High-pressure polymorphism and structural transitions of norsethite, BaMg(CO3)2

In situ high-pressure investigations on norsethite, BaMg(CO 3 ) 2 , have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P c ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C 2 /c symmetry (Z = 4; a = 8.6522(14) Å, b = 4.9774(13) Å, c = 11.1542(9) Å, β = 104.928(8)°, V = 464.20(12) Å 3 at 3.00 GPa), and the structure refinement ( R 1 = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO 3 groups were found to get tilted off the ab- plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K 0 = 66.2 ± 2.3 GPa and d K/ d P = 2.0 ± 1.8 for α-norsethite and K 0 = 41.9 ± 0.4 GPa and d K/ d P = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β a −1 = 444(53) GPa, β c −1 = 76(2) GPa; γ: β a −1 = 5.1(1.3) × 10 3 GPa, β b −1 = 193(6) GPa β c −1 = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius–Clapeyron slope of 9.8(7) × 10 −3 GPa K −1 . An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated R 3 ¯ m space-group symmetry. The same superstructure reflections, which disappear as temperature in