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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

Title data

Dinnebier, Robert E. ; Dollase, Wayne A. ; Helluy, Xavier ; Kümmerlen, Jörg ; Sebald, Angelika ; Schmidt, Martin U. ; Pagola, Silvina ; Stephens, Peter W. ; van Smaalen, Sander:
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.
In: Acta Crystallographica Section B. Vol. 55 (1999) Issue 6 . - pp. 1014-1029.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768199006126

Abstract in another language

The compounds tetrakis(trimethylsilyl)methane C[Si(CH₃)₃]₄ (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH₃)₃]₄ (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm3m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) ų for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) ų 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 Tc = 225 K, as is characterized by a jump of the lattice parameter of Δa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with ΔH = 11.7 kJ mol-1 and ΔS = 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 P213 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å3. 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 Tc1 = 268 K, with Δa1 = 0.065 Å, ΔH1 = 3.63 kJ mol-1 and ΔS1 = 13.0 J mol-1 K-1. A second first-order phase transition occurs at Tc2 = 225 K, characterized by Δa2 = 0.073 Å, ΔH2 = 6.9 kJ mol-1 and ΔS2 = 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 Tc2. Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P213, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa3, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

Further data

Item Type: Article in a journal
Refereed: Yes
Keywords: Domain size broadening; Molecular modelling; Order-disorder; Powder diffraction
Institutions of the University: Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography
Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography > Chair Crystallography - Univ.-Prof. Dr. Sander van Smaalen
Faculties
Faculties > Faculty of Mathematics, Physics und Computer Science
Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences
Result of work at the UBT: Yes
DDC Subjects: 500 Science > 530 Physics
Date Deposited: 10 Jun 2016 07:20
Last Modified: 10 Jun 2016 07:20
URI: https://eref.uni-bayreuth.de/id/eprint/32567