Effects of internal molecular degrees of freedom on the thermal conductivity of some glasses and disordered crystals

Abstract

The thermal conductivity κ(T ) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C 6 H 11 OH) has been measured under equilibrium vapor pressure within the 2-200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C 6 H 11 CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar.

Thermal properties of halogen-ethane glassy crystals: Effects of orientational disorder and the role of internal molecular degrees of freedom

“…On the other hand, it can be noticed that the additional terms B are smaller than the values found for orientationally ordered molecular crystals. 39,58,59 An additional striking difference in the behavior of the thermal conductivity of these two materials concerns how this magnitude changes at both sides of the glasstransition temperatures. Data for F-113 display a well defined minimum at such temperature whereas those for F-112 do not show any well defined feature.…”

“…The same conclusion can be achieved when compared with some structural glasses such as 2-propanol (5.4 × 10 −4 ), 1-propanol (3.1 × 10 −4 ), or ethanol (8.8 × 10 −4 for its structural glass). 17,39 To account for a better description of the thermal conductivity at temperatures higher than k B T > W , we will consider an additional phonon-scattering process which becomes thermally activated at those temperatures, corresponding to resonant scattering of phonons by single oscillators. Such additional contribution was already suggested for F-112 and is described by…”

Instability and thermal conductivity of pressure-densified and elastically altered orientational glass of Buckminsterfullerene

“…We stress that the origin of the weak T-dependence of κ of our polycrystalline C60 is different from that of the plastic crystal phases, 58 e.g. those of the high temperature phases of cyclo-hexanol and cyclo-octanol ODICs, 31,32,48 whose entropy differs little from that of their liquids state, and similarly of some other ODICs with a significant entropy from orientational configurations. 49 One expects that change in T would change the inter-crystal surface area in our samples and thus change the static sample-effect, but this change would be small in comparison to change due to change in intermolecular interactions at the surface.…”

“…Importance of thermal conductivity for investigating the effect of the nature of disorder on phonon propagation is currently a subject of interest, and several kinetically frozen ODICs have been studied, [48][49][50] and their features compared against those of glassy state of the same material. 49 It has also been used to study the vibrational density of states at low temperatures.…”

Dynamic characterization of crystalline and glass phases of deuterated 1,1,2,2 tetrachloroethane

“…The complexity of the problem can be reduced by regarding simpler systems in which only orientational degrees of freedom are frozen giving rise to orientational glasses or glassy crystals by supercooling the orientationally disordered (or plastic) phases. [17][18][19] The complexity can be further reduced analyzing systems in which the orientational disorder is due to the existence of an intrinsic statistical disorder involving the site occupancy of several atoms of the molecular entities. In some of these cases, it has been demonstrated that the main glass features show up, pointing out the relevance of the orientational degrees of freedom as far as the glass properties are considered.…”