Evolution of the α-BaMg(CO₃)₂ low-temperature superstructure and the tricritical nature of its α-β phase transition

The crystal structure of the synthetic double carbonate norsethite [BaMg(CO₃)₂] has been reinvestigated using X-ray diffraction data within the temperature range 100-500 K using a high-sensitivity PILATUS pixel detector. The previously assumed positional shift of the crystallographically unique oxygen atom is confirmed. The shift is associated with a coupled rotation of symmetry-equivalent carbonate groups. It was possible to follow the shift using high-accuracy experiments under varying temperature conditions between 100 K and the critical transition temperature occurring at T_c = 363 ± 3 K. The transition of the α-form (space group R{\bar 3}c; below T_c), which represents a superstructure of the β-form (space group R{\bar 3}m, with c' = c/2; above T_c) was studied in detail. The tricritical order character of this displacive phase transition was verified by tracking the intensities of the recorded superstructure reflections (l = 2n + 1) from single-crystal diffraction and using high-precision lattice parameters obtained from powder diffraction in transmission geometry. Thermodynamic properties suggest both rotation of the CO₃ group and a coordination change of the BaO₁₂ coordination polyhedra as the order parameters driving the temperature-dependent α-β phase transition. Nevertheless, a detailed structural analysis reveals the coordination change of the barium atoms to be the main driving force for the observed transformation.