  We have performed a combined experimental and theoretical study of ethane and
methane at high pressures up to 120 GPa at 300 K using x-ray diffraction and
Raman spectroscopy and the USPEX ab-initio evolutionary structural search
algorithm, respectively. For ethane, we have determined the crystallization
point, for room temperature, at 2.7 GPa and also the low pressure crystal
structure (Phase A). This crystal structure is orientationally disordered
(plastic phase) and deviates from the known crystal structures for ethane at
low temperatures. Moreover, a pressure induced phase transition has been
identified, for the first time, at 13.6 GPa to a monoclinic phase B, the
structure of which is solved based on a good agreement of the experimental
results and theoretical predictions. For methane, our XRD measurements are in
agreement with the previously reported high-pressure structures and EOS. We
have determined the equations of state of ethane and methane, which provides a
solid basis for the discussion of their relative stability at high pressures.
