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The crystal structure of the symmetric 1,3,5-trichloro-2,4,6-trifluorobenzene, C6CIaF a, was determined at room temperature by single-crystal X-ray diffraction and at 4.2 K by neutron powder diffraction. The space group is P63/m with a hexagonal primitive cell of dimensions a = b = 8.541 (3) and c = 6.282 (2)/k [which become a = b = 8.441 (2) and c = 6.052 (2)/k at 4.2 K], V = 397/k 3 and Z = 2. The cell contains two planar molecules at 6 (2d) sites. The single-crystal R factor of 3.9% was reached with unconstrained refinement on 401 symmetry-independent Bragg intensities. The structure was solved using constrained refinement on the molecular orientation. Constrained refinement on the thermal parameters has indicated significant internal vibrations in the molecule. The low-temperature study showed there to be no phase transition when the powder is cooled.

Raman scattering spectra of the cubic and monoclinic phases of tetracyanoethylene in the external-mode region have been investigated as a function of hydrostatic pressure in a diamond-anvil cell. There is evidence of a possible transition from the cubic phase to a new unknown phase at 13 kbar. The intensities of the Raman spectra are found to vanish at high pressures. Calculation of phonon frequencies as a function of pressure has been carried out in both the cubic and monoclinic phases using atom-atom potentials with a semi-rigid molecule model which takes account of both the external vibrations and those internal vibrations up to 300 cm -1. The calculated frequencies and mode-Gr/ineisen parameters are compared with the experiments, and Zallen's vibrational scaling law has been successfully investigated.