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).

Hopping Precession of Molecules in Crystalline Carbon Dioxide Films

“…In principle, such order parameters can be readily derived from the diffraction intensities. However, in the experimental studies [5][6][7] of the orientational order in pure CO 2 crystals the authors met certain problems in attaining a good agreement between the calculated and experimental diffraction intensities. They have emphasized [5] that the consistent description of the orientational order in such triatomic cryocrystals may not be successful unless higher fourth-and even sixthdegree Legendre polynomials are used.…”

“…However, in the experimental studies [5][6][7] of the orientational order in pure CO 2 crystals the authors met certain problems in attaining a good agreement between the calculated and experimental diffraction intensities. They have emphasized [5] that the consistent description of the orientational order in such triatomic cryocrystals may not be successful unless higher fourth-and even sixthdegree Legendre polynomials are used. In addition, in order to find a good fit between experiment and calculations they propose to extend the molecule length by ∼4% [6] although this length was determined with high accuracy [8].…”

“…1). However, some distinct experimental observations show that such a concept may not be completely true [5][6][7]. Indeed, according to a theory [1] the orientational order in such crystals can be well described and analyzed with the help of the orientational order parameter η =< P 2 (cos θ) > (here P 2 (x) is the second-degree Legendre polynomial) depending on the small librational amplitude θ.…”