Oxides are ubiquitous as they are present on earth and in every soil and sediment as well as in aerosols, aquatic biota, and waste streams. They come from a variety of sources, both natural and anthropogenic.

For most bulk oxides the crystal structures are well established, and for many systems the thermodynamics are well documented from experiments and theoretical calculations. At the surface much less is understood. While one can easily perform theoretical calculations on simple bulk-like (e.g. 1x1) terminations, the actual thermodynamically stable surface and/or experimentally observed structures are often larger and more complicated. Until these surface structures have been unambiguously experimentally determined, the problem can be confused. Even for such a simple system as the (100) surface of the archetypal perovskite strontium titanate, there are at least six different experimentally determined surface reconstructions in addition to the simple 1x1 bulk terminations. Not all of these structures have yet been solved at the atomic level, and to date there is not convincing agreement between experimental observations and theoretical analyses as to which surface structures should be stable under what conditions. One example is the 2x1 reconstruction of the SrTiO₃ (001)shown here, with the red atoms oxygen and grey atoms titanium. Strontium titanate is a important material both as a potential photocatalyst and for thin film growth. Before this surface structure was solved it was commonly believed that the (001) surface was a simple termination of the bulk. It turns out not to be at all like this, instead it has two layers of TiO₂ at the surface.