Shape Dependence of Two-Body Interatomic Potential Functions on Phase Stability of Close-Packed Polytype Structures

Many crystalline compounds are composed of one or more structural units. When these units can be “stacked” in various ways to form stable or metastable phases, the resulting phases are known as polytypes [1]. Among these polytypes, the crystal systems composed of close-packed (CP) layers have received particular attention over the years due to their fundamental and technological significance. While the demand for material engineering in controlling polytype structures is spanning across various fields such as wide-gap semiconductors [2], catalytic science [3], lightweight structural materials [4], and more, the scientific control of the polytype phase is still incomplete in the realm of industrial practical materials. Despite the fact that there are mathematically countless stacking arrangements with equal packing densities, most observed CP crystal structures exhibit short repeating sequences such as face-centered cubic (fcc) or hexagonal CP (hcp). There is, however, currently no general theory explaining why these structures can take the robust ground state.<br/>Since the descriptors for the energetics of crystal structures are typically discrete physical quantities such as atomic configurations, we are forced to search their energetic stability in a discontinuous descriptor space, which makes it challenging to completely predict the most stable structure. For polytype structures, the convergent series lattice theories, such as the axial next-nearest neighbor Ising (ANNNI) model [5] and the model recently submitted by Loach and Ackland [6], are useful for representing the infinite continuous energetic space spanned by the expansion parameters. The computational analyses based on these models have suggest that it is necessary to consider at least the interactions involving the third-nearest-neighbor distance or longer-range interlayer interactions to accurately estimate the total energetics of metallic CP polytypes [6, 7]. Furthermore, molecular dynamics (MD) simulations have shown that the ground state structures based on the finite-range Lennard-Jonesium can encompass not only fcc and hcp arrangements, but also a wide range of more complex stacking sequences, depending on the interatomic interaction distance and cutoff function [8].<br/>In the previous work, in order to investigate the bifurcation of polytype energetics as a function of interaction distance, we have presented the interlayer partial energy model where the total energy constructed from the two-body interactions is projected onto the interlayer interactions in CP polytype structures [9]. While the polytype structural energies tend to be degenerate with respect to hexagonality in the systems with the short interaction distance, the energetic degeneracy manifested by the short-range interactions has been found to split in the systems with the third neighbor interlayer interactions based on this analytical model [9]. To confirm the theoretical results, this work has systematically investigated the shape dependence of the interatomic potentials on the phase stability of the CP polytype structures by constructing specific two-body potential functions, referring to [10]. In the presentation on the day, we will discuss the variations in the phase diagram of the polytype ground state in the space defined by the potential variables associated.<br/><br/><br/>[1] A. L. Ortiz, et al., J. Appl. Cryst. 46, 242 (2013).<br/>[2] E. M. T. Fadaly et al., Nature. 580, 205 (2020).<br/>[3] Z. Fan et al., Nat. Commun. 6, 7684 (2015).<br/>[4] E. Abe et al. Philos. Mag. Lett. 91, 690 (2011).<br/>[5] W. Selke, Phys. Repo. 170, 213 (1988).<br/>[6] C. H. Loach, and G. J. Ackland, Phys. Rev. Let. 119,205701 (2017).<br/>[7] K. Moriguchi et al., MRS Advances 6, 163 (2021).<br/>[8] L. B. Pártay et al., Phys. Chem. Chem. Phys. 19, 19369 (2017).<br/>[9] S. Ogane and K. Moriguchi, MRS Advances 6, 170 (2021).<br/>[10] S. Erkoc, et al., Chem. Phys. Lett. 314, 203 (1999).