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The isochoric heat conductivity in plastic phases I and II solid cyclopentane(C5H10) is investigated. Various temperature dependents of heat conductivity indicate various characteristics of a rotary motion of molecules in these phases. In phase II, the motion of the molecules has mainly character of libration, and in phase I, is close to the hindered rotation. In a phase I of cyclopentane an increase in the slope of isochoric heat conductivity with the growth of the density of samples is revealed. Such behavior of heat conductivity is not characteristic of molecular crystals and can point to a contribution of “pseudorotation” to heat transfer. In phase II, cyclopentane the behavior of heat conductivity is close to a case of strong scattering of phonons.

Phase V– T diagrams of solid halomethanes R12 (CF2Cl2), R20 (CHCl3), R22 (CHF2Cl), R23 (CHF3), and R30 (CH2Cl2) were constructed using both literature and our experimental data obtained during the study of isochoric thermal conductivity. The carbon tetrachloride V– T phase diagram was presented earlier. The jumps in molar volume during melting and the boundaries of the existence of phases have been determined. At pressures up to several kbar halomethanes R12, R20, R23, and R30 exist in only one crystal modification, whereas R22 shows a shift type phase transition near 59 K, and the high-temperature phase of CCl4 is dynamically orientationaly disordered. Experimental values of thermal pressures ( dP/ dT) V and dPm/ dT on melting line were measured, and they are compared with those calculated by the Clapeyron-Clausius formula.

Phase V– T diagrams of freons of the ethane series R152a (CH3-CHF2), R134a (CF3-CH2F), and hexafluoro-ethane (C2F6) were constructed using both literature and our own experimental data. The ethane V– T phase diagram was presented earlier. The jumps in the molar volume during melting and the boundaries of the existence of dynamically orientationally disordered phases have been determined. The V– T phase diagrams of freons of the ethane series turned out to be similar in type, with the exception of ethane, which has three phases near melting.

The isochoric thermal conductivity of Kr₀.₆₃Xe₀.₃₇ solid solution has been investigated for three samples with different molar volumes in the temperature range from 80 K to the onset of melting, as well as the thermal conductivity of the ternary (Kr₀.₆₃Ar₀.₁₈₅Xe₀.₁₈₅) and quaternary (Kr₀.₆₃Ar₀.₁₂Xe₀.₁₂(CH₄)0.₁₃) solid solutions. It is found that for multi-component alloys, the thermal conductivity reaches a minimum, whereby adding more impurity atoms in the solid solution does not affect the thermal conductivity in agreement with the recent nonequilibrium molecular dynamics calculations for argon. The phonon contribution to the total thermal conductivity depends significantly on molar volume of sample and impurity concentration, in contrast to the contribution of diffusive modes, which is practically independent of these factors. Досліджено ізохорну теплопровідність твердого розчину Kr₀.₆₃Xe₀.₃₇ для трьох зразків з різними молярними об’ємами в інтервалі температур від 80 К до початку плавлення, а також теплопровідність потрійного (Kr₀.₆₃Ar₀.₁₈₅Xe₀.₁₈₅) та чотирьохкомпонентного (Kr₀.₆₃Ar₀.₁₂Xe₀.₁₂(CH₄)0.₁₃) твердих розчинів. Виявлено, що для багатокомпонентних розчинів теплопровідність досягає мінімуму, в результаті чого подальше додавання домішкових атомів в твердий розчин не впливає на теплопровідність відповідно до недавніх розрахунків з використанням методів нерівноважної молекулярної динаміки (NEMD) для аргону. Фононний внесок в теплопровідність істотно залежить від молярного об’єму зразка та концентрації домішки, на відміну від вкладу дифузних мод, який практично не залежить від цих чинників Исследована изохорная теплопроводность твердого раствора Kr₀.₆₃Xe₀.₃₇ для трех образцов с разными молярными объемами в интервале температур от 80 К до начала плавления, а также теплопроводность тройного (Kr₀.₆₃Ar₀.₁₈₅Xe₀.₁₈₅) и четырехкомпонентного (Kr₀.₆₃Ar₀.₁₂Xe₀.₁₂(CH₄)0.₁₃) твердых растворов. Обнаружено, что для многокомпонентных растворов теплопроводность достигает минимума, в результате чего дальнейшее добавление примесных атомов в твердый раствор не влияет на теплопроводность в соответствии с недавними расчетами с использованием методов неравновесной молекулярной динамики (NEMD) для аргона. Фононный вклад в теплопроводность существенно зависит от молярного объема образца и концентрации примеси, в отличие от вклада диффузных мод, который практически не зависит от этих факторов.

Isochoric thermal conductivity of solid thiophene is measured in “plastic” phase I, on three samples with different molar volumes at temperatures ranging from 170 K to the melting point. The thermal conductivity increased with temperature for all samples. We conducted a comparison with the isochoric thermal conductivity of other cyclic hydrocarbons in “plastic” phases. It is shown that the observed effect correlates to the changes in the magnitude of the NMR second moment in the “plastic” phases, and, respectively, with the degree of disinhibition in the rotational motion of molecules.

