High-pressure and high-temperature polymorphism of Na₂CO₃ up to 10 GPa

We report two new high-pressure polymorphs of sodium carbonate, ε and ε-II, stable over ∼2 GPa and 11 GPa, respectively. The ε polymorph exhibits a doubling of the c unit-cell parameter relative to the γ and β phases, crystallizes in space group Cc, and has a bulk modulus of 47.6 (8) GPa, comparable to that of the γ phase. Its stability is confirmed using in situ single-crystal and large-volume press (LVP) experiments. Phase transitions observed over a broad pressure-temperature range allow us to refine the topology of the Na₂CO₃ phase diagram. By integrating precise single-crystal constraints with LVP observations, we propose an updated phase diagram that shows an expanded stability field for the incommensurate γ phase, and reveals a wide stability field for the ε phase at conditions relevant for the Earth's mantle. These results provide improved experimental constraints for the high-pressure behavior of sodium carbonate.