NMR has been used to study the orientational behavior of registered lattices of ortho-H₂ and para-D₂ molecules adsorbed on graphite in the range 1.3-4.2 K. The results show a weak crystal field (0.56 K for o-H₂ and 2.4 K for p-D₂). In addition, p-D₂ molecules experience a temperature-dependent molecular field arising from electrostatic quadrupole-quadrupole interactions. No orientational ordering occurs down to 1.3 K, a factor of 2 below the mean-field transition temperature.
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Kubik et al. (1978) studied this question.
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