Quasiperiodic crystals, or, in short, quasicrystals (QCs) were discovered in 1982 and reported in 1984 by Shechtman, Blech, Gratias & Cahn.1 QCs are materials with perfect long-range order and display strong Bragg reflections, but with no three-dimensional translational periodicity, instead they possess non-crystallographic rotational symmetry (Figure 1). QCs constitute an intermediate state between amorphous solids and regular crystalline materials.2 With their atypical symmetry and structure, QCs have been a “crystallographer′s dream” with lot of work being done to structurally characterize these materials. The exotic structural properties of this class of materials are accompanied by unique physical properties that are unexpected for metallic alloys.3 In the last two decades, QCs have emerged as a unique and important class of materials, particularly given the fact that the 2011 Nobel Prize in Chemistry has been given to one of the scientists, Prof. Dan Shechtman, who pioneered this field.
Quasicrystals can be classified into two structural groups:
Polygonal or dihedral quasicrystals (quasiperiodic in two dimensions), have an axis of 8- (octagonal), 10- (decagonal) or 12- (dodecagonal) fold local symmetry. They are periodic along this axis, which is perpendicular to quasiperiodic layers.
Icosahedral quasicrystals (quasiperiodic in three dimensions) with 5 fold or broken symmetry. They have no periodic direction. Figure 2 shows a photograph and electron diffraction pattern of single-grain icosahedral Ho-Mg-Zn quasicrystal grown from the ternary melt: by using the self-flux method (excess Mg), and slowly cooling from 700°C to 480°C.4
Along with their novel structures and symmetries, QCs exhibit unusual properties. The properties of QCs also vary depending on their direction.
Quasicrystals have remarkable electronic properties (such as strong anisotropy in electronic transport), arising from the interplay between short-range and long-range order in these materials. Electrically, they behave in a very peculiar way depending on temperature. QCs provide relatively low electrical and thermal conductivity at room temperature; however, the electrical conductivity of QCs increases linearly with temperature. QCs usually show metal-insulator transition. At low temperatures, their resistance changes markedly in response to changing magnetic fields, which makes them interesting for applications in magnetic devices.5, 6
Their interesting optical properties also find some applications as a solar selective absorber, which absorbs solar radiation and converts it to heat.7
Another interesting application of QC is as reversible hydrogen storage medium. QCs based on Ti-Zr-Ni is considered to be new, promising hydrogen-storage material.8
Quasicrystals are hard and brittle at room temperature, also show a low surface friction and high oxidation and corrosion resistance. They are usually ductile at high temperatures and exhibit work softening. The brittleness of bulk QCs can be overcome while still preserving some of their useful properties by using these particles in the nanometer range. Most applications use QCs as coatings, thin films or small particles embedded in another material. Such materials are of great technological interest as they can be strong but much lighter than other materials with comparable physical properties.9
Quasicrystals have already found some commercial applications in surgical instruments, LED lights, heat insulation in engines and nonstick frying pans.
Aldrich Materials Science now offers high purity aluminum-based quasicrystals with ultra-fine particle size and narrow particle size distribution, as seen in the table below.
1. Shechtman, D.; Blech, I.; Gratias, D.; Cahn, J. W. Phys. Rev. Lett.1984, 53, 1951. 2. Jannot, C. Quasicrystals: A. Primer 2nd Ed., Osford University Press, 1994. 3. Shechtman, D.; Lang, C. I. MRS Bulletin1997, 22(11), 40. 4. Fisher, I. R.; Cheon, K. O.; Panchula, A. F.; Canfield, P. C. Phys. Rev. B1999, 59, 308. 5. Poon, S. J. Advances in Phys. 1992, 41(4), 303. 6. Janot, C.; de Boissieu, M. Phys. Rev. Lett. 1994, 72(11), 1674. 7. Eisenhammer, T. Thin Solid Films1995, 270(1-2), 1. 8. Kim, J. Y.; Hennig, R.; Huett, V. T.; Gibbons, P. C.; Kelton, K. F. J. Alloys and Compounds2005, 404-406, 388. 9. Lang, C. I.; Shechtman, D.; Gonzalez, E. Bulletin of Materials Science1999, 22(3), 189.