Thermal expansion of solid solutions Kr-CH₄ at temperatures of liquid helium

A negative contribution of the CH₄ impurity to the thermal expansion of the solution has been revealed in the dilatometric studies of solid Kr + 0.76% CH4, Kr + 5.25% CH₄, and Kr + 10.5% CH₄ solutions at 1-23 K. It is shown that the negative contribution results from changes in the occupancy of the ground state of the A-modifications of isolated CH₄ molecules. Assuming that the CH₄ impurity singles and clusters contribute to the thermal expansion independently, we can estimate their contributions. The contribution of the singles to the thermal expansion of the solid solution is negative. The energies of the first excitational rotational states were determined for singles and two-body and three-body clusters of the CH₄ molecules

Argon effect on thermal expansion of fullerite C₆₀

The linear thermal expansion of compacted Ar-doped fullerite C₆₀(ArxC₆₀) is investigated at 2-12 K using dilatometric method. The thermal expansion of ArxC₆₀ was also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in the considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed

Low temperature thermal expansion of fullerite C₆₀ alloyed with argon and neon

Динамика кристаллической решетки The linear thermal expansion of compacted fullerite C₆₀ alloyed with argon (ArxC₆₀) and neon (NexC₆₀) are investigated by a dilatometric method. The experimental temperature is 2-12 K. In the same temperature interval the thermal expansion of ArxC₆₀ and NexC₆₀ are examined after partial desaturation of the gases from fullerite. It is found that Ar and Ne alloying affects the temperature dependence of the thermal expansion coefficient of C₆₀ quite appreciably. The libration and translation contributions to the thermal expansion of pure C₆₀ are separated. The experimental results on the thermal expansion are used to obtain the Debye temperature of pure C₆₀. The effects observed are tentatively interpreted

Low-temperature thermal expansion of pure and inert gas-doped fullerite C₆₀

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 has been investigated using the 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 has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems

Negative thermal expansion of fullerite C₆₀ at liquid helium temperatures

The thermal expansion of fullerite C₆₀ has been measured in the temperature range 2-9 К A compacted fullerite 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 К the linear thermal expansion coefficient becomes negath re. At temperatures above 5 К our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed