The phase diagrams of Na ${}_2$ CO ${}_3$ and K ${}_2$ CO ${}_3$ have been determined with multianvil (MA) and diamond anvil cell (DAC) techniques. In MA experiments with heating, $\gamma$ -Na ${}_2$ CO ${}_3$ is stable up to 12 GPa and above this pressure transforms to $P{6}_3$ /mcm-phase. At 26 GPa, Na ${}_2$ CO ${}_3$ - $P{6}_3$ /mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na ${}_2$ CO ${}_3$ - $P{2}_1$ /m. On cold compression in DAC experiments, $\gamma$ -Na ${}_2$ CO ${}_3$ is stable up to the maximum pressure reached of 25 GPa. K ${}_2$ CO ${}_3$ shows a more complex sequence of phase transitions. Unlike $\gamma$ -Na ${}_2$ CO ${}_3$ , $\gamma$ -K ${}_2$ CO ${}_3$ has a narrow stability field. At 3 GPa, K ${}_2$ CO ${}_3$ presents in the form of the new phase, called K ${}_2$ CO ${}_3$ -III, which transforms into another new phase, K ${}_2$ CO ${}_3$ -IV, above 9 GPa. In the pressure range of 9–15 GPa, another new phase or the mixture of phases III and IV is observed. The diffraction pattern of K ${}_2$ CO ${}_3$ -IV has similarities with that of the theoretically predicted K ${}_2$ CO ${}_3$ - $P{2}_1$ /m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K ${}_2$ CO ${}_3$ . Reconstruction of the diffraction pattern of $\gamma$ -K ${}_2$ CO ${}_3$ is observed at pressures of 0.5–3.1 GPa if the DAC is loaded on the air.

Using ab initio crystal structure prediction technique, we reveal novel structure of CaC ₂ O ₅ , Ca-pyrocarbonate with symmetry Cc , which stabilizes at 10 GPa. This structure is characterized by the isolated [C ₂ O ₅ ] groups formed by polymerization of two [CO ₃ ] triangles.