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

Abstract

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