Orientational order in solid hydrogen at reduced ortho concentration

The splitting of the NMR line in solid hydrogen has been observed during 145 h while it aged at temperatures below 0.1°K. For ortho-hydrogen mole fractions $X$ less than 0.55, the NMR splitting is present but disappears abruptly at transition temperatures fitting a straight line $T_c=(0.54\mathrm{}\pm 0.06)X+(0.02\pm 0.02)$. The concentration dependence of the splitting is compared to a model which includes only a zero-point admixture to the $J_{\zeta }=0\mathrm{}$ ground state from the $J_{\zeta }=\pm 1$ wave functions of the libron band, and a zero-point admixture of the $J=3\mathrm{}$ state, both of which admixtures scale according to the observed concentration dependence of the transition temperature.