Theory of Phase Transitions in Solid Methanes. VIII

The effects of rotational ordering on the thermal expansion in solid CH4 are studied on the basis of the quantum-statistical mechanics of the James-Keenan model. We assume that solid methane is an equilibrium mixture of nuclear spin species. Then it is shown that the thermal expansion coefficient becomes negative below 11K, in agreement with experiment. This is understood as a direct result of interspecies conversion. As the temperature is lowered, more and more molecules convert into species A, so that the rotational ordering tends to be destroyed and thus its effect of squeezing the lattice is weakened. The anomaly just below the transition temperature is also discussed.

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