Raman scattering study of monoclinic and cubic tetracyanoethylene under high pressures

Abstract

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-Grüneisen parameters are compared with the experiments, and Zallen's vibrational scaling law has been successfully investigated.