Dominant mechanisms of phonon scattering in low-temperature phases of solid methanes

Measurements of thermal conductivity of solid deuterated methane at equilibrium vapor pressure in the temperature range 1.2-30 K are reported. Two phonon scattering mechanisms are sufficient to describe correctly the obtained dependence of thermal conductivity on temperature in the orientationally ordered phase III of the CD4. Unlike the situation in other simple dielectric crystals, the exponential decrease of thermal conductivity with increasing temperature is caused not by U processes, but rather by scattering of phonons on excitations of high-energy spin-librational levels of spatially oriented molecules. From comparison of these results with the measurements of thermal conductivity of methane (CH4) some information on temperature dependence of phonon scattering on almost freely rotating methane molecule has been inferred.