Temperature- and pressure-induced phase transition in tetrafluoro-1,4-benzoquinone crystal

The temperature and pressure effects on the Raman-active inter- and intramo lecular vibrations of tetrafluoro-1,4-benzoquinone crystal were studied. Th e diffuse Raman bands observed in the intermolecular vibrational spectrum c ould be resolved with decreasing temperature or increasing pressure. The ob served spectral structure due to the intermolecular vibrations changed as t he temperature fell and also as the pressure increased. The vibrational fre quencies of some intramolecular vibrational bands changed discontinuously u nder a certain pressure when the applied pressure was increased. The calcul ated pressure-induced frequency shifts for these vibrations deviate from th e observed frequency shifts under high pressure. The DSC curve shows no dis tinct endothermic or exothermic peaks at temperatures between 77 and 520 K. These observations indicate that a displacive phase transition takes place at a temperature of around 180 K under 1 atm or under a pressure of 0.8 GP a at 300 K in a tetrafluoro-1,4-benzoquinone crystal.