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 tetrakis(trimethylsilyl) silane Si[Si(CH3)(4)], (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. Al room temperature 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 be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at T-c = 225 K, as is characterized by a jump of the lattice parameter of Delta a = 0.182 Angstrom and by an exothermal maximum in differential scanning calorimetry (DSC) with Delta H = 11.7 kJ mol(-1) and Delta S = 50.0 J mol(-1) K-1. The structure of the low-temperature 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 steric interactions between neighboring molecules, as is shown by analyzing distances 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 Delta 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 show a small anomaly in the temperature dependence of the peak positions in NMR to occur at T-c2. Rietveld refinements were performed for the low-temperature 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 orientational disorder.