HIGH-PRESSURE RAMAN-STUDY OF DENSE METHANE - CH4 AND CD4

High-pressure Raman spectra of liquid and solid CH4 and CD4 were measured up to 18 and 13 GPa, respectively, at 300 K in a gasketed diamond-anvil cell. On the basis of visual observations under the polarizing microscope and changes in the Raman spectra, it was confirmed that there are only two solid phases (I and VII) for CH4 and CD4 at 300 K up to 18 GPa; no solid-solid phase transition was found around 12 GPa, in contrast to earlier claims. In phase VII, some molecules are orientationally ordered, whereas others are orientationally disordered, just like the low-temperature solid phases II, III, IV and V.