Abstract
The paraelectric to antiferroelectric phase transition in titanite at ∼500 K involves a displacement of the titanium atom from the center of the [TiO6] octahedron in the paraelectric phase (A2/a) to an off-center position in the antiferroelectric (P2 1/a) phase. We have carried out a detailed single crystal high temperature x-ray diffraction study of the phase transition including structure refinements at 294, 350, 400, 430, 440, 450, 500, 600, and 700 K. The unit cell dimensions show a pronounced hysteresis effect in the 450–500 K range on heating and cooling during the first cycle along with a reduction of the transition temperature, T c from 495 ± 5 K on heating to 445 ± 5 K on cooling. The hysteresis effect disappears on further heating and the superstructure reflections show residual intensities above T c (445 K). An order parameter treatment of the phase transition is presented in terms of Landau theory and induced representation theory. The Ti-displacements parallel and antiparallel to a are taken as the primary order parameter η, which transforms as the Y +2 representation. A coupling of Y +2 with T +1 results in the linear-quadratic coupling of the spontaneous strain components, ɛ ij with η. The Ti-displacements are coupled linearly to the Cadisplacements. Both sets of displacements predicted from induced representation theory are observed experimentally. The phase transition is initially driven by the soft mode at the zone boundary point Y +2 ; near T c critical fluctuations set in and an order-disorder mechanism finally drives the phase transition, whereby parallel and antiparallel Ti-displacements related by [0, 1/2, 1/2] in adjacent domains are dynamically interchanged. Immediately above T c , the high temperature (A2/a) phase is a statistical average of small dynamic antiphase domains of the low temperature (P2 1/a) phase. Vacancies and defects pinning the domain boundaries may drastically alter the transition behavior and affect the domain mobility.
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Ghose, S., Ito, Y. & Hatch, D.M. Paraelectric-antiferroelectric phase transition in titanite, CaTiSiO5 . Phys Chem Minerals 17, 591–603 (1991). https://doi.org/10.1007/BF00203838
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DOI: https://doi.org/10.1007/BF00203838