The isochoric thermal conductivity of solid tetrahydrofuran C4H8O (a cyclic hydrocarbon with pseudorotational motion of molecules) was studied using samples of different density in the temperature range 120−180 K. For all the samples, the isochoric thermal conductivity was found to decrease with temperature following a much weaker dependence than 1/T. The experimental data were interpreted in terms of the Debye model of thermal conductivity considering heat transfer by both low-frequency phonons and “diffuse” modes. No significant contribution to the thermal conductivity by pseudorotation motion of the molecules was found.

Thermal conductivity of solid furan has been measured at isochoric conditions in the high-temperature orien-tationally-disordered phase I for samples of different densities. Our isochoric data show a gradual increase of ΛV with temperature whereas isobaric thermal conductivity goes down in this temperature range. The above effect is most clearly expressed in furan, where the atoms in the ring plane are not equivalent, as compared with earlier studied C₆H₆ and C₆H₁₂. The increase of ΛV with temperature can be attributed to weakening of the translation-al-orientational interaction which, in turn, leads to a decrease of phonon scattering on rotational excitations. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both low-frequency phonons and ―diffuse‖ modes.

The isochoric thermal conductivity of solid n-hexane C6H14 is studied using three samples with different densities for temperatures ranging from 100 K to the onset of melting. In all cases, the isochoric thermal conductivity varies more weakly than Λ∝1/T. The present results are compared with the thermal conductivities of other representatives of the n-alkanes. The contributions of low-frequency phonons and “diffuse modes” to the thermal conductivity are calculated.

The thermal conductivity of solid hexafluoroethane C2F6 is investigated in three samples of different density in the temperature interval from 90K up to the onset of melting. The isochroic thermal conductivity in the β phase is practically constant immediately after the α→β transition, and as the temperature is increased further it grows weakly on up to the onset of melting. This behavior is due to weakening of the translation-orientation interaction as the rotational motion of the C2F6 molecules around the C–C axis becomes less hindered.

The thermal conductivity of solid ethane (C2H6) is investigated for three samples of different density in the temperature range from 35K to the onset of melting. In all cases the isochoric thermal conductivity in the α and β phases decreases with increasing temperature by a dependence weaker than Λ∝1∕T. Such behavior is typical for orientationally ordered phases of molecular crystals and does not reveal any clear features due to rotation of the methyl groups. The thermal conductivities of ethane and some of its halogen derivatives (freons) are compared. The experimental results are discussed in the framework of a model in which heat is transported by low-frequency phonons, and, above the mobility edge, by high-frequency “diffusive” modes.

The dependence of the thermal conductivity Λ of solid solutions of cryocrystals on the molar volume is investigated theoretically in the Callaway approach under the assumption that the heat is transferred by mobile low-frequency phonons and, above the phonon mobility edge, by “localized” modes which migrate randomly from site to site. The phonon mobility edge ω0 is found from the condition that the phonon mean free path, which is governed by umklapp processes and scattering on point defects, cannot become smaller than one-half the phonon wavelength. The Bridgman coefficient g=−(∂ ln Λ/∂ ln V)T is the weighted mean over these modes, which have strongly different volume dependences. For the example of the solid solution Kr1−c(CH4)c, where c is the mole fraction of the components, it is shown that with increasing methane concentration in krypton the Bridgman coefficient decreases, in good agreement with experiment, from g≈9, characteristic for the pure crystals, to g≈4.

The isochoric thermal conductivity of (CH4)1−ξKrξ solid solutions is studied between 40 K and ∼150 K over a wide range of concentrations (ξ=0.013, 0.032, 0.07, 0.115, 0.34, 0.71, 0.855, 0.937, and 0.97). A gradual transition from the thermal conductivity of a highly perfect crystal to the minimum thermal conductivity is observed as the crystal becomes increasingly more disordered. A qualitative description is given in the framework of Debye model of thermal conductivity, which takes into consideration the fact that phonon mean free path cannot decrease indefinitely.

The isochoric thermal conductivity of solid propane C3H8 is investigated on three samples of different densities in the temperature interval from 50K to the onset of melting. In all cases the isochoric thermal conductivity exhibits dependences that are weaker than Λ∝1∕T. The results obtained are compared with those for other representatives of n-alkanes. The experimental data are discussed within a model in which the heat is transported by low-frequency phonons and by “diffusive” modes above the phonon mobility edge.

The isochoric thermal conductivity of the solid solutions Kr1−ξXeξ (ξ=0.14 and 0.37) is investigated in the temperature interval from 80 K to the onset of melting. Its value and temperature dependence are in good agreement with the lower limit of the thermal conductivity of the crystal lattice.

The isochoric thermal conductivities of the solid solutions (CO2)0.905Kr0.095 and (CO2)1−xXex (x=0.052 and 0.097) of different densities are investigated in the temperature interval from 150K to the onset of melting. An unusual effect of point defects on the thermal conductivity has been detected. In pure CO2 at T>150K the isochoric thermal conductivity decreases smoothly with increasing temperature. In contrast, the thermal conductivity of CO2∕Kr and CO2∕Xe solid solutions first decreases, passing through a minimum at 200–210K, and then starts to increase with temperature up to the onset of melting. This behavior of the isochoric thermal conductivity is attributed to the rotation of the CO2 molecules, which gain more freedom as the spherically symmetrical inert gas atoms are dissolved in the CO2.