The cubic/tetragonal phase transition in D3C-THF: An optical and x-ray powder diffraction study

The high-temperature phase transition in the clathrasil Dodecasil-3C (17SiO(2) .(CH2)(4)O) has been studied by high-temperature, high-resolution powder diffraction and by following the temperature evolution of the optical birefringence. The transition changes the crystal system from cubic to tetragonal at 359(1) K. A detailed discussion of the symmetry properties of the phase transition is given. The transition behavior was characterized by group theoretical considerations as well as in the framework of Landau theory. The phase transition could be classified as improper ferroelastic and of first order. In addition to the tetragonal ferroelastic lattice strain, a symmetry-allowed volume reduction could be observed. (C) 1998 Academic Press.