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

We investigate the regions of thermodynamic stability of possible modifications of the alkali oxides M2O as a function of pressure and type of alkali metal (M=Li,Na,K,Rb,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 (CaF2- and CdCl2-type) to one or more high-pressure modifications exhibiting structures belonging to the extended cotunnite family, e.g., the high-pressure (hp)-BaF2-, the PbCl2-, the gamma-US2-, or the Theta-Ni2Si-type. Metastable modifications that would be stable at sufficiently large negative pressures exhibit the CaCl2- or the CdCl2-structure type.