Molecular dynamics study of structural properties of amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$

The structural properties of amorphous aluminum oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ have been investigated by means of the molecular dynamics technique. The simulations were done in a microcanonical ensemble, using a pairwise potential, on systems with up to 1800 particles. Three different systems, at densities ranging from 3.0 to $3.3\hspace{0.5em}{\mathrm{g}/\mathrm{c}\mathrm{m}}^3,$ were prepared by quenching from the melt. The network topology of our system is analyzed through partial pair correlations, coordination numbers, angle distributions, and ring statistics. A detailed analysis of the interatomic distances reveals that in the amorphous state there is a short-range order dominated by a slightly distorted $({\mathrm{AlO}}_4{)}^{5-}$ tetrahedron, in agreement with recent experimental results. This conclusion is supported by the distribution of nearest-neighbor coordination numbers, where more than $75\%$ of Al atoms have four O as nearest-neighbors. Ring statistics reveal the presence of two- to fivefold rings, with a peak at the fourfold ring and where the two- and threefold rings are planar. Comparison with available experimental data and earlier calculations shows that the structures of amorphous and liquid alumina are very similar. The amorphous structure also presents close similarities to the surface structure of γ-alumina at room temperature. Simulations for systems at different densities show that the coordination number of the elementary unit increases as the density increases, suggesting that the tetrahedrally and octahedrally coordinated forms of amorphous alumina found experimentally correspond to different densities.