| In 1984 D. Schechtman announced
the discovery of a new class of materials (Schechtman et al. Phys.
Rev. Lett. 53 p.1951 1984). These bi- and tri-metallic
alloys appeared to have icosahedral symmetry (including 5- and 10- fold
rotations) and could not therefore be crystals in the conventional sense,
since crystals can have 1-, 2-, 3-, 4- and 6- fold symmetries only.
The irrational ratio of spot separations in diffraction for these materials
means that local lattice translations are not possible. These materials
were therefore dubbed `quasicrystals'. Schechtman was awarded the
1999 Wolf prize in Physics for his discovery.
Quasicrystals have since
been found to have many unusual tribological properties. These include
low coefficients of friction that rival teflon, good wear-resistance, and
good corrosion resistance (Dubois, Phys. Scripta T49A p.17 1993).
These properties make them an attractive alternative to conventional, chemically-based
platings for machine parts and other coatings applications. Quasicrystals
also display anomalous electrical properties, including the presence of
a pseudogap at the Fermi level, which makes the comparison of their electronic
structure with crystalline alloys of similar composition of obvious interest.
The almost semiconductor-like electrical behaviour has led to the suggestion
that they could be incorporated in sensors.
These properties and the
recent availability of large single grains have led to interest in the
surface properties of these materials. This has been led by the group
of Thiel and collaborators in Ames laboratory, by Urban and co-workers
in Julich, and others. In the past 18 months McGrath's group in Liverpool
have initiated studies of the clean and adsorbate-covered icosahedral Al-Pd-Mn
(and recently decagonal Al-Ni-Co) quasicrystal surfaces using STM, AES
and LEED. The clean surfaces of Al-Pd-Mn have been characterized
(Ledieu, McGrath et al., Surf. Sci. 433 p.665 1999; Proc. Mat, Res.
Soc. 553 p.237 1999; Mat. Sci. Eng. A, accepted). STM studies
of C60 adsorption reveal that the molecules
adsorb in a unique site at the surface and decorate the quasicrystal
lattice (Ledieu, McGrath et al., Surf. Sci. accepted). |