Theory of Phase Transitions in Solid Methanes. II. Nuclear Spin Species A and T of Light Methane

The statistical mechanical properties of the James–Keenan model are studied with the purpose of elucidating the mechanism of phase transitions in the solid light methane. All the calculations are made on the basis of quantum statistical mechanics in the subspace with $\text{J\hspace{0.17em}≦\hspace{0.17em}4,}$ $J$ being the rotational quantum number. The molecular field approximation method is employed. The low and high temperature‐ordered phases are assumed to have the same sublattice structure as that which James and Keenan predicted in their classical theory. Under the provisional assumption of no interspecies conversion, nuclear spin species $A$ and $T$ are treated separately. The results of the numerical calculations indicate that the James–Keenan model is quite hopeful as a working model to understand the nature of the phase transitions in the whole family of the isotopic solid methanes.