Structure and compression of crystalline methane at high pressure and room temperature

Methane, CH4, crystallizes in the face-centered cubic metal structure (space group Fm3m) at 15.9 kbar and 20 °C. Cubic unit-cell edges at 16.1, 28.9, 39.5, and 52.1 kbar are 5.4434, 5.3064, 5.1963, and 5.0947 Å, respectively. The isothermal bulk modulus, calculated from a first-order Murnaghan equation of state (K′=4), is 49±9 kbar. Methane transforms to a second as yet unidentified high-pressure solid phase at 52.5 kbar (20 °C).