Pressure-induced phase transition in malayaite, CaSnOSiO4
-
Stephanie Rath
,
Martin Kunz
Abstract
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 Pc = 4.95(1) GPa. No discontinuity was observed for the individual crystallographic axes or the volume compressibility. Instead, the A2/a - A1- phase transition reveals itself by significant deviations of the α and γ 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) Å3, K0 = 121(1) GPa, and K' = 4.2(5); A1̅: V0 = 390.3(1) Å3, K0 = 118.3(7) GPa, K' = 4). Fitting the compressibility to all data gives values of V0 = 389.64(3) Å3, 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 A1̅ 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 [1 - 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 (1̅11).
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, α = 89.078(7)°, β = 112.745(5)°, γ = 91.230(7)° , V = 369.10(5) Å3; space-group A1̅.
© 2015 by Walter de Gruyter Berlin/Boston
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