ORIENTATIONAL ORDERING IN SOLID HYDROGEN - DEPENDENCE OF CRITICAL-TEMPERATURE AND CONCENTRATION ON DENSITY

The orientational ordering phase transition in solid hydrogen has been studied as a function of para concentration and pressure up to 6 kbar. Measurements are consistent with the Pa3 structure for all densities and high ortho concentration. The density dependence of the critical temperature is consistent with that for the electric quadrupole-quadrupole interaction. When doped with p-H2, with increasing density the critical concentration decreases anomalously below the zero-pressure value of 0.56. © 1979 The American Physical Society.