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 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 CH4 molecules.Comment: 10 pages, 7 figure

Cluster approach to formation of nitrogen–rare gas cryoalloys

Structure of solid binary N₂–Kr and N₂–Ar solutions were studied by transmission electron diffraction (THEED). The samples were prepared in situ by deposition of gaseous mixture onto an Al substrate cooled to 20 K. The lattice parameters for low concentrations of one of the components were measured. The relative change of the lattice parameter per unit concentration is determined for N₂ impurity in Kr and Ar as well as for Kr and Ar in N₂. Analysis of the concentration dependence of the lattice parameters for low fractions of both components was performed within a cluster model in the three-particle approximation

Properties of solid hydrogen doped by heavy atomic and molecular impurities

Using powder x-ray diffraction we studied the structural characteristics of normal and para hydrogen crystals doped with Ar, Kr, N₂, and O₂ impurities over the range from 5 K to the melting point of the hydrogen matrix. It has been established that in spite of very low solubility of the dopants in solid hydrogen, these impurities appreciably affect the structural characteristics. In particular, only nitrogen impurities do not change the molar volume of the matrix, the other three make the matrix expand. The Ar and Kr impurities also change the c/a ratio of the hcp matrix. The fact that both Ar and O₂ have smaller molar volumes than hydrogen may be treated as evidence that these impurities form van der Waals complexes with the hydrogen lattice environment

Quantum effects in the thermal conductivity of solid krypton–methane solutions

The dynamic interaction of a quantum rotor with its crystalline environment has been studied by measurement of the thermal conductivity of solid Kr1_c(CH4)_c solutions at c = 0.05-0.75 in the temperature region from 2 up to 40K. The thermal resistance of the solutions was mainly determined by the resonance scattering of phonons by CH4 molecules with the nuclear spin I=1 (the nuclear spin of T-species). The influence of the nuclear spin conversion on the temperature dependence of the thermal conductivity k(T) was found: a clearly defined minimum on k(T), its temperature position depending on the CH4 concentration. It was shown that the anisotropy molecular field not increase monotonously with the CH4 concentration. A compensation effect in the mutual orientation arrangement of the neighboring rotors is observed at c > 0.5. The temperature dependence of Kr1_c(CH4)_c is described within the Debye model of thermal conductivity taking into account the lower limit of the phonon mean free path. The anomalous temperature dependence of the thermal resistance shows the evolution of the phonon-rotation coupling at varying temperature. It increases strongly when the character of CH4 rotation changes from the quantum at low temperatures to classical at high temperatures. Also, a jump of thermal conductivity (a sharp increase in k(T) within a narrow temperature range) was observed, whose position varies from 9.7 K to 8.4 K when the CH4 concentration changes from 0.25 to 0.45.Comment: 13 pages, 4 figure

Heat capacity of methane–krypton solid solutions. Conversion effect

The heat capacity of Kr-nCH₄ solid solutions with the concentrations n= 1; 5; 10% and of the solid solution Kr-1%CH₄-0.2%O₂ has been studied at 0.7-8 K. The contributions of Crot to the heat capacity of the solutions caused by the rotation of the CH₄ molecules are estimated. The deviations of the measured Crot from the values corresponding to the equilibrium distribution of the nuclear spin CH₄ modifications are dependent on the correlation between the characteristic times of conversion and of the calorimetric experiment. The effects of temperature, O₂ impurities, and CH₄ clusters upon the conversion rate are studied. It is shown that the hybrid mechanism of conversion proposed by Berlinsky and Nijman, which takes into account both intramolecular and intermolecular interactions of the proton spins, is predominant