Second-order P<i>6c</i>2-P31<i>c</i> transition and structural crystallography of the cyclosilicate benitoite, BaTiSi₃O₉, at high pressure

Author(s)

Clivia Hejny, Ronald Miletich-Pawliczek, A. Jasser, Pascal Schouwink, Wilson A. Crichton, Volker Kahlenberg

Abstract

Experimental high-pressure investigations on benitoite in the diamond-anvil cell reveal a second-order phase transition at a critical transition pressure Pc = 4.24(3) GPa, as determined from synchrotron powder diffraction, single-crystal X-ray diffraction, and Raman spectroscopy. Diffraction experiments indicate a non-isomorphous transition from P6c2 to P31c space-group symmetry with a' = av3 and c' = c relative to the P62c subcell below Pc. The high-pressure polymorph is characterized by a larger compressibility compared to the compressional behavior of benitoite below Pc. Fitting second-order Birch-Murnaghan equations of state to the experimental data sets, the parameters obtained are V0 = 372.34(4) Å3, K0 = 117.9(7) GPa, with a0 = 6.6387(3) Å, Ka = 108.1(7) GPa, and c0 = 9.7554(4) Å, Kc = 143.3(1.1) GPa for the low-pressure form (P <Pc), and V0 = 376.1(4) Å3, K0 = 88.9(1.6) GPa, with a0 = 11.516(4) Å, Ka = 95.4(1.8) GPa, and c0 = 9.826(4) Å, Kc = 77.2(1.6) GPa for the high-pressure form (P > Pc). One of the most significant structural changes is related to the coordination of Ba atoms, changing from an irregular [6+6] coordination to a more regular ninefold. Simultaneously, the Si3O9 rings are distorted due to no longer being constrained by mirror-plane symmetry, and the Si atoms occupy three independent sites. The higher compressibility along the c-axis direction is explained by the relative displacement of the Ba position to the Si3O9 rings, which is coupled to the lateral displacement of the non-bridging O2-type atoms of the ring unit. A symmetry mode analysis revealed that the transition is induced by the onset of a primary order parameter transforming according to the K6 irreducible representation of P6c2.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Leopold-Franzens-Universität Innsbruck, Scientific Software Center, European Synchrotron Radiation Facility ESRF

Journal

American Mineralogist: an international journal of earth and planetary materials

Volume

97

Pages

1749-1763

No. of pages

15

ISSN

0003-004X

Publication date

2012

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy

Portal url

https://ucrisportal.univie.ac.at/en/publications/e1a0b51a-2c81-4c6d-9059-43c940420f9e