Pressure-induced phase transition in malayaite, CaSnOSiO4

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).