Heat capacities of neon in one- and three-dimensional channels at low temperatures

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Heat capacities of Ne adsorbed in the one- and three-dimensional channels in the high-silica zeolites ZSM-23 and ZSM-5 have been observed in the temperature range from 2 K to 23 K for various concentrations of Ne. For lower concentrations of Ne, the low-temperature heat capacities for both the one-dimensional and the three-dimensional channels, both about 5.5 AA in diameter, are described by the Einstein model, which implies that Ne atoms are localized independently or construct clusters of minute size in the channels. However, for the higher concentration where channels are nearly filled with Ne atoms, the heat capacities of Ne reflect the difference between the topological channel structures of the two zeolites. In the case of three-dimensional channels, the heat capacity is revealed to be well reproduced by the Debye model for a continuous solid with the characteristic temperature Theta D=29 K, which is unexpectedly smaller than the values Theta 3D approximately=67 K for the bulk solid Ne and Theta 2D approximately=50 K for the two-dimensional solid Ne. This suggests the possibility of an unusual softening effect of phonons in solid of Ne in the three-dimensional network of channels. While the heat capacity of Ne in the one-dimensional channels cannot be understood in terms of the one-dimensional Debye model, it can be explained using the Einstein model. These results for the classical Ne particle are significant when compared with the thermal behaviour of He in the same zeolites, which reflects the characteristic quantum effect at low temperatures.