Orientational order parameter in CO2–Kr solid solutions

Abstract

Orientational order in binary CO2–Kr cryoalloys is studied at a fixed temperature of 30K as a function of the krypton content x in the deposited gas mixture ranging from 0to59mol.% by transmission electron diffraction. The integrated intensities of two regular reflections, (111) and (200), and two superlattice reflections, (210) and (211), are determined. The absolute values of the orientational order parameter η in the CO2 subsystem is recovered for all x using a previously proposed method modified for binary solid substitution solutions with randomly distributed components. For small x⩽0.05, where the predominant clusters are isolated krypton atoms, η decreases linearly with x and for 0.05<x⩽0.25 the orientational order parameter is constant to within the measurement error. For larger x, η manifests an unphysical increase. This last point means that for x⩾0.25 the assumption of a random distribution of the components is not valid. An analysis of these data shows that the first signs of a deviation from a random distribution (an indication of the onset of a phase transition) appear at x≃0.258. The maximum concentration of krypton in a regular solution is about 33mol.%, rather than the 38mol.% derived from an analysis of the dependence of the lattice parameter on x. Values of the rank-4 orientational order parameter η4 are recovered. Peculiarities in the behavior of η4 for x>0.25 confirm the results of this analysis of η(x).