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

ABSTRACT In this work, we performed crystal structure searches for rubidium and cesium carbonates (RbCO and CsCO) 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, and , were predicted for both carbonates. The MCO‐ (M = Rb, Cs) phase is isostructural with the high‐pressure KCO‐ phase, whereas the MCO‐ 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: . For RbCO, the transition pressures are 4.9 GPa and 23.4 GPa, and for CsCO, 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 RbCO and CsCO 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‐type configuration, with the transition pressure increasing systematically with the ionic radius of the cation.