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

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.