Structural aspects of the threefold surface of icosahedral Al–Pd–Mn

We report the first STM study of a threefold surface of an icosahedral quasicrystal. We find that a rough, cluster-dominated structure evolves into a terrace-step morphology, with increasing temperature. The terraces display a fine structure whose long-range order is consistent with threefold symmetry. The fine structure includes small, deep holes. The steps can be very straight, serving to bound equilateral triangles (or portions thereof). These straight steps can cut directly across meandering step edges, superimposing a triangular ‘shadow’ upon the other terrace-step landscape. The data suggest that the triangles grow outward from a special type of central point. The triangles may represent the initial stages of facetting.