Thermal expansion of Kr–CH 4 solid solutions at liquid-helium temperatures

A negative contribution of the CH 4 impurity to the thermal expansion of a solution was observed in dilatometric studies of the solid solutions Kr +0.76% CH 4 , Kr +5.25% CH 4 , and Kr +10.5% CH 4 at 1–23 K. It was shown that the negative contribution is due to changes in the occupancy of the ground state of the A modification of isolated CH 4 molecules . The contributions of the CH 4 impurity singles and clusters to the thermal expansion were estimated assuming them to be independent of one another. The contribution of the singles to the thermal expansion of the solid solution was found to be negative. The energies of the first excited rotational states were determined for singles and two- and three-body clusters of CH 4 molecules.