The thermal conductivity of (CH 4 ) 1-c (CD 4 ) c solid solutions with c = 0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 has been measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II) and completely ordered (phase III). The temperature range is 1.3-30 K. It is shown that the thermal conductivity has different temperature dependences k( ) T in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in k( ) T is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the k( ) T curve of phase II approaches the dependence k( ) T typical of phase I. There is a hysteresis in the vicinity of the II«III phase transition. In phase III the impurity effect in k( ) T can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH 4 and CD 4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH 4 -CD 4 system. PACS: 63.20.-e Phonons in crystal lattices; 66.70.+f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves.
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 CD 4 . 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 ͑CH 4 ͒ some information on temperature dependence of phonon scattering on almost freely rotating methane molecule has been inferred.
Some mixed crystals show a low-temperature thermal-conductivity dependence on temperature similar to that of amorphous solids. This phenomenon is usually ascribed to a disorder of another kind. In the current paper we report results of measurements of the thermal conductivity of CH4-CD4 solid solution which also displays a glass-like behaviour for some concentrations of deuterated methane. Here, however, the disorder appearing at these concentrations of CD4 cannot account for the characteristic plateau. Instead, we explain it in terms of an enhancement of the phonon-rotation interaction resulting from lowering the symmetry of the molecular field.
The paper reports preliminary results on the kinetics of the molecular spin conversion in solid methane (CH 4 ) and deuterated methane (CD 4 ), obtained through thermal conductivity measurements in the temperature range 2-10 K.PACS: 63.20.-e Phonons in crystal lattices; 66.70.+f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves.
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