Raman spectra of solid CH4 under pressure. I. Phase transition between phases II and III

Raman spectra of the intramolecular vibrations in solid CH4 have been observed at 4.2 K under pressure. The observed ν1−ν3 vibration–rotation band structures in phase II agree well with previous experimental and theoretical data. The ν4 band was detected in both phases II and III. A remarkable change of each spectrum was found at 0.4 kbar, indicating the II–III phase transition. The rotational states of methanes in phase III have been deduced from the observed data. It is concluded that in phase III there are at least three kinds of sites with different site symmetries.