  Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first
principles calculations have been used to identify the structure of the
high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The
structure of HP phase is trigonal R3, which can be represented as a distortion
of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The
positions of hydrogen atoms in HP phase have been 19 obtained from first
principles calculations. The molecules occupy four different crystallographic
sites in phases B and eleven sites in the HP phase, which result in splitting
of molecular stretching modes detected in Raman spectroscopy and assigned here
based on 22 a good agreement with the Raman spectra calculated from the first
principles.
