HIGH-PRESSURE RAMAN-STUDY OF LIQUID AND CRYSTALLINE CH3F UP TO 12-GPA

High-pressure Raman spectra of liquid and crystalline CH3F were measured 4p to 12 GPa at 300 K in a gasketed diamond-anvil cell. Two solid phases have been found; the transition pressures of liquid to solid phase I and phase I to phase II were determined to be 2.75 and 3.63 GPa, respectively. Among these, the solid phase I is an orientationally disordered (plastic) phase, while the solid phases II is an orientationally ordered phase. The frequency of CF stretching ν3(A1) vibration shows a large red shift with a slope dν/dP of about -2.6 cm-1/GPa in the liquid phase, and it splits into the TO and LO modes in the two solid phases. The Fermi resonances between the same symmetry vibrations of ν1(A 1) and 2ν5(A1), and of ν4(E) and 2v5(E) have been observed and their behaviors have been analyzed by the Fermi resonance theory. The pressure dependence of the CH stretching mode is compared with those of the other fluorinated methanes CH2F 2, CHF2Cl, and CHF3. © 1995 American Institute of Physics.