New structures of Rb2O and Cs2O stable at high pressures

In this work, a search for stable structures of Rb${}_2$O and Cs${}_2$O was carried out using evolutionary algorithms within the density functional theory. In the studied pressure range of 0–100 GPa, for Rb${}_2$O, the transitions $Fm\bar{3}m$ $\leftrightarrow$ $Pnma$-I $\leftrightarrow$ $Pnma$-II were detected, which are realized at pressures of 2.6 and 40 GPa, respectively. For Cs${}_2$O, the transitions $R\bar{3}m$ $\leftrightarrow$ $Pnma$-I $\leftrightarrow$ $I4/mmm$ were revealed, which are realized at pressures of 6.3 and 63 GPa. As a result, previously unknown structures were found for these oxides Rb${}_2$O-$Pnma$-II and Cs${}_2$O-$I4/mmm$. According to the calculated phonon dispersion curves, the predicted structures are dynamically stable. Also, the P–T diagram for Rb${}_2$O and Cs${}_2$O was calculated for the first time. The electronic properties of the newly predicted high-pressure structures Rb${}_2$O-$Pnma$-II and Cs${}_2$O-$I4/mmm$ were investigated, and the band structures and electron density of states (DOS) were calculated.