research papers
The structure of dodecasil 3C-tetrahydrofuran [Si68O136]·4M, M = (CH2)4O, at room temperature was determined from a merohedrally twinned crystal in the tetragonal space group I41/a. The deformation of the ideal framework at the cubic tetragonal phase transition at Tc ≃ 365 K could be explained mainly by two different symmetry-breaking processes. (i) A tetragonal tetrahedron distortion of the Si(5) tetrahedra and (ii) a hitherto unknown local one-dimensional tilt mechanism, localized in the tetrahedral network. The location of the axes of this tilt system coincides with the positions of the fourfold inversion axes in the space group I41/a. At room temperature the tilt angle is = 24°. The symmetry properties of the tilt system can explain the reduction of space-group symmetry from the space group of the ideal structure Fdm to the space group at ambient conditions I41/a. The guest molecule tetrahydrofuran does not fit the cage symmetry and has been found to be dynamically disordered. The average structure shows an off-center location in the [51264] cage and follows the local symmetry of the cage.
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CCDC reference: 131618