Pressure-induced Fermi resonance between fundamental modes in phthalic anhydride.

In situ high-pressure Raman spectra of phthalic anhydride (PA) have been measured up to 16 GPa through diamond anvil cell technique. The results show that all the Raman bands are blue-shifted with the increase of pressure, accompanied by appearance of some new bands. A Fermi resonance phenomenon of the two Raman fundamental modes of PA at 773 cm -1 and 801 cm -1 is proposed at pressures above 6.6 GPa, where a possible first-order phase transition occurs. The pressure-induced changes of Fermi resonance parameters, e.g., intensity ratio, coupling coefficient and frequency gap of unperturbed transition, are discussed.