Solid methane has been a favourite subject for the study of phase transitions ever since the discovery by Clusius [1] of the phase transition α → β at 20.4 K in CH4. The reasons for the sustained interest in solid CH4 may be attributed to the following facts.
i. The CH4 molecule has an almost spherical shape with a slight TD anisotropy. As a result, CH4 crystallizes below 90.7 K into a plastic crystal, α, of cubic symmetry, exhibiting almost complete rotational freedom of the molecules.
ii. The CH4 molecule is very small in size with a low moment of inertia (I × 1040 g cm2 = 5.34 in CH4 and 10.64 in CD4), leading to a large rotational kinetic energy and related quantum effects at low temperature.
iii. The protons in CH4 and the deuterons in CD4 carry a nuclear spin resp. Ip = 1/2 and I. = 1, which gives rise to spin-isomerism. Thus, CH4 has three spin varieties, meta (A1, In = 2), ortho (T2, in = 1) and para (E, In = 0). The isomerism may be expected to have profound influence on the phase diagram of solid methane.
The Crystal to Plastic Crystal Phase Transition of Tetraethylammonium Difluorophosphate and Tetrafluoroborate
