The crystal structure of methane B at 8 GPa-An α-Mn arrangement of molecules

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of similar to 8 GPa. We find this substructure to be cubic with space group I (4) over bar 3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) angstrom at 8.3(2) GPa, which is a factor of root 2 larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter 14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of a-manganese at ambient conditions. (C) 2014 AIP Publishing LLC.