  First-principles calculations based on density functional theory (DFT) and
the pseudopotential method have been used to study the stoichiometric and
reduced SnO2 (110) surface. The ionic relaxations are found to be moderate for
both the stoichiometric and reduced surfaces, and are very similar to those
found in recent DFT-pseudopotential work on TiO2. Removal of neutral oxygen
leaves two electrons per oxygen on the surface, which are distributed in
channels passing through bridging oxygen sites. The associated electron density
can be attributed to reduction of tin from Sn4+ to Sn2+, but only if the charge
distribution on Sn2+ is recognized to be highly asymmetric. Reduction of the
surface gives rise to a broad distribution of gap states, in qualitative
agreement with spectroscopic measurements.
