Abstract
Quasicrystals are a class of ordered solids made of typical metallic atoms but they do not exhibit the physical properties that usually signal the presence of metallic bonding, and their electrical and thermal transport properties resemble a more semiconductor-like than metallic character. In this paper I first review a number of experimental results and numerical simulations suggesting that the origin of the unusual properties of these compounds can be traced back to two main features. For one thing, we have the formation of covalent bonds among certain atoms grouped into clusters at a local scale. Thus, the nature of chemical bonding among certain constituent atoms should play a significant role in the onset of non-metallic physical properties of quasicrystals bearing transition-metal elements. On the other hand, the self-similar symmetry of the underlying structure gives rise to the presence of an extended chemical bonding network due to a hierarchical nesting of clusters. This novel structural design leads to the existence of quite diverse wave functions, whose transmission characteristics range from extended to almost localized ones. Finally, the potential of quasicrystals as thermoelectric materials is discussed on the basis of their specific transport properties.
Highlights
Daniel Shechtman was awarded the Nobel Prize in Chemistry 2011 “for the discovery of quasicrystals”, a novel phase of matter first reported in an Al-Mn alloy on 12 November 1984 [1], characterized by the presence of a quasiperiodic atomic long-range order [2], along with point group symmetries which are not allowed by the classical crystallography restriction theorem, namely, icosahedral, octagonal, decagonal, or dodecagonal ones
Albeit the Al-Mn alloy discovered by Shechtman was thermodynamically unstable, transforming into a periodic crystal phase upon heat treatment, the existence of thermodynamically-stable QCs was subsequently reported in the Al-Li-Cu and Zn-Mg-Ga alloy systems in 1986 and 1987, respectively [7,8]
One must take into account the existence of a richer behavior for electronic states in QCs: On the one hand, we have extended electronic wave functions (Figure 3a) able to open a pseudogap close to the Fermi level via diffraction and interference processes with quasiperiodically-stacked arrangements of atomic planes; on the other hand, we have localized electronic states as well (Figure 3b), stemming from resonant effects involving nested atomic clusters exhibiting self-similar geometries at different scales
Summary
Daniel Shechtman was awarded the Nobel Prize in Chemistry 2011 “for the discovery of quasicrystals”, a novel phase of matter first reported in an Al-Mn alloy on 12 November 1984 [1], characterized by the presence of a quasiperiodic atomic long-range order [2], along with point group symmetries which are not allowed by the classical crystallography restriction theorem, namely, icosahedral, octagonal, decagonal, or dodecagonal ones. QCs provide an intriguingexample exampleof of ordered ordered solids made which do do notnot exhibit the physical an intriguing made of oftypical typicalmetallic metallicatoms atoms which exhibit the properties usually related to the of metallic bonding. Is this behavior uniquely physical properties usually related to presence the presence of metallic bonding. Is unusual this unusual behavior related to the long-range quasiperiodic order (QPO). Figure comparing electrical conductivity temperature dependences different quasicrystalline samples i-AlCuFe(),. From top to bottom their chemical compositions read as follows: Al63 Cu24.5 Fe12.5 , Al62.8 Cu24.812.5
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