Stability of alkali-metal oxides as a function of pressure: Theoretical calculations

Abstract

We investigate the regions of thermodynamic stability of possible modifications of the alkali oxides ${M}_{2}\mathrm{O}$ as a function of pressure and type of alkali metal $(M=\mathrm{Li},\mathrm{Na},\mathrm{K},\mathrm{Rb},\mathrm{Cs})$. The results are in good agreement with experiment at standard pressure, and we find that at elevated pressures, all systems should exhibit a transition from the experimentally observed modifications ($\mathrm{Ca}{\mathrm{F}}_{2}$- and $\mathrm{Cd}{\mathrm{Cl}}_{2}$-type) to one or more high-pressure modifications exhibiting structures belonging to the extended cotunnite family, e.g., the high-pressure (hp)-$\mathrm{Ba}{\mathrm{F}}_{2}$-, the $\mathrm{Pb}{\mathrm{Cl}}_{2}$-, the $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{U}{\mathrm{S}}_{2}$-, or the $\ensuremath{\Theta}\ensuremath{-}{\mathrm{Ni}}_{2}\mathrm{Si}$-type. Metastable modifications that would be stable at sufficiently large negative pressures exhibit the $\mathrm{Ca}{\mathrm{Cl}}_{2}$- or the $\mathrm{Cd}{\mathrm{Cl}}_{2}$-structure type.