(56e) A Simple Chemical View of Relaxations at (110) Surfaces of Rutile Structure-Type Oxides

First-principles
electronic structure calculations have been used to examine the geometric and
electronic structure of the bulk and (110) surface of two rutile structure-type
oxides: stannic oxide, SnO₂, and stishovite, SiO₂.  Examination
of the 3D valence charge density distributions and the electron localization
function (ELF) provides a simple view of the electronic structure in terms of
local electron bond pairs and lone pairs.  For stoichiometric (110) surfaces,
variations in the spatial distribution of electron pairs about the oxygen
anions can be interpreted in simple chemical terms as rehybridization of
surface oxygen anions, and the driving force for relaxations can be understood
qualitatively in terms of the simple valence shell electron pair repulsion
(VSEPR) model, independent of the nature of the cation in the structure.  For the
case of SnO₂, the stability of highly oxygen-deficient surfaces can
be understood in terms of the coordination chemistry of the surface Sn cations.