Thermoactivated heat transfer mechanism in molecular crystals: Thermal conductivity of benzophenone single crystals

Abstract

Thermal conductivities of two benzophenone single crystals have been measured at temperatures from 4.7 to 270 K. The experimental data for both are consistent for temperatures above 15 K. The thermal conductivity of benzophenone can be represented as a sum of two contributions: κ1 + κTA where κ1 is due to the standard phonon mechanisms accepted for ordered crystals and κTA takes into account the heat flow due to intermolecular hopping of thermally activated intramolecular vibrational modes. The thermal activation contribution in unsubstituted benzophenone is substantially smaller when compared to that in any of the two para-bromobenzophenone polymorphs studied previously. Unlike in the 4-bromobenzophenone crystals, the microscopic agent responsible for intramolecular excitation(s) was not determined. The characteristic intramolecular excitation energy was evaluated to be 220 K, about three times less compared to 4-bromobenzophenone.

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

“…Later on, a similar effect was observed in monoclinic parabromobenzophenone polymorph [12]. A slight thermally activated increase of thermal conductivity in the high temperature region has also been observed in non-bromizated benzophenone compound in its stable orthorhombic α-form [13].…”

“…The contribution of intramolecular modes can be described within a simple model in which a thermally activated contribution κTA(T) to the thermal conductivity is approximated by an Arrhenius law with activation energy E [11][12][13]:…”

“…where κ0 is the pre-exponential factor characterizing the intensity of the thermally activated heat transfer and kB is the Boltzmann constant. The pre-exponential factor κ0 and the activation energy E of the studied samples were determined following the same procedure as that used in [11][12][13]. Thermal conductivity, neglecting the contribution of low temperature defects, can be described as: Symbols (identical to those in Fig.…”

“…Such a contribution to the total thermal conductivity can be described by the Arrhenius equation: κТА(Т) = κ0 ехр(-Е/kBТ), where E is the activation energy and κ0 is the pre -exponential factor characterizing the intensity of the thermally activated heat transfer. The thermally activated heat transfer was also observed in a monoclinic modification of this substance [12], benzophenone (BP, (C6H5)2CO) [13] and 2-adamantanone (C10H14O) [18].…”

“…can be neglected. The extensive experimental evidence currently available on isochoric and isobaric thermal conductivities of molecular crystals shows that the dependence κ(T) is also influenced by crystal lattice anharmonicities [2] and intensive hybridization (coupling) of acoustic phonons and low-frequency optical excitations (e.g., see [3][4][5][6][7][8][9][10][11][12][13]). It has been found that for many molecular crystals the high-temperature thermal conductivity can be approximated as a sum of two terms κ(T) = А/Т + В, where A/T refers to the effect of propagating phonons (Eucken law) and В = const refers to an additional temperatureindependent contribution of localized or diffusive short-wavelength vibrational modes.…”