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Evolution of the α‐BaMg(CO 3 ) 2 low‐temperature superstructure and the tricritical nature of its α–β phase transition
Author(s) -
Ende Martin,
Effenberger Herta,
Miletich Ronald
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520617009295
Subject(s) - superstructure , phase transition , diffraction , crystal structure , chemistry , crystallography , bar (unit) , single crystal , space group , atmospheric temperature range , x ray crystallography , transition temperature , materials science , condensed matter physics , thermodynamics , physics , optics , superconductivity , meteorology
The crystal structure of the synthetic double carbonate norsethite [BaMg(CO 3 ) 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 c ; below T c ), which represents a superstructure of the β‐form (space group R 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 = 2 n + 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 CO 3 group and a coordination change of the BaO 12 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.