Quasicrystals are a form of solid that differ from crystalline and amorphous materials in that they exhibit aperiodic long-range order. They have been the focus of interest for the past two decades because of the presence of novel atomic arrangements, but also because of the interesting surface properties they exhibit, such as low friction, low adhesion, corrosion resistance and catalytic activity. Most stable quasicrystalline phases have icosahedral symmetry and exhibit three high-symmetry surfaces: fivefold, threefold and twofold. The relative stability of the three high-symmetry surfaces of the icosahedral quasicrystals is still a matter of debate. In J. Phys.: Condens. Matter 26 015001 , we present the structure of the twofold surface of icosahedral Ag-In-Yb quasicrystal and compare it with the structure of the other two high-symmetry surfaces of the same system.

We used scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED) to investigate the surface structure of the twofold i-Ag-In-Yb surface. Both the LEED and STM images confirm that the surface exhibits quasicrystalline long-range order, with the twofold symmetry expected from the bulk. STM images reveal a step–terrace structure with a terrace size comparable to that of the threefold and fivefold surfaces of the same quasicrystal (figure 1). The distribution of step heights and the high-resolution STM images of the terraces suggest that the twofold surface terminates at bulk planes intersecting the centers of the rhombic triacontahedral (RTH) clusters that make up the bulk structure of the system. The other high-symmetry surfaces also terminate at bulk planes intersecting the center of the RTH clusters.

The twofold surface is formed at the highest density bulk planes, whereas the fivefold and threefold surfaces are formed at moderate density planes. A common observation in all of the surfaces is that the surface planes are rich in Yb and In. This suggests that surface termination is also influenced by the chemical composition of the planes and is not exclusively determined by atomic density. No facets are observed on the surface, suggesting that the twofold surface is as stable as the other high-symmetry surfaces.