A simple chemical view of relaxations at stoichiometric (1 1 0) surfaces of rutile-structure type oxides: A first-principles study of stishovite, SiO2

First-principles electronic structure calculations have been used to examine the geometric and electronic structure of the bulk and (1 1 0) surface of stishovite, the rutile-structure polymorph of SiO2. The primary changes in geometric and electronic structure associated with surface relaxation are similar to those predicted for stoichiometric (1 1 0) surfaces of other rutile-structure oxides: TiO2, SnO2, RuO2. Occupied surface states can be attributed primarily to changes in the local coordination environment (hybridization) of surface oxygen anions, and the relaxations that lead to “rumpling” of the stoichiometric (1 1 0) surface can be viewed as a change in hybridization of 3-coordinated in-plane oxygen from a planar (sp2) bulk local coordination environment to a lower-energy, non-planar, pyramidal (sp3) surface geometry, following earlier descriptions by Godin and LaFemina for SnO2(1 1 0). It is demonstrated that these descriptions follow naturally from a visual examination of the 3D valence charge density distributions and the electron localization function (ELF) which provide a view of the electronic structure in terms of electron bond pairs and lone pairs. Consideration of the surface relaxations in terms of molecular analogs suggests that the simple valence shell electron pair repulsion (VSEPR) model provides insight into the chemical driving force for surface relaxation and oxygen rehybridization.