Predicted structures and superconductivity of hypothetical Mg-CH4 compounds under high pressures

According to the fact that methane has not been metalized and may be dissociated under high pressure, we present a structural characterization of compressed Mg-methane compounds obtained using first-principles molecular dynamics within density-functional theory. In this article, we propose two structures with space groups orthorhombic Pban and tetragonal P4/nmm, to be stable in the pressure ranges of 0-75 GPa and 75-120 GPa, respectively. The calculated results indicate that the orthorhombic Pban phase is a semiconductor and the tetragonal P4/nmm phase is a superconductor with T_c of 84-121 K in the pressure range of 75-120 GPa.