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The first quasicrystalline samples were found in rapidly solidified binary alloys, an icosahedral [SBGC84] and a decagonal [Bend85] phase in Al-Mn and the dodecagonal phase in Ni-Cr [INF85]. These phases are metastable, as it is the case for a sizable fraction of the known quasicrystals. However, the assumption that quasicrystals generally are metastable phases that can only be produced by rapid quenching from the melt was disproved soon. The first thermodynamically stable icosahedral quasicrystals were found in a ternary Al-Cu-Li alloy [SD86], and by today quite a collection of stable icosahedral and decagonal phases is known.
Typically, stable quasicrystals occur in ternary or quaternary alloys. A large class consists of Al-based alloys, the most prominent examples being Al-Cu-Fe [TIM87, TIM88a, TIM88d], Al-Pd-Mn [TIYM90, BNR91], and Al-Pd-Re [TIYM90], which show highly ordered icosahedral phases. There is also a stable decagonal phase in the Al-Pd-Mn system [BNR91, BH94], other important examples of decagonal quasicrystals are the decagonal phases of the Al-Co-Cu [HZWK88, HWMK90, GSLR+98] and the Al-Co-Ni [T:Sche, YKST90, RBN96, RBNG+96, GSLR+98, RBNG+98, SGLR+98] system. Particularly the Al-Co-Ni system has been studied in much detail, and by now eight different structural modifications of the decagonal phase have been reported [RBNG+98], showing different degrees of perfection, see also figure 1 above.
Although Al-based quasicrystals still play a major role, it is by no means true that these exhaust all quasicrystalline materials. A new class of stable icosahedral quasicrystals was discovered in Mg-Zn alloys containing rare earth elements [LZTZ93, TNIM+94]. These are very interesting from the physical point of view because the rare earth elements, due to their large magnetic moments, are expected to lead to interesting magnetic properties of these materials. Other materials that may be important for applications are Ti-Zr-based quasicrystals [SVGK+96}, see the discussion below. Further icosahedral and stable octagonal quasicrystals have been found in Ga alloys, see~[GK98] and references therein, and a dodecagonal quasicrystalline phase and several approximant phases have been discovered in the binary Ta-Te system [CH98, CHK98, KCH98].
With the advent of stable quasicrystalline alloys, it became possible to grow single quasicrystals of reasonable size, see for instance [BN93, ZES97, FIPC+98, LA98, SHT98, STT98, Tsai98, FKIR+99] and references therein. Thus one was able to perform experimental measurements on macroscopic single quasicrystals. This has promoted the investigation of quasicrystal properties to a new level. Nevertheless, the preparation and characterization of the samples remains difficult and still poses one of the experimental challenges when dealing with quasicrystals.
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Uwe Grimm,
last modified on
14 September 2005