Effect of the rotational motion of N2 impurity molecules on the thermal expansion of solid Ar
Abstract
An excess coefficient of linear expansion Δα of solid Ar with N2 as an impurity is discovered in the temperature range 2–12° K. The quantity Δα depends linearly on the concentration in the interval n = 1.02–2.03% N2; for 3.27% a small derivation from linearity is observed, which is due to the interaction between impurity molecules. The anomaly in the thermal expansion is explained by librations of N2 impurity molecules in Ar up to their free rotation. The relative increase in the volume of the crystal, necessary for free rotation of the impurity molecule, corresponding to the effect, is calculated. Its small magnitude is related to the large angles of zero-point librations of a diatomic molecule in an environment of spherically symmetric neighbors.