Can the Lennard-Jones solid be expected to be fcc?

Abstract

The structure of the Lennard-Jones solid, obtained by molecular-dynamics simulation of crystallization in the supercooled liquid, may be fee, although the hep structure is energetically more favorable. This could derive from the cubic symmetry of the fee lattice, allowing lattice defects that are not possible in the hep arrangement, but are essential to crystal growth in the simulated liquid. Two crossing stacking faults in a small fee crystallite can produce nonvanishing, growth-promoting, but stacking-faultresisting, surface steps. PACS numbers: 61.20.Ja, 36.40. +d, 61.50.Cj It is well known that the Lennard-Jones (LJ) potential favors the hep structure over the fee structure for the solid [1]. The difference in cohesive energy (^0.01%) appears to be too small, however, to provide a basis for an explanation of an observed preference for one of the two structures in molecular-dynamics (MD) simulations of crystallization in the supercooled LJ liquid. In fact, until very recently, such a preference has never been found [2][3][4][5][6][7], suggesting the inadequacy of the LJ potential to model the interatomic interactions in a simulation of either fee or hep crystal growth.It is the purpose of this Letter to investigate the role that lattice defects may play in the simulated crystallization process, and, in particular, to demonstrate that growth-stimulating defects are much more probable to occur in fee crystallites than in hep crystallites. Moreover, it will be shown that such defects exclusively stimulate fee growth, without further assumptions regarding the interatomic potential other than that it is isotropic and short ranged.That growth characteristics can be decisive in the choice of crystal structure of a substance, rather than a difference in cohesive energy, can be illustrated by a comparison with the method of static lattice energy calculations, aimed at structure prediction. Here, the evaluation and subsequent minimization of lattice sums involves inclusion of all interactions between a representative central atom (the reference atom) with all its close and more distant neighbors within a limiting sphere. However, the reference atom has been incorporated in the crystal lattice under completely different conditions, notably in the absence of at least half of its ultimate neighbors. Moreover, according to accepted theories of crystal growth [8], the motions of surface-migrating atoms are governed by short-range forces and are rather insensitive to the detailed shape of the potential. Trapping sites that are (almost) equally favorable (but, possibly, not equivalent, as on close-packed faces of fee or hep crystals) will have equal a priori occupation probability. The possibility of complete layers of atoms shifting to "better" positions in response to the arrival of new neighbors can be ruled out.Both the fee and the hep structures consist of plane hexagonal arrays of atoms that are stacked in an orderly way, with atoms in one layer over three-coordinated sites in the preceding layer. Consequently, each l...

Preference for fcc atom stacking observed during growth of defect‐free LJ3281 clusters

“…The internal structure in new cluster regions built at the low temperature is characterized by many stacking faults resulting in the formation of fcc and hcp parallel layers [11]. This is caused by approximately the same binding energies for fcc and hcp structure, leading to an ambiguity in determining the position of newly added atoms on a close-packed layer [9]. Moreover, two hcp monolayers are often inclined one to each other at an acute angle close to α = 70.5° (see the hcp structure in Fig.…”

“…As predicted by van de Waal [8], the growth of an ideal fcc cluster would be complicated by difficulty in the formation of growth islands on the close packed (111) faces and possibility for wrong position of the islands resulting in the formation of hcp stacking faults [9]. Therefore, following his previous suggestion on the decisive role of lattice defects promoting the growth of the fcc structure [9], he proposed the defected fcc cluster composed of 3281 atoms [8] with "non-disappearing" fcc growth sites (here also called as the van de Waal cluster). The importance of defects in determining the fcc structure has been clearly observed recently [10] in case of the proposed LJ 3281 cluster, where, instead of energetically more preferred hcp structure, overgrowth of fcc structure takes place.…”

Solid-phase structures of the Dzugutov pair potential

“…We find that the hexagonal close-packed (hcp) and other stacking variants are considerably less stable than fcc at high pressures where fcc is more stable than bcc and the σ-phase. This result is remarkable since for the Lennard-Jones potential hcp is known to be slightly more stable than fcc [25].…”

On the origin of second-peak splitting in the static structure factor of metallic glasses

“…The atomic arrangement in this cluster is the same as that found in the crossing region of intersecting stacking faults in (otherwise perfect) fcc crystals. If such clusters are used as seeds for sequential building algorithms [13], they will not produce a glass but rather a (nearly perfect) fcc crystal, provided new atoms are placed in sites with fourfold coordination, rather than tetrahedral sites [14]. If loops, such as those visible in Fig.…”