Structure of Periodic Crystals and Quasicrystals in Ultrathin Films of Ba-Ti-O
Eric J. Cockayne, Marek Mihalkovic, Christopher L. Henley
We model the remarkable thin-film Ba-Ti-O structures formed by heat treatment of an initial perovskite BaTiO$_3$ thin film on a Pt(111) surface. All structures contain a rumpled Ti-O network with all Ti threefold coordinated with O, and with Ba occupying the larger. mainly Ti_7O_7, pores. The quasicrystal structures is a simple decoration of three types of tiles: square, triangle and 30^o rhombus, with edge lengths 6.85 Angstroms, joined edge-to-edge in a quasicrystalline pattern; observed crystalline Ba-Ti-O structures are built from these and other tiles. Simulated scanning tunneling microscopy images reproduce the patterns seen experimentally, and identify the bright protrusions as Ba atoms. The models are consistent with all experimental observations.