Combining single-crystal XRD and LVP experiments under HP-HT conditions: an updated Na₂CO₃ phase diagram

Sodium carbonate (Na₂CO₃) represents a model system for investigating complex polymorphic behavior, owing to its multiple temperature-induced phase transitions [1] and the occurrence of an incommensurately modulated ambient-pressure structure [2-4]. Although its crystallography has been extensively explored, its structural evolution under simultaneous high-pressure and high-temperature conditions is still not fully defined, particularly in the 0-10 GPa range. Here we adopt a multi-technique strategy that integrates single-crystal X-ray diffraction with Diamond Anvil Cell and monochromatic X-ray powder diffraction measurements using large-volume press (LVP) to constrain the phase stability and thermoelastic behavior of Na₂CO₃ up to 10 GPa and high temperatures. The combination of these complementary approaches provides a coherent and detailed picture of its HP-HT phase relations. Our experiments lead to the identification of two previously unreported high-pressure phases, labelled ε and ε-II. The ε phase becomes stable above approximately 2 GPa and crystallizes in the monoclinic space group Cc, showing a doubling of the c lattice parameter relative to both the β and γ structures. Its measured bulk modulus of 47.6(8) GPa closely matches that of the γ phase, suggesting comparable compressibility. At higher pressures, around 11 GPa, we observe the ε-II phase, which likely corresponds to a metastable, distorted derivative of the ε structure. The integration of in-situ single-crystal diffraction data with constraints derived from LVP experiments allows us to refine the topology of the Na₂CO₃ phase diagram. The revised diagram (Figure 1) indicates an enlarged stability field for the incommensurate γ phase, together with a significant pressure stability range for the ε polymorph. This study demonstrates the effectiveness of combining single-crystal XRD and large-volume press techniques for resolving complex phase relationships, providing improved constraints on Na₂CO₃ phase diagram and a more consistent basis for future high-pressure experimental and computational studies. [1] Harris, M. J., & Salje, E. K. H., Journal of Physics: Condensed Matter, 1992, 4(18), 4399. [2] De Wolff, P. M., Foundations of Crystallography, 1974, 30(6), 777-785. [3] Van Aalst, W., Den Holander, J., Peterse, W. J. A. M., & De Wolff, P. M., Acta Cryst., 1976, B32(1), 47-58. [4] Dušek, M., Chapuis, G., Meyer, M., & Petricek, V., Acta Cryst., 2003, B59(3), 337-352.