Molecular crystals have weak Van der Waals bonds and high lattice anharmonicity. Their thermal expansion coefficients far exceed those of other solids. As a result, the isobaric and isochoric data on the thermo physical properties of molecular crystals differ appreciably as the temperature rises. This is well known, for example, for heat capacity. The studies of thermal conductivity at constant density in the region about and above Debye temperatures permit a direct comparison of measurement and theory and furnish extra information about the dependence of thermal conductivity upon the molar volume.

The researchers of the group have constructed an original setup for investigation of the isochoric thermal conductivity of molecular crystals in the temperature interval 20-300K under pressures up to 800MPa. The method of steady radial heat flow has been used.

The objects were: inert gases (Ar, Kr, Xe), molecular crystals CO, CO₂, C₂H₆, N₂O, N₂, O₂, CHCl₃, CH₂Cl₂, CF₂Cl₂, CHF₂Cl, CH₄, CCl₄, SF₆, C₆H₆,

The goal was to find general regularities of heat transfer in these objects and in particular to study the effect of molecule rotation degree of freedom on thermal conductivity.

Main results:

It is found that at T і Q_D molecular crystals have appreciably different temperature dependences of isochoric thermal conductivities. It is shown that the dependence Lµ1/T  is obeyed quite well for isobaric thermal conductivity due to partial counterbalance of two processes - the weak decrease in the thermal conductivity on the approach to its lower limit and the more rapid decrease in the thermal conductivity due to the vibrational mode "softening" in the process of crystal expansion.

It is found that the behavior of the thermal conductivity of molecular crystals at T і Q_D is much determined by the approach of the thermal conductivity to its lower limit . In the vicinity of the melting temperature the thermal conductivity of molecular crystals is only two or three times higher than its lower limit.

The correlation between the character of molecular rotation and thermal conductivity behavior has been studied. It is found that with the increase in the molecular reorientation frequency at growing temperature the isochoric thermal conductivity becomes higher too. It is shown that this effect occurs because the phonon scattering at collective rotational excitations decreases sharply as the rotational correlations of the neighboring molecules grow weak.

The gradual change from pure phonon heat transfer to the heat transfer by "diffusion" modes at rising temperatures and impurity concentrations has been investigated on solid cryocrystals solutions. It is shown that the Bridgman coefficient characterizing the dependence of thermal conductivity on the molar volume is determined by the competition of the phonon and "diffusion" mode mechanisms of heat transfer.

Selected publications:

1. Konstantinov V.A. Heat transfer by low-frequency phonons and "diffusive" modes in molecular crystals // Low temperature physics. - 2003. - Vol. 29. - P. 422 - 428.

2. Konstantinov V.A., Manzhelii V.G., Pohl R.O.,Revyakin V.P. Search for the minimum thermal conductivity in mixed cryocrystals (CH)_1-xKr_x // Low temperature physics. - 2001. - Vol. 27.- № 9 - P. 858 - 865.

3. V.A. Konstantinov, E.S. Orel, V.P. Revyakin. Molar volume dependence of the thermal conductivity in mixed cryocrystals. // Low temperature physics. - 2003. - Vol. 28. - № 2 - P. 136 - 139.