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Pressure-induced phase transition in malayaite, CaSnOSiO₄

¹ Labor für Kristallographie, ETH Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
² Museum of Natural History, Augustinergasse 2, CH-4001 Basel, and Department of Earth Sciences, University of Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
³ Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236,D-69120 Heidelberg, Germany

A single crystal high-pressure diffraction study in a diamond-anvil cell shows that monoclinic malayaite (CaSnOSiO₄) transforms into a triclinic high-pressure polymorph at P_c = 4.95(1) GPa. No discontinuity was observed for the individual crystallographic axes or the volume compressibility. Instead, the A2/a – A phase transition reveals itself by significant deviations of the and angles from 90°. The bulk elastic properties of the triclinic phase cannot be distinguished from those of the monoclinic structure within experimental uncertainty (A2/a: V₀ = 389.68(3) ų, K₀ = 121(1) GPa, and K’ = 4.2(5); A: V₀ = 390.3(1) ų, K₀ = 118.3(7) GPa, K’ = 4). Fitting the compressibility to all data gives values of V₀ = 389.64(3) ų, K₀ = 121.6(7) GPa and K’ = 4.6(2). Structure refinements at four different pressures reveal the structural details of the monoclinic A2/a and triclinic A phases. Below the transition temperature the SiO₄ polyhedra show some non-rigid distortion, whereas the SnO₆ polyhedra remain almost unchanged. At the phase transition, the SiO₄ tetrahedra show further angular twisting while the SnO₆ chains shift parallel to [01], inducing a reduction in symmetry. Furthermore, at pressures above 5 GPa the Ca atoms are displaced almost parallel to [100], causing a change in coordination from CaO₇ polyhedra to sheets of CaO₈ parallel to (11).

At 7.394(4) GPa the cell parameters of the triclinic structure are a = 6.9958(4) Å, b = 8.8080(9) Å, c = 6.4968(4) Å, = 89.078(7)°, ß = 112.745(5)°, = 91.230(7)° , V = 369.10(5) ų; space-group A.

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