New High-Pressure Polymorphs of Rb2CO3 and Cs2CO3: Crystal Structure Prediction and P–T Phase Diagrams

Abstract

In this work, we performed crystal structure searches for rubidium and cesium carbonates (Rb${}_2$CO${}_3$ and Cs${}_2$CO${}_3$) in the pressure range of 0–100 GPa using evolutionary algorithms based on the density functional theory. As a result, two new stable high-pressure polymorphs, $Cc$ and $C2/c$, were predicted for both carbonates. The M${}_2$CO${}_3$-$C2/c$ (M = Rb, Cs) phase is isostructural with the high-pressure K${}_2$CO${}_3$-$C2/c$ phase, whereas the M${}_2$CO${}_3$-$Cc$ phase has no known structural analogs and represents a novel phase for alkali metal carbonates. A common sequence of phase transitions was established for both compounds: $P{2}_1/c$ $\leftrightarrow$ $Cc$ $\leftrightarrow$ $C2/c$. For Rb${}_2$CO${}_3$, the transition pressures are 4.9 GPa and 23.4 GPa, and for Cs${}_2$CO${}_3$, they are 5.2 GPa and 35.5 GPa. The phonon calculations confirmed the dynamic stability of all predicted phases. It was also shown that these high-pressure phases cannot be quenched to ambient pressure. Within the quasi-harmonic approximation, P–T phase diagrams of Rb${}_2$CO${}_3$ and Cs${}_2$CO${}_3$ were constructed for the first time, revealing a weak temperature dependence of the phase boundaries. The obtained results elucidate a general trend in the phase transitions of alkali metal carbonates: under high pressure, the cation sublattices of all alkali carbonates tend to adopt an AlB${}_2$-type configuration, with the transition pressure increasing systematically with the ionic radius of the cation.