Interaction potentials for modelling GaN precipitation and solid state polymorphism

Abstract

We outline a molecular mechanics model for the interaction of gallium and nitride ions ranging from small complexes to nanoparticles and bulk crystals. While the current GaN force fields allow the modelling of either bulk crystals or single ions dispersed in solution, our model covers both and hence paves the way to describing aggregate formation and crystal growth processes from molecular simulations. The key to this is the use of formal +3 and −3 charges on the gallium and nitride ions, whilst accounting for the charge transfer in GaN crystals by means of additional potential energy terms. The latter are fitted against experimental data of GaN in the wurtzite structure and benchmarked for the zinc-blende and rock-salt polymorphs. Comparison to quantum chemical references and experiment shows reasonable agreement of structures and formation energy of [GaN] n aggregates, elastic properties of the bulk crystal, the transition pressure of the wurtzite to rock-salt transformation and intrinsic point defects. Furthermore, we demonstrate force field transferability towards the modelling of GaN nanoparticles from simulated annealing runs.

On the Role of Amides and Imides for Understanding GaN Syntheses from Ammonia Solution: Molecular Mechanics Models of Ammonia, Amide and Imide Interactions with Gallium Nitride

“…[22] For the gallium nitride interactions we used a recently developed force-field model that is based on full + 3 and À 3 partial charges, respectively. [13] This enables consistent modelling of the Ga 3 + interactions with imide (-2 charged), amide (-1) and ammonia species, respectively, as discussed in the results section. To assess the electrostatics, we use explicit Coulomb summation for isolated complexes, whilst the damped-shifted potential [23] with a damping parameter of 0.05 Å À 1 was chosen for MD simulations of periodic systems.…”

“…For the Ga 3 + species, an approximate approach to modeling GaÀ N interactions is i) to employ molecular mechanics models originally designed for bulk gallium nitride. [13] On the other hand, ii) the formulation of tailor-made GaÀ N interaction potentials with individual parameters for ammonia, amide, imide and nitride could provide more detailed account of the different coordination of Ga 3 + . See Table 2 for an overview of the different molecular mechanics models used for describing the GaÀ N interactions.…”

“…a GaÀ N interaction model based on directly adopting the vander-Waals term from the Ga 3 + À N 3À potential developed for bulk GaN. [13] b Specific Lennard-Jones parameters optimized for describing the interactions between Ga 3 + and NH 3 , NH 2 À , and NH 2À , respectively. The models were optimized to best reproduce the structure and formation energy (see Table 3) of the Ga-based complexes shown in Figure 1.…”

“…[5] In what follows, we revisit the molecular mechanics modelling of Ga••N interactions for Ga solution complexes in ammonia, thus considering NH 3 , amide and imide ions, respectively. On this basis, improved GAFF parameters [11,12] are obtained and combined with our earlier force-field for bulk gallium nitride [13] to enable ns scale molecular dynamics simulation of (NH 2 ) À and (NH) 2À interactions in bulk GaN. Finally, we demonstrate the unprejudiced analyses of amide and imide defects in GaN crystal faces from simulated annealing runs.…”

Diabat method for polymorph free energies: Extension to molecular crystals

“…3 Polymorphs are different crystal structures for the same molecule or compound. Different polymorphs have different properties, e.g., different catalytic activities, [4][5][6][7] different photophysical properties, [8][9][10] different mechanical properties, 11,12 and different thermodynamic stabilities. For active pharmaceutical ingredients (APIs), which are often formulated as molecular crystals, thermodynamic stability is of paramount importance.…”