Orientational Ordering in Solid Hydrogen: Dependence of Critical Temperature and Concentration on Density

Abstract

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 $\mathrm{Pa}3\mathrm{}$ 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 $p$-${\mathrm{H}}_2$, with increasing density the critical concentration decreases anomalously below the zero-pressure value of 0.56.