PRESSURE EFFECT ON THE INTERMOLECULAR AND INTRAMOLECULAR VIBRATIONS OF 1,2,4,5-TETRACHLOROBENZENE CRYSTAL

The effect of pressure on the frequencies and band structures of the R aman active inter- and intramolecular vibrations of the 1,2,4,5-tetrac hlorobenzene crystal was studied under hydrostatic pressures upto 4 GP a at 300 K in a gasketed diamond anvil cell. Changes of the Raman freq uencies and band structures of the vibrations suggest that the phase t ransition takes place under about 0.7 GPa. The frequency shift of the intramolecular vibrations induced by pressure was calculated using an intermolecular potential of the atom-atom type based on the two-site m olecular exciton theory. The calculation shows that the observed press ure-induced frequency shift is mainly caused by the repulsive force be tween the chlorine atoms belonging to the adjacent molecules.