Metal oxide surface dynamics from molecular dynamics simulations: the α-Al2O3(0 0 0 1) surface

Constant-stress, constant-temperature (10, 300 and 700 K) molecular dynamics simulations were carried out with shell-model potentials for an infinite crystalline Al-terminated α-Al2O3(0 0 0 1) slab of ∼25 Å thickness. The surface undergoes considerable relaxation at 10 K, but exhibits ordered surface structures at all three temperatures. The relaxation causes the (0 0 0 1) surface at 10 K to appear oxygen-terminated. The ionic motion within the central and surface regions of the slab system has been analysed in terms of mean-square displacements. At room temperature the 〈u2〉surface/〈u2〉bulk ratio for the Al ions is ≈2.5 and when only the out-of-plane surface motion is considered the ratio is as large as ≈3.5. The O ion motion at the surface is slightly smaller than that of the cations.