Atomistic simulation and virtual diffraction characterization of stable and metastable alumina surfaces

The structures of select alumina surfaces are studied using molecular statics and molecular dynamics simulations and are characterized using virtual diffraction methods. First, bulk alumina simulations are performed to validate the transferability of the ReaxFF potential to model different alumina phases. Bulk alumina simulations accurately predict α-Al2O3 as the lowest energy crystalline phase; however, they unexpectedly predict an even lower-energy amorphous phase. At 0 K, virtual X-ray diffraction patterns of the bulk crystalline phases and select alumina surfaces are validated by experimental studies. Molecular statics simulations of select alumina surfaces are consistent with prior first-principles studies. However, molecular dynamics simulations show that many surfaces experience significant reconstructions at temperatures below what is expected from experiments. It is believed that premature surface reconstructions are biased by the predicted lower-energy amorphous phase and occur due to the extra degrees of freedom allowed by the free surfaces as well as the available thermal energy during dynamics. Discrete peaks appearing in virtual selected-area electron diffraction patterns indicate that the reconstructions are not fully amorphous due to lattice constraints imposed by the internal bulk structure. Bulk and surface energies are tabulated for each simulation to be used in future predictive mesoscale models of polymorphic alumina.