The paper presents an analysis of extensive data set of mechanical, structural, thermophysical, and spectral properties of solid methane in the temperature interval 0.5？·？Ttr–Ttr (Ttr is the triple point temperature) under equilibrium vapor pressure. It is shown that the anomalies of the studied properties (or lack of reliable data) at temperatures 60–70？K have been observed in the body of the reviewed papers. We proposed that the observed anomalies are due to a transition between classical and quantum regimes of collective rotational degrees of freedom of methane molecules in this temperature interval. 1. Introduction The development of far cosmos research and discovery of atmospheres at some planets of the Solar System have inspired new large interest to mechanical and thermodynamic properties of solidified gases, in particular, solid methane [1]. Solid methane is the lightest representative among the group of the simplest molecular crystals, formed by tetrahedral molecules with 4-3m symmetry [2], where X is hydrogen isotopes or halogen atoms F, Cl, and so forth or their isotopes. In this paper, we present a detailed analysis of the experimental data of many authors on mechanical, structural, thermophysical, and spectral properties of solid methane in temperature interval ( is the triple point temperature) under equilibrium vapor pressure. The body of reviewed papers shows the presence of anomalies of above properties at temperatures 60-70？K, or the reliable data are absent. We suppose that these anomalies are caused by a sudden change of rotational degrees of freedom of molecules at temperatures between 60 and 70？K [3]. 2. Some Data on Thermodynamic Properties of Solid Methane Methane is the most examined crystal of group. The well-known reviews [4, 5] have presented extensive data on thermodynamic, structural, and mechanical characteristics of solid methane. However, the anomalies of a series of solid methane properties at temperatures higher than 0.5 were not mentioned in these reviews. Under equilibrium vapor pressure, crystalline methane (triple point temperature ？K [4]) undergoes a -type phase transition at ？K [3]. In both phases, low-temperature II and high-temperature I, carbon atoms form a FCC lattice (space group Fm3m [4]). It is commonly supposed that in low-temperature phase carbon atoms are ordered in the lattice (expected space group Fm3c or P4-3m [4]) and make certain librational vibrations relative to the center of inertia of methane molecule and also reorientational hopping from one equilibrium position to another. In phase I,
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