New High‐Pressure Polymorphs of Rb ₂ CO ₃ and Cs ₂ CO ₃ : Crystal Structure Prediction and P–T Phase Diagrams

In this work, we performed crystal structure searches for rubidium and cesium carbonates (Rb 2 ₂ CO 3 ₃ and Cs 2 ₂ CO 3 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, C c $$ Cc $$ and C 2 / c $$ C2/c $$ , were predicted for both carbonates. The M 2 ₂ CO 3 ₃ - C 2 / c $$ C2/c $$ (M = Rb, Cs) phase is isostructural with the high-pressure K 2 ₂ CO 3 ₃ - C 2 / c $$ C2/c $$ phase, whereas the M 2 ₂ CO 3 ₃ - C c $$ Cc $$ 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 P2₁/c ↔ ↔ C c $$ Cc $$ ↔ ↔ C 2 / c $$ C2/c $$ . For Rb 2 ₂ CO 3 ₃ , the transition pressures are 4.9 GPa and 23.4 GPa, and for Cs 2 ₂ CO 3 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 ₂ CO 3 ₃ and Cs 2 ₂ CO 3 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 ₂ -type configuration, with the transition pressure increasing systematically with the ionic radius of the cation.