1. Experimental Discovery

Quasicrystals were discovered in 1982, when Dan Shechtman observed a pattern of sharp spots with icosahedral symmetry in selected-area electron diffraction patterns of a rapidly cooled Al-Mn alloy. It took about two years before the experimental account of this icosahedral phase appeared in print [SBGC84], reflecting the reservations of at least part of the scientific community.

Independently, another type of quasicrystal with a twelvefold rotational axis was found in a Ni-Cr alloy [INF85]. This dodecagonal phase was also characterized by sharp electron diffraction spots, a clear evidence of an ordered structure. Subsequently, two more types of quasicrystals with ten- and eightfold rotational symmetries were found. Again by selected-area electron diffraction, a decagonal phase was observed in a rapidly solidified Al-Mn alloy [Bend85] with a somewhat higher Mn content than those used by Shechtman et al. [SBGC84]. An octagonal phase was first found in rapidly solidified V-Ni-Si and Cr-Ni-Si alloys [WCK87]. These three phases with an axis of twelve-, ten-, or eightfold symmetry are sometimes referred to as T phases, they share the common feature that they are periodic in one space direction which is the direction of the high-symmetry axis.

Thus, the peculiar property of quasicrystals that led to their experimental discovery is the occurrence of sharp spots in electron diffraction patterns with symmetries that are not found in "conventional" crystals. In other words, quasicrystals are well-ordered solids with unusual symmetry properties. As an example, electron diffraction patterns for eight different structural modifications of decagonal Al-Co-Ni are displayed in figure 1, showing the richness of quasicrystalline phases that appear in this intermetallic alloy. The top left pattern corresponds to the so-called "basic Ni-rich state", which is one of the best ordered quasicrystalline structures known to date [RBNG+96, JBR97]. For more detailed information on the Al-Co-Ni system, which serves as the prototype for decagonal quasicrystals, we refer to [T:Sche, YKST90, ETYS+94, GU94, RBN96, RBNG+96, RNB96, GrM97a, GrM97b, JBR97, ZES97, CW98b, HHS98, GSLR+98, RBNG+98, SGLR+98, HPR99] and references therein.

Figure 1: Electron diffraction patterns of eight structural modifications of decagonal Al-Co-Ni perpendicular to the unique periodic axis [RBNG+98].

Uwe Grimm, last modified on 14 September 2005