Evidence from x-ray diffraction of orientational ordering in phase III of solid hydrogen at pressures up to 183 GPa

X-ray powder-diffraction experiments on solid hydrogen are performed at pressures up to 183 GPa and low temperature of 100 K. A transition into phase III is confirmed by in situ Raman-scattering measurements. Two diffraction peaks corresponding to the 100 and 101 lines of the hcp structure are observed above the II-III phase-transition pressure of 160 GPa, indicating that the hydrogen molecules in phases II and III are still in the vicinity of the hcp lattice point up to 183 GPa. Assuming a hexagonal structure for these phases, the lattice parameter, c decreases by 1.5% at the II-III phase transition. The behavior is consistent with a theoretically proposed feature of classical orientational ordering of rotating hydrogen molecules for phase III.