Surface of Strontium Titanate

We report the first complete determination, using surface x-ray diffraction, of the surface structure of TiO₂-terminated SrTiO₃(001), both at room temperature in vacuum, and also hot, under typical conditions used for thin film growth. The cold structure consists of a mixture of a (1×1) relaxation and (2×1) and (2×2) reconstructions. The latter disappear over several minutes upon heating. The structures are best modeled by a TiO₂-rich surface similar to that proposed by Erdman et al. [Nature (London) 419, 55 (2002).]. Both reconstructions have been shown by density functional theory to be energetically favorable. The calculated (1×1) surface energy is higher, indicating that it may be a disordered mixture of the reconstructions. Atomic displacements are significant down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO₃.