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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.

The thermal conductivity of solid cyclohexene C 6 H 10 has been measured in two independent experiments in five different stable and metastable phase states: orientational glass (Ig), orientational glass (IIIg) with a partial order, dynamically orientationally disordered state (III) with a partial order, completely orientationally ordered phase (II) and “plastic” phase (I). The measurements were carried out at saturated vapor pressure in the temperature range 2–120 K and at isochoric conditions in “plastic” and orientationally ordered phases on samples of different densities. The isochoric thermal conductivity of “plastic” phase increases smoothly with temperature. It can be attributed to weakening of the translational orientational coupling which, in turn, leads to a decrease in phonon scattering on rotational excitations. The thermal conductivity of cyclohexene measured at saturated vapor pressure exhibits a similar behavior in phases Ig, IIIg, and II. At low temperatures ( T T 2 dependence , passes through a maximum and decreases further with increasing temperature following the dependence, which is somewhat different from 1 /T. It was found that the thermal conductivity can be represented as a sum of two contributions κ ( T )= κ 1 (T )+ κ 2 ( T ), where κ 1 ( T ) is due to propagating phonons whose mean-free path exceeds half the phonon wavelength, and κ 2 ( T ) is attributed to localized short-wavelength or “diffusive” vibrational modes.

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.

Thermal conductivity Λ P of solid cyclohexane is measured at a pressure P = 0.1 MPa in the temperature range from 80 K to the melting point, which covers the ranges of low-temperature orientationally ordered phase II and high-temperature orientationally disordered phase I. Thermal conductivity Λ V is measured at a constant volume in orientationally disordered phase I. The thermal conductivity measured at atmospheric pressure decreases with increasing temperature as Λ P ∝ T −1.15 in phase II, whereas Λ P ∝ T −0.3 in phase I. As temperature increases, isochoric thermal conductivity Λ V in phase I increases gradually. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both phonons and “diffuse” modes.

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 isochoric thermal conductivity of solid CO was investigated in three samples of different densities in the interval from 35 K to the onset of melting. In α-CO the temperature dependence of the isochoric thermal conductivity is significantly weaker than ; in β-CO it increases slightly with temperature. A quantitative description of the experimental results is given within the Debye model of thermal conductivity in the approximation of the corresponding relaxation times and which allows for the fact that the mean-free path of phonons cannot become smaller than half the phonon wavelength. On this consideration the heat is transported by both phonons and 'diffusive' modes.

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 isochoric thermal conductivity of gama-O2 has been studied on samples of varying density in the temperature interval from 44 K to the onset of melting. The thermal conductivity of nearly free sample decreases at rising temperature along the isochores. It is shown that the absolute value of thermal conductivity in the gama-phase of O2 is close to its lower limit and most of the heat is transported by "diffusive" modes. The growth of thermal conductivity in gama-O2 is attributed to the decay of the phonon scattering at the rotational excitations of the molecules and at the short-range magnetic order fluctuations at rising temperature., Comment: Postscript 9 pages, 2 figures, 1 table. To be published in 2005

The isochoric thermal conductivity of Kr1−ξXeξ solid solutions (ξ=0.034, 0.072, and 0.14) is investigated in the temperature range from 80 K to Tm (Tm is the melting temperature) for samples of different densities. It is found that an increase in disordering of the crystal leads to a gradual crossover from the thermal conductivity of a perfect crystal to the lower limit of the thermal conductivity of the lattice. A quantitative description of the results obtained is performed within the Debye model of thermal conductivity and allows for the fact that the mean free path of phonons cannot be less than half the phonon wavelength.

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 the solid solution Kr–CH4 with methane concentrations of 3.0 and 6.3 mol% has been studied at various densities in the temperature range 40–170 K. The additional contribution to the thermal resistance due to impurities is proportional to the impurity concentration and decreases with increasing temperature. The observed effect is explained with a simple model according to which the mean free path of high-frequency phonons is limited by the interatomic distance.

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 orientationally disordered (OD) phase of solid methane has been studied at molar volumes 30.5 and 31.1 cm3/mol from 35 to 140 K. Above the order–disorder transition temperature, the isochoric thermal conductivity of methane first grows with temperature, passes through a smooth maximum and then decreases up to the melting temperature. The thermal conductivity maximum shifts towards higher temperatures as the density of the samples increases. The unusual temperature dependence of the isochoric thermal conductivity of methane is attributed to the interaction between phonons and correlated molecular rotations. The contributions of phonon–phonon scattering (Umklapp-processes) and phonon scattering at excitations of the rotational molecule motion to thermal resistance are discussed.

Thermal conductivity of solid cyclopentane C5H10 has been measured at isochoric conditions in the plastic phases I and II for samples of different densities. Isochoric thermal conductivity is nearly constant in phase II and increases with temperature in phase I. Such behaviour is attributed to weakening of the translational orientational coupling 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 diffusive modes.

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 conductivity of solid trifluoromethane was investigated for three samples of different densities in the interval from 75 K to the onset of melting. The isochoric thermal conductivity first decreases with increasing temperature, passes through a minimum at T ~ 100 K, and then starts to increase slowly. The results obtained are compared with the thermal conductivities of other freons of methane series. The correlation between the temperature dependence of isochoric thermal conductivity and the character of the rotational molecular motion is discussed.

The isochoric thermal conductivity of solid nitrogen has been investigated on four samples of different densities in the temperature interval from 20 K to the onset of melting. In alfa-N2 the isochoric thermal conductivity exhibits a dependence weaker than 1/T; in beta-N2 it increases slightly with temperature. The experimental results are discussed within a model in which the heat is transported by low-frequency phonons or by "diffusive" modes above the mobility boundary. The growth of the thermal conductivity in beta-N2 is attributed to the decreasing "rotational" component of the total thermal resistance, which occurs as the rotational correlations between the neighboring molecules become weaker., Postscript 12 pages, 3 figures, 1 table. To be published in 2005

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.