Anomalous behavior of thermal conductivity at high temperatures for molecular crystals composed of flexible molecules

The temperature dependence of thermal conductivity k(T) of selected molecular polycrystals consisting of flexible molecules was investigated within 20-260 K. The materials were para-chloronitrobenzene, pentachloronitrobenzene and freons F-112 and F-113. The k(T) of these crystals increased with temperature in the interval where processes of phonon phonon scattering were dominant. The increase was observed both in the orientationally-ordered and-disordered phases and is typical of the thermally activated heat transfer by localized molecular excitations [M. A. Strzhemechny et al., Chem. Phys. Lett. 647 (2016) 55]. In a wide interval of temperatures, irrespective of the glass transition temperature, the k(T) could be described by a sum of three components: k(T) = A/T + B + k(TA)(T). The term A/T accounts for phonon phonon scattering processes and B accounts for diffuse phonon scattering. The third contribution is described by the Arrhenius equation, k(TA)(T) = k(0) exp(-E/k(B)T), where E is the activation energy and k(0) is the pre-exponential factor characterizing intensity of the activation process. A comparative analysis of anomalous thermal conductivities of some other molecular crystals was carried out. It was found that k(0) linearly depended on E and a similar relationship was evident for a series of quasicrystals.