The hexagonal-to-cubic-to-hexagonal structure changes in hydrogen and deuterium were studied by x-ray diffraction over many cycles of the transitions for orthohydrogen and paradeuterium concentrations up to 95%. Results of the x-ray study can be closely correlated with those of infrared, neutron-diffraction, heat-capacity, nuclear-magnetic-resonance, and volume-change measurements. The structure changes take place by a shifting of the hexagonal nets, which appears to be incomplete after the first transition and may cause intermediate close-packed structures to form. Repeated cylcing through the transition stabilizes the cubic structure, possibly because orientation of the $J=0\mathrm{}$ molecules takes place.

• Received 11 September 1967

DOI:https://doi.org/10.1103/PhysRev.165.1032