Structure of Ultrathin Films of Barium Titanate

BaTiO$_3$ is one of the best-known perovskite ferroelectrics, and has been extensively studied in its bulk and thin-film forms. Remarkably, a variety of new structures (including a dodecagonal quasicrystal) have recently been observed in ultrathin Ba-Ti-O films fabricated via various multistep heat treatments of an initial thin film perovskite BaTiO$_3$ layer deposited on a Pt(111) surface. We have found a tiling decoration model for the ultrathin film Ba-Ti-O structures that is fully consistent with experimental observations. The structures consist of rumpled Ti-O networks with each Ti threefold coordinated with O, and Ba nestled in the larger, mainly Ti$_7$O$_7$, pores. Simulated STM images show that the circular protrusions observed experimentally are the Ba ions. Ab initio molecular dynamics calculations demonstrate the stability of our model with respect to competing models for these thin film structures.