Two-Dimensional Wetting Layer Structures of Reduced Ternary Oxides on Ru(0001) and Pt(111)

Long-range ordered structures of reduced oxide films with monolayer thickness derived from BaTiO3 and SrTiO3 on Ru(0001) and Pt(111) are investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Upon ultrahigh vacuum annealing at 1100 K, a hexagonal phase is observed for BaTiO3 on Ru(0001), which forms similarly from SrTiO3 on Pt(111). At higher temperatures, a triangle-square tiling called sigma-phase develops in the BaTiO3/Ru(0001) system, with a unit cell rotation of 15 degrees against the Ru(0001) substrate. Furthermore, it is shown that this 15 degrees rotated sigma-phase also forms in the BaTiO3/Pt(111) system in addition to the already known 8 degrees rotated sigma-phase. The results emphasize a strong flexibility in the structural parameters of the reduced oxide wetting layers in response to the substrate interaction strength.