Deductions about the structure of phase III from thermodynamic measurements on solid isotopic methanes

Abstract

In a continuing study of the solid isotopic methanes, the heat capacity of solid CHD₃ has been measured in the range 0.15<TK. Some structure is found in a Schottky anomaly in the region of the measurements and it is related to the composition of nuclear spin symmetry species in the solid. There is no evidence of spin conversion. The entropy of CHD₃ has been calculated as a function of temperature from the heat capacity and other data, and is used to make deductions about quantum disorder in the solid. Combined results for CH₃D, CH₂D₂ and CHD₃ lead to the conclusion that the structure of phase III of solid methane is quantum disordered and that it must contain at least three types of sublattice. A model consisting of two sublattices with tetrahedral molecular fields and one with symmetry lower than tetrahedral, accounts for the experimental observations satisfactorily.