Modeling the surface structure and stability of alpha-quartz

Atomistic simulation techniques were employed to model the {0001}, {10 (1) over bar 0}, {10 (1) over bar 1}, and {10 (1) over bar (1) over bar} surfac es of alpha-quartz. The effect of associative and dissociative adsorption o f water onto the surface structure is studied, and it is found that associa tive adsorption of water onto the {10 (1) over bar 1} surface induces the f ormation of SiO-Si bridges, similar to those found on the very stable unhyd rated {0001} surface. Dissociative adsorption of water is energetically fav orable on all four surfaces, and hydration energies agree with experiment. Surface H+ ions were replaced by Na+ ions in two consecutive steps. Replaci ng only half the surface H+ ions by Na+ ions is energetically favorable, bu t when all H+ ions are replaced, the surface energies and hence stabilities of the four surfaces diverge widely, which has implications for the crysta l morphology. On the {10 (1) over bar (1) over bar} surface Na-O-Na bridges are formed which has a stabilizing effect.