3 years ago

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

Evolution of the α-BaMg(CO3)2 low-temperature superstructure and the tricritical nature of its α–β phase transition
Herta Effenberger, Ronald Miletich, Martin Ende
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 Tc = 363 ± 3 K. The transition of the α-form (space group Rc; below Tc), which represents a superstructure of the β-form (space group Rm, with c′ = c/2; above Tc) 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.Using a high-sensitivity silicon detector and a micro-focus source, the norsethite [BaMg(CO3)2] low-temperature superstructure has been tracked across the temperature-driven α–β phase transition using single-crystal diffraction experiments. Systematic analysis of physical potential order parameters reveals the structure-related driving force and that the transition is tricritical in nature.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1107/S2052520617009295

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