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

Author(s)

Martin Ende, Herta Silvia Effenberger, Ronald Miletich

Abstract

The crystal structure of the synthetic double carbonate norsethite [BaMg(CO3) 2] has been reinvestigated using X- ray diffraction data within the temp. range 100- 500 K using a high- sensitivity PILATUS pixel detector. The previously assumed positional shift of the crystallog. unique oxygen atom is confirmed. The shift is assocd. with a coupled rotation of symmetry- equiv. carbonate groups. It was possible to follow the shift using high- accuracy expts. under varying temp. conditions between 100 K and the crit. transition temp. occurring at Tc = 363 ± 3 K. The transition of the a- form (space group R [inline formula omitted] c; below Tc) , which represents a superstructure of the ß- form (space group R [inline formula omitted] m, with c' = c /2; above Tc) was studied in detail. The tricrit. 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. Thermodn. properties suggest both rotation of the CO3 group and a coordination change of the BaO12 coordination polyhedra as the order parameters driving the temp.- dependent a- ß phase transition. Nevertheless, a detailed structural anal. reveals the coordination change of the barium atoms to be the main driving force for the obsd. transformation.

Organisation(s)

Department of Mineralogy and Crystallography

Journal

Acta Crystallographica. Section B: Structural Science

Volume

73

Pages

827-835

No. of pages

9

ISSN

0108-7681

DOI

https://doi.org/10.1107/S2052520617009295

Publication date

10-2017

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 104011 Materials chemistry, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Electronic, Optical and Magnetic Materials, Metals and Alloys, Materials Chemistry, Atomic and Molecular Physics, and Optics

Portal url

https://ucrisportal.univie.ac.at/en/publications/2961f5e4-3f3c-4f08-9b6f-52888417135a