Structural studies of solid methane at high pressures

High-pressure X-ray powder diffraction measurements for solid methane were performed at pressures up to 13 GPa. The diffraction pattern at 7.0 GPa is assigned to a rhombohedral lattice with lattice constants of a=8.643(1) Å and α=89.40(2)° and with a unit cell that contains 21 molecules. The bulk modulus of the rhombohedral phase is estimated to be 7.9(3) GPa.