Surface structure of SrTiO3(001)

We report on the structural determination of the surface of TiO2-terminated SrTiO3(001) using surface x-ray diffraction. The detailed analysis of two surface diffraction data sets are presented, one (cold) taken at room temperature in vacuum, and the other (hot) under typical conditions used for thin film growth. 49 different combinations of possible surface terminations are described for the cold structure, from which the final structure was chosen, consisting of a weighted mixture of a (1x1) relaxation and (2x1) and (2x2) reconstructions, simultaneously present at the surface. The structures are best modeled by a TiO2-rich surface similar to that proposed by Erdman [Nature (London) 419, 55 (2002)]. The reconstructions are energetically favorable according to density functional theory. They disappear within several minutes upon heating to the hot conditions, forming a termination very similar to the cold (1x1), but more puckered and higher in energy. Six additional models, suggested by direct methods and the literature, to describe the hot surface are also discussed. Direct methods confirm the TiO2-rich termination and the atomic positions of the hot surface. The atomic coordinates for the two TiO2-rich surfaces exhibit significant displacements down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO3. Surface energy considerations suggest a temperature-induced order-disorder transition, produced by a mixing of the (2x1) and (2x2) reconstructions, to form the hot pseudo (1x1) structure. Electrostatic stability arguments provide circumstantial support for the experimentally determined TiO2-rich surfaces.