transitions are studied using the density-functional perturbation theory. It is found that at low pressure, both the bce and fee Cs are dynamically stable, but fee is thermodynamically metastable. The high-pressure phase transitions of Cs from bcc to fcc((1)) and from fcc((1)) to fcc((2)) are related to the phonon instabilities of the long-wavelength acoustic modes resulted from the negative tetragonal shear elastic constant C'=1/2(C-11-C-12), During the fcc((1))-to-fcc((2)) phase transition, all the phonon modes become soft, making it the most striking feature of this isostructural phase transition.