Abstract
As a prototype of an initial or intermediate structure between hcp and fcc lattices we consider a distorted bcc crystal. We calculate the temperature and pressure dependences of the lattice parameters for the heavier rare gas solids Ar, Kr, Xe in a quasiharmonic approximation with Aziz potentials, and confirm earlier predictions that the hcp structure predominates over fcc in the bulk within wide ranges of P and T. The situation is different for confined clusters with up to 105 atoms, where, owing to the specific surface energetics and terminations, structures with five-fold symmetry made up of fcc fragments are dominant. As a next step we consider the free relaxation of differently distorted bcc clusters, and show that two types (monoclinic and orthorhombic) of initial distortion are a driving force for the final hcp vs fcc configurations. Possible energy relationships between the initial and final structures are obtained and analyzed.
Highlights
The problem of the hcp vs fcc competition in rare gas solids (RGS) has a long history [1]
Some specific theories based on a first-principle local density approximation [2,3] or density-functional perturbation theory [4] confirm that the fcc structure may be expected at ambient pressures but it is still unclear why the theories based on the pair potentials like LennardJones (LJ) predict that the hcp structure is the only possibility while experimental observations evidence in favor of the absolutely dominant fcc structure
Some certain progress is attained accounting two- and manybody interactions [1,5,6]. The explanation of such a discrepancy based on the specific surface energetics and terminations elaborated in particular in our works [7,8], which show that the structures with five-fold symmetry comprising fcc fragments are dominant up to 105 atoms, is valid
Summary
The problem of the hcp vs fcc competition in rare gas solids (RGS) has a long history [1]. Some specific theories based on a first-principle local density approximation [2,3] or density-functional perturbation theory [4] confirm that the fcc structure may be expected at ambient pressures (in line with experiment) but it is still unclear why the theories based on the pair potentials like LennardJones (LJ) predict that the hcp structure is the only possibility while experimental observations evidence in favor of the absolutely dominant fcc structure. The transformations of the fcc structure towards the hcp meet large barriers, for larger sizes they are impeded. Within last two and half decades several experimental groups observed the transition from the fcc to the hcp structure in RGS under high
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