PHASE TRANSITION IN DI-TETRAMETHYLAMMONIUM HEXAFLUORO-PLATINATE(IV). H. Küppers⁽¹⁾, C. Bruhn⁽²⁾, and W. Preetz⁽²⁾. ⁽¹⁾Mineralogisches Institut, ⁽²⁾Institut für Anorganische Chemie, Universität Kiel, D-24098 Kiel, Germany.

(TMA)₂[PtF₆] has been reported as being cubic (Fm3m) and belonging to the K₂[PtCl₆]type. Crystals grown at 5^deg.C from a water-acetone solution exhibit quasi-cubic morphology (cube, octahedron, rhombic dodecahedron). Between crossed polarizers they show, however, interference colours in accord with tetragonal symmetry (tetragonal c axis pointing along one of the quasi-cubic axes). The refractive indices n_o = 1.4788(6) and n_e =1.4768(6) were measured at 589 nm on a spindle stage. The double refraction (measured with an Ehringhaus compensator) amounts to -0.0020(2) at room temperature and disappears, on heating, at 164(2)^deg.C, following a parabolic decay with a critical exponent [[gamma]] = 0.58. DTA measurements show a small endothermal peak at 163(2)^deg.C. Twins or trillings are often observed with sharp {110} twin boundaries. Application of stress induces ferroelastic switching. In particular, uniaxial pressure along [100] causes the change from a to c. Lattice parameters, referred to the quasi-cubic face-centred cell, determined at room temperature, differ only slightly : a = 11.420(4), c = 11.406(4) Å. A preliminary single crystal X-ray structure analysis shows only small deviations from the K₂[PtCl₆] structure. Equatorial fluorine atoms have enlarged displacement parameters. The hydrogen atoms of the methyl groups are directed towards the fluorine atoms with distances C...F = 3.36Å.