Antiphase Domain Boundary Formation in 2D Ba-Ti-O on Pd(111): An Alternative to Phase Separation

2D oxide quasicrystals (OQCs) are unique structures arising from atoms positioned at the vertices of a dodecagonal triangle-square-rhombus tiling. The prototypical example for OQCs is derived from BaTiO3 on Pt(111). Herein, scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) investigations of 2D oxide layers derived from BaTiO3 on Pd(111) are reported. Upon ultrahigh vacuum (UHV) annealing, different long-range ordered structures are observed with a base of four vertex atoms forming two edge-sharing equilateral triangles. By a periodic repetition of this base in either quadratic or rectangular unit cells, a triangle-square tiling (known as sigma-phase approximant or 3(2).4.3.4 Archimedean tiling) or a triangle-rhombus tiling is formed. Both structures vary strongly in their vertex density. In addition, the formation of antiphase domain boundaries in the sigma phase is observed resulting in a well-ordered incorporation of rhombuses in the triangle-square tiling. A systematic variation of the frequency of these domain boundaries is identified as a mechanism for an incremental increase in the global vertex density, mediating between pure triangle-square and triangle-square-rhombus tilings.