Order-disorder phenomena determined by high-resolution powder diffraction:the structures of tetrakis(trimethylsilyl)methane C[Si(CH3)(3)](4) and tetrakis(trimethylsilyl)silane Si[Si(CH3)(3)](4)

The compounds tetrakis(trimethylsilyl)methane C[Si(CH3)(3)](4) (TC) and tet rakis(trimethylsilyl) silane Si[Si(CH3)(4)], (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. Al room temp erature both structures have space group Fm(3) over bar m (Z = 4) with a = 13.5218 (1) Angstrom, V = 2472.3 (1) Angstrom(3) for TSi, and a = 12.8902 ( 2) Angstrom, V = 2141.8 (1) Angstrom(3) for TC. X-ray scattering data can b e described by a molecule with approximately sixfold orientational disorder , ruling out a structure with free rotating molecules. Upon cooling, TSi ex hibits a first-order phase transition at T-c = 225 K, as is characterized b y a jump of the lattice parameter of Delta a = 0.182 Angstrom and by an exo thermal maximum in differential scanning calorimetry (DSC) with Delta H = 1 1.7 kJ mol(-1) and Delta S = 50.0 J mol(-1) K-1. The structure of the low-t emperature phase is refined against X-ray powder data measured at 200 K. It has space group P2(1)3 (Z = 4), a = 13.17158 (6) Angstrom and V = 2285.15 (2) Angstrom(3). The molecules are found to be ordered as a result of steri c interactions between neighboring molecules, as is shown by analyzing dist ances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at T-c1 = 268 K, with Delta a(1) = 0.065 Angstrom, Delta H-1 = 3.63 kJ mol(-1) and De lta S-1 = 13.0 J mol(-1) K-1. A second first-order phase transition occurs at T-c2 = 225 K, characterized by Delta a(2) = 0.073 Angstrom, Delta H-2 = 6.9 kJ mol(-1) and Delta S-2 = 30.0 J mol(-1) K-1. The phase transition at higher temperature has not been reported previously. New NMR experiments sh ow a small anomaly in the temperature dependence of the peak positions in N MR to occur at T-c2. Rietveld refinements were performed for the low-temper ature phase measured at T = 150 K [space group P2(1)3, lattice parameter a 12.609 (3) Angstrom], and for the intermediate phase measured at T = 260 K [space group Pa(3) over bar, lattice parameter a 12.7876 (1) Angstrom]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. Tn the intermediate phase the molecules exhibit a twofold or ientational disorder.