The dilatometric investigation in the temperature range of 2-28K shows that a first-order polyamorphous transition occurs in the orientational glasses based on C60 doped with H2, D2 and Xe. A polyamorphous transition was also detected in C60 doped with Kr and He. It is observed that the hysteresis of thermal expansion caused by the polyamorphous transition (and, hence, the transition temperature) is essentially dependent on the type of doping gas. Both positive and negative contributions to the thermal expansion were observed in the low temperature phase of the glasses. The relaxation time of the negative contribution occurs to be much longer than that of the positive contribution. The positive contribution is found to be due to phonon and libron modes, whilst the negative contribution is attributed to tunneling states of the C60 molecules. The characteristic time of the phase transformation from the low-T phase to the high-T phase has been found for the C60-H2 system at 12K. A theoretical model is proposed to interpret these observed phenomena. The theoretical model proposed, includes a consideration of the nature of polyamorphism in glasses, as well as the thermodynamics and kinetics of the transition. A model of non-interacting tunneling states is used to explain the negative contribution to the thermal expansion. The experimental data obtained is considered within the framework of the theoretical model. From the theoretical model the order of magnitude of the polyamorphous transition temperature has been estimated. It is found that the late stage of the polyamorphous transformation is described well by the Kolmogorov law with an exponent of n=1. At this stage of the transformation, the two-dimensional phase boundary moves along the normal, and the nucleation is not important.Comment: 29 pages, 14 figures, added references, corrected typo

The effective thermal conductivity of the powder samples of xenon hydrate was measured in the interval 2 -170 K using the steady-state method. The thermal conductivity of the homogeneous Xe clathrate hydrate was estimated from the effective thermal conductivity using an empirical expression. The applicability of the formula was checked by comparing two powder samples with different grain size and porosity. The temperature dependence of the thermal conductivity ͑T͒ϳT n of Xe clathrate hydrate is divided into four distinct temperature regimes ͑I-IV͒ with different n. In the interval 55-97 K ͑III͒ the behavior of ͑T͒ shows an anomaly, where the thermal conductivity decreases by almost 50% as the temperature increases. This observation is attributed to the resonant scattering where the coupling of the lattice with "rattling" motions of Xe atom dominates the thermal resistivity at high temperature. Since the observed vibrational energy of Xe in the small cages is ϳ4 meV ͑or Ϸ46 K͒ the resonant scattering contribution to the thermal resistivity is expected to decrease in an interval of comparable temperature. The thermal conductivity in the low temperature regime ͑regimes I and II͒ is found to follow the prediction of the soft-potential model. The data on thermal conductivities of several gas clathrate hydrates are compared.

The radial thermal expansion coefficient α r of pure and Xe-saturated bundles of singlewalled carbon nanotubes has been measured in the interval 2.2-120 K. The coefficient is positive above T = 5.5 K and negative at lower temperatures. The experiment was made using a low temperature capacitance dilatometer with a sensitivity of 2·10 -9 cm and the sample was prepared by compacting a CNT powder such that the pressure applied oriented the nanotube axes perpendicular to the axis of the cylindrical sample. The data show that individual nanotubes have a negative thermal expansion while the solid compacted material has a positive expansion coefficient due to expansion of the intertube volume in the bundles. Doping the nanotubes with Xe caused a sharp increase in the magnitude of α r in the whole range of temperatures used, and a peak in the dependence α r (T) in the interval 50-65 K. A subsequent decrease in the Xe concentration lowered the peak considerably but had little effect on the thermal expansion coefficient of the sample outside the region of the peak. The features revealed have been explained qualitatively.

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The coefficients of thermal expansion (13 to 139 OK), heat capacity (2 to 20°K), andvelocity of longitudinal and transversal ultrasonic waves (88 to 190 OK) for solid CO, have been determined along the solid-vapour equilibrium line. Heat capacity of constant volume, adiabatic and isothermal compressibilities, the Griineisen and Poisson coefficients were calculated using the above data. The contributions of various types of heat motion to heat capacity, thermal expansion and compressibility have been separated, and the parameters of isolated orientational defects in crystalline CO, determined. The connection between the orientational defect parameters and the extent of non-centricity of intermolecular interactions in solids is discussed.Die thermischen Ausdehnungskoeffizienten (13 bis 139 OK), die Wiirmekapazitiit (2 bis 20 "K) und die longitudinale und transversale Ultraschallgeschwindigkeit (88 bis 190°K) wurden fur festes CO, ander Festkorper-Dampf-Gleichgewichtslinie bestimmt. Mit den erhaltenen Werten wurden die Wirmekapazitit bei konstantem Volumen, die adiabatischen und isothermen Kompressibilititen und die Griineisen-und Poissonkoeffizienten berechnet. Die Beitriige verschiedener Arten der Wirmebewegung zur Wirme-. kapazitiit, thermischen Ausdehnung und Kompressibilitiit wurden separiert und die Parameter von isolierten Orientierungsdefekten in kristallinem CO, bestimmt. Die Verbindung zwischen den Parametern der Orientierungsdefekte und dem AusmaD der Nichtzentrizitit von intermolekularen Wechselwirkungen in Festkorpern wird diskutiert.

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...

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The low-temperature (2–24 K) thermal expansion of pure (single-crystal and polycrystalline) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr is investigated using a high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations ΔL(t) on heating by ΔT shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C60 molecules. It is assumed that the reorientation is of a quantum character. The inert gas impurities affect the reorientation of the C60 molecules very strongly, especially at liquid-helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr– and He–C60 solutions is revealed. The hysteresis is attributed to orientational polymorphous transformation in these systems.