MOLECULAR REARRANGEMENTS IN TETRAMETHYLAMMONIUM TETRAFLUOROBORATE (TMT). Thomas M. Schultz, Department of Solid State Physics, Risø National Laboratory, DK-4000 Roskilde, Denmark, and Finn K. Larsen, Department of Chemistry, University of Aarhus, DK-8000 Århus C, Denmark
The thermal behaviour of tetraalkylammonium tetrafluoroborates includes series of phase transitions.1 The present study is part of an attempt to characterize the molecular rearrangements over an extended temperature range in terms of constrained rigid body motion. TMT at room temperature crystallizes in a tetragonal space group, P4/nmm. The (CH3)4N+ ions sit ordered at special positions (3/4, 1/4, 1/2) of point symmetry 42m, while the BF4- ions are placed at general positions near a 4mm axis. This imposes an eight-fold disorder on the BF4- ion. Furthermore, two equilibrium positions exist for each group. They are nearly related by a rotation of approximately 40 degrees around an axis near one of the BF bonds. A rigid body TLX refinement gives R(F) = 3.7% with 78% occupation for the major set. Lowering the temperature to 160 K, just above the phase transition at 154 K, depopulates the minor set completely, R(F) = 3.1 %.
The phase transition is accompanied by a doubling of unit cell volume and a change to monoclinic space group symmetry. Twinning is involved and most diffraction peaks split up at the phase transition. A twin model allowed to establish a fully ordered structure for data collected at 140 K.
1. Zabinska, G., Ferloni, P. and Sanesi, M. Thermochimica Acta 122 (1987) 87-94.