High-Pressure Compounds in Methane-Hydrogen Mixtures

The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science. Diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH4(H2)2, (CH4)2H2, CH4(H2)4, and CH4H2, at pressures up to 10 gigapascals. These have been characterized by synchrotron single-crystal x-ray diffraction, polycrystalline x-ray diffraction, Raman spectroscopy, and visual observation. Although CH4(H2)2 crystallizes in the MgZn2-type, hexagonal Laves phase structure, (CH4)H2 has a body-centered tetragonal structure that is similar to that of Al2Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.