Scattering of acoustic phonons in disordered matter: A quantitative evaluation of the effects of positional versus orientational disorder

Abstract

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...

Anomaly in temperature dependence of thermal transport of two hydrogen-bonded glass-forming liquids

“…26 and 35͒ show that ⌫ B ͑T͒ strongly increases with increasing temperature well within the deep glass phase, and such an increase becomes more marked for T Ͼ 40 K, which lies above the plateau in the thermal conductivity. 15 Within such region of temperatures, the fractional change of Brillouin frequencies ⌬ B / B shows a departure from a T −1 law. In contrast, linewidth data for glycerol show a rather mild temperature dependence upon crossing T g and strongly increases above Ϸ250 K, which corresponds to a temperature region where stochastic molecular motions set in.…”

“…1 shows that ͑T͒ exhibits a well defined maximum for temperatures close to T m Ϸ 159 K for EtOH and T m Ϸ 149 K for 1Pr, as well as a broad minimum that matches the calorimetric glass→ SCL and ODC→ RPC transitions 14 for both materials. The decrease with temperature of ͑T͒ within the glass and ODC phases is expected because of strong scattering of heatcarrying transverse phonons operative at such temperatures 15 and shows the behavior of these materials similar to LDA. 20 The presence within the ODC of an underlying crystalline lattice allows us to test the predictive capability of some results where the crystal anharmonicity is explicitly taken into account and yield in the high temperature limit, 27…”

“…15,23 The sample container was a 40 mm long, 22 mm diameter stainless steel tube having a copper bottom and a cap. Two Cu wires with 1 mm diameter were passed through the container in order to provide good thermal con-tact.…”

“…͑3͒ and ͑4͒ for liquid ethanol, a system extensively studied both dynamically 14 and thermodynamically, 15 has been carried out for temperatures above T = 273 K by means of computer simulations. 16 The results which are some 10% below experiment are able to reproduce the quasilinear decrease with temperature of ͑T͒.…”

Deuteration effects in the thermal conductivity of molecular glasses

“…The phases of deuterated ethyl alcohols can be obtained by the same technique as for the protonated alcohol [6]. Since the structural and orientational glasses of the deuterated alcohol exist in a wide range of temperatures, it is quite easy to compare the behavior of the thermal conductivity in theses phases within the same temperature interval.…”