Thermal conductivity of solid thiophene in an incommensurate orientational state

Abstract

The thermal conductivity of solid thiophene at equilibrium vapor pressure between 2 K < T < 170 K, has been measured in a sequence of incommensurate metastable orientationally disordered phases II, II1, II2, and II2g with different degrees of orientational ordering of the molecules. It is found that in phase states II, II1 and II2 with dynamic orientational disorder of the molecules, the thermal conductivity does not depend on the temperature. It is shown that the temperature dependence of the thermal conductivity κ(T) of orientational glass Vg and II2g (incommensurate) does not have any of the anomalies that are typical for amorphous materials and glasses. The temperature dependence κ(T) of the incommensurate state of orientational glass II2g is bell-shaped, which is typical for the thermal conductivity of crystals with long-range orientational order. In the II2g state, as temperature drops from Tg to almost 10 K, the thermal conductivity increases according to κ(T) = A/T + B, where the first term describes the input of the propagating phonons, wherein the average length of their mean free path is greater than half of the phonon wavelength. The B term is associated with the input of localized short-wave, or “diffuse” vibrational modes. At low temperatures T ≤ 7 K, κ(T) ∝ T3 is observed with increasing temperatures, which corresponds to the boundary scattering of phonons.