Compressibility and Anisotropy of Trona: Unveiling the Structure of a Dense Na₃H(CO₃)₂·2H₂O Polymorph

Understanding the structural stability of hydrated carbonates and bicarbonates under different thermodynamic conditions is crucial, as they play an important role in the carbon cycle, in environmental chemistry, and geochemistry. Sodium sesquicarbonate dihydrate, Na₃H(CO₃)₂·2H₂O trona, is a naturally occurring evaporite mineral also found in magmatic environments. In this work, we carried out high-pressure (HP) in situ synchrotron powder and single-crystal X-ray diffraction (XRD) measurements on a naturally occurring trona specimen. A high-pressure monoclinic to triclinic phase transition occurred at 12.8 GPa, and the structure of the dense HP trona phase was solved. The coordination number of sodium atoms increased from 6 in the low-pressure polymorph to 7-8 in the HP one. Additionally, our results did not show any indication of the phase transition previously reported at 7 GPa based on Raman spectroscopy. The compressibility and anisotropy of low- and high-pressure phases were determined. Birch-Murnaghan equation of state parameters were fitted to the pressure-volume data sets, yielding a bulk modulus of 35(2) GPa (K' = 4.4(9)) for trona using neon as pressure-transmitting medium. Density functional theory calculations supported the experimental results, confirming the structural stability of the phases obtained by XRD.