Voids and pits on sputter-annealed fivefold terraces of icosahedral Al–Pd–Mn quasicrystals

Abstract

Our scanning tunneling microscope studies on clean surfaces of fivefold icosahedral Al_70.2Pd_20.7Mn_9.1 reveal that, after annealing at 900 K, two types of flat depressions are visible on terraces: voids (small, shallow, flat-bottomed depressions bordered by 2.5 Å steps) and pits (deeper, and often larger, depressions). They are different not only in their dimensions but in their behavior: voids preferentially nucleate on some of the terraces and, after annealing at 925 K, their coalescence and growth leads a new type of termination to be exposed. Pits, however, do not destroy the terraces on which they exist, under the experimental conditions. At the bottom of 4.1 Å-deep pits, 2.5 Å-deep voids nucleate, so that the total depth of the pit is 6.6 Å. We propose that 2.5 Å void-rich terraces are metastable terminations, and that these metastable terminations are also exposed at the bottom of the 4.1 Å-deep pits.

Acknowledgment

This work was supported by the Director, Office of Science of Basic Energy Science, Materials Science Division of the U.S. Department of Energy under Contract No. W-405-Eng-82.