Construction of Long-Range EAM-Type Interatomic Interaction Model for Dynamical Analyses on Long-Periodic Polytypism

Many crystalline compounds are composed of one or more structural units. When these units can be stacked in different ways to form stable or metastable phases, the resulting phases are known as polytypes [1]. Among these polytypes, the crystalline systems composed of close-packed (CP) layers have especially attracted attention on their fundamental and technological properties for many years. The material engineering demand for polytype phase control is diverse [2-4]. However, the scientific control of the polytype phase is still incomplete in the field of practical materials engineering and predicting polytype phase stability for a material has also been a long-standing issue in condensed matter physics and/or materials science. This is because the polytype control technology has many uncertainties since the polytype formation mechanism has yet been physically unsolved.<br/>To provide a useful theoretical tool in this situation, we have constructed EAM-type potentials that can be applied to atomic-level dynamical analyses on long-period polytypism. In this study, the metallic lanthanum (La) system has been selected as a prototype system analyzing the dynamical processes of polytype formations for the following reasons; (i) the ground state for pure La is the 4H structure which is the simplest long-periodic polytype, (ii) La has the phase stability in both 4H and 3C phases below 583K [6], and (iii) it is, therefore, expected that the phase transition phenomena including 4H and/or 3C polytype structures can be investigated by the molecular dynamical (MD) studies such as crystal growth kinetic simulations.<br/>We have recently presented a theoretical consideration on the CP polytype total energetics using the geometrical analysis on the correlation between interlayer interactions and interatomic ones [7]. These results suggest that short-range interactions are not enough to describe the CP polytype energetics and provide significant insights for creating interatomic models successfully showing the polytypes other than 3C and 2H structures as a ground state. We have succeeded in constructing an EAM-type potential that mimics the energetics of metallic La based on the first-principles calculations [8]. The following three procedures are found to be essentially important for deriving an interatomic interaction model with the 4H structure as the ground state; (i) The cutoff radius should be set to have a relatively longer distance (three or more interlayer distances in the 4H stacking). (ii) The equations of state for the energetics of 2H, 3C, and 4H structures need to be reproduced as accurately as possible. (iii) The adiabatic potential along important transition paths among crystal structures such as the Bain path for the bcc-fcc transition must be exactly considered [9,10].<br/>We have calculated the Generalized stacking fault energy (GSFE) surface for some CP structures using the present potential constructed. Both unstable stacking fault and stable fault energies for La are much lower than ones for Al calculated by another EAM-type potential [11]. Therefore, La has low energy barrier required for forming stacking defects, and polytype formations can easily appear. The presentation of the day will also discuss other basic properties, the transferability for the potential constructed, and some of the dynamical phenomena by the MD analyses.<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] Murray S. Daw and M. I. Baskes, Phys. Rev. B 29, 6443 (1984).<br/>[6] A. B. Lysenko et al., Crystallogr. Rep. 50, S10 (2005).<br/>[7] S. Ogane and K. Moriguchi, MRS Advances 6, 170 (2021).<br/>[8] K. Moriguchi et al., MRS Advances 6, 163 (2021).<br/>[9] K Moriguchi and M Igarashi, Phys. Rev. B 74, 024111 (2006).<br/>[10] G. Grimvall et al., Rev. Mod. Phys. 84, 945 (2012).<br/>[11] Y. Mishin <i>et al</i>., Phys. Rev. B 59, 3393(1999).