Structure, twinning, and ferroelasticity of tetramethylammonium-hexafluoroplatinate(IV), (TMA)2[PtF6]

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Abstract
(TMA)2[PtF6] is found to be tetragonal at room temperature (space group I4/m, a = 8.073(9) Å, c = 11.401(2) Å). The structure is related to the K2[PtCl6] structure type (space group Fm[unk]m) and is characterized by a ferrorotative tilt of the [PtF6] octahedra by an angle of 12.6(1)° and of the TMA tetrahedra of 12.1(2)° with respect to their position in the cubic aristotype structure. The double refraction (amounting to −0.0019(1) at room temperature) decreases parabolically, when the crystal is heated, and vanishes at 436 K indicating a phase transition to the cubic K2[PtCl6] structure type. Twinning with domains having straight twin boundaries and differently directed tetragonal c-axes (pseudo-cubic axes) sometimes occurs in as-grown crystals. Ferroelastic behaviour can be demonstrated under the polarizing microscope by application of external stress: (i) domain boundaries can be moved through the crystal; (ii) if a cube-shaped crystal is pressed perpendicular to the c-axis, the c-axis switches into the direction of pressure.