Thermodynamic stability boundaries of “classical” noble-gas crystals and the polymorphism problem

Abstract

The equations of statistical crystal theory with the pair interaction by Lennard-Jones are applied to the calculation of a thermodynamic stability region of hypothetical bcc phase discussed in the literature for crystals of a type similar to Ar. It is shown that this region is completely within a similar region previously calculated by authors for a real fcc phase of such crystals. This result makes improbable the realization of the bcc modification, predicted by a group of authors in computer “experiments” on Xe at high temperatures and pressures, and provoked a critical discussion in the literature. The quantum corrections to the thermodynamic functions of “classical” noble-gas crystals were also taken into consideration and the deviation of the spinodal line of Ne from the “universal” curve is calculated. A good quantitative agreement of obtained results with the data of numerical “experiments” on model (with the Lennard-Jones interaction) “crystals” of Ar, Kr, Xe, as well as of Ne in the “exotic” region of negative pressures is found. Although, at present, the indications of the polymorphic transformation of neon to a modification different than fcc have not been experimentally observed, the discovery of this phenomenon in the predicted range of temperatures T < 70 K and pressures > 0.3 GPa would be of principal interest.