The thermal conductivity of all three disordered solid phases of ethyl alcohol has been measured. That for the orientationally disordered bcc phase is found to be remarkably close to that for the structurally amorphous solid, especially at low temperatures. The results, which emphasize the role of orientational disorder in phonon scattering, are discussed with the aid of computer simulations on single-crystalline models of both bcc and monoclinic crystals. DOI: 10.1103/PhysRevB.74.060201 PACS number͑s͒: 66.70.ϩf, 61.43.Ϫj, 63.50.ϩx, 65.60.ϩa Our current understanding of the mechanisms of heat transport in disordered media rests upon concepts grounded on clean experiments showing that acoustic phonons, especially those having transverse polarization, are the main heat carriers. 1 Work carried out over the last couple of decades has evidenced striking quantitative similarities in the characteristic thermal conductivity of bulk amorphous materials 2 between, say, 0.1 and 10 K, independent of chemical composition. Furthermore, such similarity also extends to a good number of disordered crystals, including a quasicrystal, 2,3 and from the set of collected data it has been inferred that the ratio of the wavelength of the acoustic wave to the mean free path l of all these solids ranges within 10 −2 -10 −3 , which suggests the presence of "universal" behavior of some sort. On such grounds, it becomes clear that the presence of "glassy dynamics" cannot be attributed in full to the absence of static translational long-range order ͑LRO͒.Some molecular crystals where the individual molecules have random static orientations while their centers of mass are at the nodes of a three-dimensional crystalline lattice are also known to exhibit glasslike excitations. Of those, solid ethyl alcohol is perhaps the most convenient benchmark to carry out a quantitative comparison of the effects caused by the complete lack of LRO, 4 on the most sensitive property to explore the propagation of excitations in condensed matter, the thermal conductivity. The material, apart from the wellknown monoclinic ͑fully ordered͒ crystalline ͑FOC͒ modification, can be prepared in three long-lived phases, an amorphous solid or glass, an orientationally disordered crystal ͑ODC͒ ͑or orientational glass͒ showing static orientational disorder but having translational LRO since the molecules are at the nodes of a bcc lattice, and a crystal with dynamic orientational disorder ͓rotator-phase crystal ͑RPC͔͒ which retains LRO as a bcc lattice still exists. Two glass transitions take place about 97 K between the glass and supercooled liquid and the ODC and RPC. 4 Here we report on measurements of the thermal conductivity of ethyl alcohol for all the solid phases. The relevance of such an exercise is twofold. First and foremost, as stated in a recent review, 2 the measurements will provide additional tests on claims of quantitative universality of the properties of heat propagation at low and intermediate temperatures in disordered matter brought forward by a cl...

The thermal conductivity ͑T͒ of the crystalline and glassy phases of the two isomers of propyl alcohol has been measured. The two isomers differ by a minor chemical detail involving the position of the hydroxyl group with respect to the carbon backbone. Such a difference in molecular structure leads, however, to disparate behaviors for the temperature dependence of ͑T͒, for both glass and crystal states. The ͑T͒ for the glass shows for 1-propanol an anomalously large plateau region comprising temperatures within 6 -90 K, while data for isomeric 2-propanol show only a small plateau up to 10 K which is comparable to data on lower alcohols. The results emphasize the role played by internal molecular degrees of freedom as sources of strong resonant phonon scattering.

The thermal conductivity of two molecular glasses ͑ethanol and 1-propanol͒ decrease with increasing temperature up to their glass transitions at T g Ϸ 97 and 98 K, respectively. Within their supercooled liquid phases, the conductivity increases with rising temperature up to a maximum which roughly coincides with the liquidus ͑or melting temperatures T m Ϸ 159 K and T m Ϸ 149 K, respectively͒. From there on, the conductivity decreases with increasing temperature, a behavior common to most liquids examined so far, exception made of liquid water. The origin of the rather different dependencies with temperature of thermal transport is understood as a competition between phonon-assisted and diffusive transport effects which are amenable to experiments using high resolution quasielastic neutron scattering and visible and ultraviolet Brillouin light-scattering spectroscopies.

The phonon scattering processes on the three solid phases of ethanol are investigated by means of thermal conductivity, light and neutron scattering measurements as well as molecular dynamics simulations on single-crystalline models for the two crystalline modifications (fully ordered monoclinic and orientationally disordered bcc phases). The orientationally disordered crystal is found to exhibit a temperature dependence of the thermal conductivity that is remarkably close to that found for the structurally amorphous solid, specially at low temperatures. The results, together with measurements of the Brillouin linewidths as derived from light scattering measurements emphasize the role of orientational disorder on phonon scattering. The experimental results obtained on polycrystal samples are then discussed with the aid of computer simulations on single-crystalline models of both bcc and monoclinic crystals. Our findings are in good agreement with the wealth of thermodynamic and dynamic data available so far [1], but are in stark contrast with inferences made from the analysis of inelastic X-ray data on polycrystalline samples [2], where a common nature for the excitations in all phases is postulated. * Electronic address: jbermejo@we.lc.ehu.es