Room-Temperatrue Structure and Geometrical Analysis of the Cubic–Tetragonal Phase Transition in Dodecasil 3C-THF

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 T_c ≃ 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.

-1