Nitrogen vacancy, self-interstitial diffusion, and Frenkel-pair formation/dissociation inB1TiN studied byab initioand classical molecular dynamics with optimized potentials

Abstract

We use ab initio and classical molecular dynamics (AIMD and CMD) based on the modified embedded-atom method (MEAM) potential to simulate diffusion of N vacancy and N self-interstitial point defects in B1 TiN. TiN MEAM parameters are optimized to obtain CMD nitrogen point-defect jump rates in agreement with AIMD predictions, as well as an excellent description of TiN x (∼0.7 < x 1) elastic, thermal, and structural properties. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, the MD simulations presented in this paper reveal an unanticipated atomistic process, which controls the spontaneous formation of N self-interstitial/N vacancy (N I /N V ) pairs (Frenkel pairs), in defect-free TiN. This entails that the N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair N I and N V recombine within a fraction of ns; ∼50% of these processes result in the exchange of two nitrogen lattice atoms (N−N Exc ). Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired N I and N V point defects.

Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN

“…7. These are approximately one order of magnitude smaller than N vacancy migration rates, obtained from classical molecular dynamics simulations [54], at all investigated temperatures (see Fig. 7 and Table II Table II is due to lattice vibrations.…”

“…Recent classical and ab initio molecular dynamics simulations of N point-defect diffusion in bulk TiN demonstrated that N monovacancy migration is the primary mechanism for transport of N atoms [54]. Our present theoretical investigations focus on migration of Ti monovacancies at the dilute limit, as this is expected to be the most important reaction pathway for diffusion of metal atoms in overstoichiometric (TM/N ratio < 1) bulk TiN.…”

“…AIMD allows for direct visualization of reaction pathways and provides corresponding kinetic rates. In addition, this technique has revealed nonintuitive system configurations and reaction pathways at finite temperatures [53][54][55]. Nevertheless, given that AIMD is highly computationally-intensive, it has been employed in very few theoretical studies of mass transport in TM nitrides [53][54][55][56][57][58], materials characterized by inherently low defect mobilities, at least within the metal sublattice [8].…”

Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N2 Dissociative Chemisorption, N Adatom Migration, and N2 Desorption

“…This indicates that anion surface vacancies are highly stable, as expected for all TM nitrides; strong TM-N bonds hinder N diffusion. 63,121 Due to the use of periodic boundary conditions, desorbed Nad/Nsurf molecules rebound from the bottom layer of the overlying slab replica to return to the upper layer of the slab from which they desorbed. AIMD simulations show that the presence of N vacancies catalyzes N2 molecular dissociation on VN(001), as observed previously for TiN(001).…”

“…62,63 Very few theoretical studies have considered the interaction of TM nitride surfaces with gases and metal adatoms via ab initio molecular dynamics (AIMD). 64,65 Recently, we investigated mass transport of Ti and N adatoms, together with TiNx (x = 1-3) admolecules, on TiN(001) [62,[66][67][68][69] and TiN (111) [70] surfaces, and bulk diffusion in TiN, 63 by both ab initio and classical molecular dynamics. MD simulations show that isolated N adatoms (Nad) on TiN(001) favor threefold sites (TFS) near, and strongly bonded with, nitrogen surface atoms (Nsurf) and bounded by two Ti surface atoms.…”