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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

Based on a concise review of the experimental data, a theory of the structural, kinetic, and dynamic features of the Fischer cluster is put forward. This theory is based on the idea of heterophase fluctuations of glass-forming liquids. According to these ideas in a one-component liquid the solidlike and fluidlike transient associations of molecules (fluctuons) are formed. The fluctuons differ in the short-range order and, consequently, in the free energy. They form a random free energy landscape. The random field free energy functional of the Ginzburg-Landau type, which accounts for the free energy landscape, is deduced. Based on this, the phase equilibrium, critical behavior, and ordering phenomena of the heterophase liquid are considered. It is shown that in the critical region an effective attractive interaction of the fluctuons results in their gravitation and formation of correlated fractal aggregations. The aggregation consists of the fluctuons of similar short-range order. The fractal aggregation formation is a special structural state of the liquid above the glass transition temperature. The condition at which the aggregations are formed is formulated. Thus it is shown that the Fischer cluster is an array of fractal aggregations of fluctuons. The growth kinetics and dynamics of the Fischer cluster are described. The experimental data are analyzed and discussed using the theory that is put forward.

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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.

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