High-pressure phase diagrams of Na₂CO₃ and K₂CO₃

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, γ -Na 2 CO 3 is stable up to 12 GPa and above this pressure transforms to P63 /mcm-phase. At 26 GPa, Na₂ CO₃ - P6₃ /mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na₂ CO₃ - P21 /m. On cold compression in DAC experiments, γ -Na₂ CO₃ is stable up to the maximum pressure reached of 25 GPa. K₂ CO₃ shows a more complex sequence of phase transitions. Unlike γ Na₂ CO₃, γ -K₂ CO₃ has a narrow stability field. At 3 GPa, K₂ CO₃ presents in the form of the new phase, called K₂ CO₃ -III, which transforms into another new phase, K₂ CO₃ -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 K₂ CO₃ -IV has similarities with that of the theoretically predicted K₂ CO₃ - P2₁ /m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K₂ CO₃. Reconstruction of the diffraction pattern of γ -K₂ CO₃ is observed at pressures of 0.5–3.1 GPa if the DAC is loaded on the air.