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 CH 4 -H 2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH 4 (H 2 ) 2 , (CH 4 ) 2 H 2 , CH 4 (H 2 ) 4 , and CH 4 H 2 , 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 CH 4 (H 2 ) 2 crystallizes in the MgZn 2 -type, hexagonal Laves phase structure, (CH 4 )H 2 has a body-centered tetragonal structure that is similar to that of Al 2 Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.