On the polyamorphism of fullerite-based orientational glasses

Abstract

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

Hysteretic phenomena in Xe-doped C60 from x-ray diffraction

“…A qualitative explanation of both low-temperature anomalies (the hysteresis and the negative expansivities) was suggested by Aleksandrovskii et al [24]. The negative expansion coefficients, found dilatometrically at low temperatures [24], are related to the tunnel nature of the low-energy levels.…”

“…A qualitative explanation of both low-temperature anomalies (the hysteresis and the negative expansivities) was suggested by Aleksandrovskii et al [24]. The negative expansion coefficients, found dilatometrically at low temperatures [24], are related to the tunnel nature of the low-energy levels. The hysteresis is ascribed to a polyamorphic phase transition between two different orientational glass phases, in which case the non-ergodic character of the glass system can lead to hysteretic phenomena.…”

Intercalation of fullerite C60 with N2 molecules. An investigation by x-ray powder diffraction

“…It was found [8][9][10][11][12] that at the lowest (liquid helium) temperatures studied, nearly all of the C 60 molecules (90%) in the solid solution formed the lower energy pentagon-pentagon arrangements between neighboring C 60 molecules. Over this temperature region, the presence of atomic and molecular species can also significantly alter the temperature behavior of the thermal expansion as compared to that of the pure C 60 lattice, as well as introducing hysteresis effects [20,21].…”

Evidence for high saturation of porous amorphous carbon films by noble gases

“…Last years many experimental groups carry out investigations of fullerenes and nanotubes saturation by different gaseous admixtures and study various aspects of relevant processes [7][8][9][10][11][12]. One of the most important tasks connected with potential applications is to attain as high saturation as possible, which, additionally, would be stable at elevated temperatures.…”