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 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 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.

The thermal expa11sion of fullerite C 60 has been measured in the temperature range 2-9 K. A compac.t~d fullcrite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below-3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above S K our results are in good agreement with the available literature data. A qualitative cxplaPation of the results is proposed.

Abstract.The linear coefficient of the radial thermal expansion has been measured on a system of SWNT bundles in an interval of 2.2 -120K. The measurement was performed using a dilatometer with a sensitivity of 2⋅10 -9 cm. The cylindrical sample 7 mm high and 10 mm in diameter was obtained by compressing powder. The resulting bundles of the nanotubes were oriented perpendicular to the sample axis. The starting powder contained over 90% of SWNTs with the outer diameter 1.1 nm, the length varying within 5-30 μm.Since the discovery of carbon nanotubes in 1991 [1], this novel class of physical objects has been attracting immense experimental and theoretical interest. However, a wide variety of types of carbon nanotubes and the problems encountered in preparing pure samples of these types of nanotubes makes it extremely difficult to reveal regularities in the behavior of nanotubes (e.g., see [2] and the References in it). The thermal expansion of nanotubes remains among the least studied properties of carbon nanotubes. For example, the thermal expansion of single-walled nanotubes and their bundles has never been investigated experimentally below room temperature. Meanwhile, investigations at low temperatures can provide the most valuable information about the dynamics of nanotubes. The thermal expansion coefficients (TECs) predicted theoretically [3 -8] for singlewalled nanotubes (SWNT) differ in order of magnitude and sign.In this study the radial thermal expansion of bundles of closed end SWNTs (c-SWNT) was measured in an interval of 2.2 -120 K.The sample for measuring thermal expansion was prepared using the effect of aligning the SWNT axes by pressure of 1 GPa (see [9]). Under this pressure the nanotubes within a 0.4 mm thick layer are aligned in the plane normal to the pressure vector, the average deviation from the plane of alignment being ~ 4° [9].

The linear coefficients α(T) of N2–C60 solutions with 9.9% and 100% of the C60 lattice thermal expansion interstitials filled with N2 are investigated in the interval 2.2–24K. The dependence α(T) has a hysteresis suggesting co-existence of two types of orientational glasses in these solutions. The features of the glasses are compared. The characteristic times of phase transformations in the solutions and reorientation of C60 molecules are estimated.

The effect of a normal H 2 impurity upon the radial thermal expansion α r of SWNT bundles has been investigated in the interval T = 2.2-27 K using the dilatometric method. It is found that H 2 saturation of SWNT bundles causes a shift of the temperature interval of the negative thermal expansion towards lower (as compared to pure CNTs) temperatures and a sharp increase in the magnitude of α r in the whole range of temperatures investigated. The low temperature desorption of H 2 from a powder consisting of bundles of SWNTs, open and closed at the ends, has been investigated.