Structure and ferroelasticity of tetramethylammonium-hexafluoroosmate(IV), (TMA)(2)[OsF6]

(TMA)(2)[OsF6] has at room temperature a rhombohedral structure (space group R (3) over bar) which is pseudocubic (K-2[PtCl6]-type). a = 8.0565(6) Angstrom, r = 20.020(3) Angstrom (hexagonal setting). Double refraction decreases linearly with increasing temperature and disappears above 434 K which indicates cubic symmetry of the high temperature phase. The change from the cubic high temperature structure to the rhombohedral structure is characterized by a ferrorotative tilt of the [OsF6] octahedra and the TMA tetrahedra. Evidence of ferroelasticity follows from the switching of the trigonal c-axis by the application of directional pressure. The phase transitions of (TMA)(2)[OsF6] (Fm (3) over barm --> R (3) over bar) and of the recently reported (TMA)(2)[PtF6] and (TMA)(2)[IrF6] (Fm (3) over barm --> I4/m) are described by Landau's theory.