We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies 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 good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.
A.F.G. and E.S. acknowledge support from the Army Research Office (Grant Nos. 56122-CH-H and 71650-CH) and the Deep Carbon Observatory. S.S.L. acknowledges the support from the Helmholtz Young Investigators Group CLEAR (Grant No. VH-NG-1325). Part of this work was performed at GeoSoilEnviroCARS (the University of Chicago, Sector 13), Advanced Photon Source (APS), and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation Earth Sciences (Grant No. EAR-1634415) and the Department of Energy GeoSciences (Grant No. DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE), Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. A.R.O. and A.A.M. thank Russian Ministry of Science and Higher Education (grant H III-2711.2020.2 to leading scientific schools) for support. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Part of this research was carried out at PETRA III (beamline P02.2). The research leading to these results received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 312284.