Observation and Structure Determination of an Oxide Quasicrystal Approximant

We report on the first observation of an approximant structure to the recently discovered two-dimensional oxide quasicrystal. Using scanning tunneling microscopy, low-energy electron diffraction, and surface x-ray diffraction in combination with ab initio calculations, the atomic structure and the bonding scheme are determined. The oxide approximant follows a $3^2\mathrm{.4.3.4}$ Archimedean tiling. Ti atoms reside at the corners of each tiling element and are threefold coordinated to oxygen atoms. Ba atoms separate the ${\mathrm{TiO}}_3$ clusters, leading to a fundamental edge length of the tiling 6.7 Å.

Figure 1

(a) Atomically resolved STM image of the ${\mathrm{BaTiO}}_3$-derived OQC approximant ($4\times 4{\mathrm{nm}}^2$, 0.3 nA, 1.0 V). The unit cell of the snub-square tiling is marked in red. The characteristic five-vertex element is marked in blue. (b) Large-scale image of the moiré structure of the approximant on Pt(111) ($35\times 35{\mathrm{nm}}^2$, 0.5 nA, $-1\mathrm{V}$). (c) Fourier transform of (b). The spots marked by red circles reveal the moiré periodicity. (d) Schematics of the quasicrystalline tiling with emphasis on the five-vertex elements therein. (e) LEED pattern of the OQC approximant on Pt(111) at 20 eV. (f) Assignment of the LEED spots to the six domains of the approximant.

Figure 2

SXRD-derived charge density $\rho (x,y)$ contour plot of four 2D unit cells of the OQC approximant. The snub-square tiling is indicated by the solid blue lines. Peaks in $\rho (x,y)$ are identified with Ba (green), Ti (blue), and O atoms (red). In the lower right part the threefold coordination of oxygen atoms around titanium atoms is emphasized by dashed triangles.

Figure 3

Relaxed approximant structure on Pt(111) from ab initio calculations: (a) top view, (b) side view. Corresponding charge density distribution with superimposed real structure: (c) top view, (d) side view for a cut along the dashed line marked with the arrow in (c).