Quantum mechanical ab initio characterization of a simple periodic model of the silica surface

The electronic, structural, and vibrational properties of a thin silica fil m are studied by means of periodic ab initio techniques, both Hartree-Fock and density functional. The film is cut from bulk edingtonite, a tetragonal silica structure with five SiO2 groups per unit cell; the dangling bonds a re saturated so as to obtain a fully hydroxylated surface. This model syste m is proved to possess a number of attractive properties which make it suit able for the study of some properties of silica surfaces: (1) the parent st ructure is rather stable, its energy per unit SiO2 being comparable to that of common zeolites such as faujasite; (2) the formation of the surface inv olves low energy, no reconstruction, and scarce relaxation; (3) the simples t two-dimensional structure contains only five SiO2 groups in the unit cell and can therefore be treated with limited computational effort; (4) the bu ilding block of the two-dimensional periodic structure is easily related to standard cluster models of silica; (5) the density of surface hydroxyls is comparable to that experimentally observed on dehydrated silica; (6) the s tretching frequency of the OH group at the surface is very close to that ob tained with the best cluster models and to the experimental value for isola ted hydroxyls on amorphous silica; (7) a few variants of the fundamental st ructure are possible, and different low-energy surfaces can be cut from the m, which allow the simulation of a number of local situations within the sa me basic model.