Quasicrystal

Quasicrystals were first observed by Dan Schechtman in experiments on electron diffraction on a fast-cooled Al ₆ Mn alloy , ^[1] conducted on April 8, 1982, for which he was awarded the Nobel Prize in Chemistry in 2011. The first quasicrystalline alloy discovered by him was called “ shechtmanite ” ( English Shechtmanite ) ^[2] . Shechtman’s article was not accepted for publication twice and in an abbreviated form was eventually published in collaboration with well-known experts Blekh, D. Gratias and J. Kahn. ^[3] The resulting diffraction pattern contained sharp ( Bragg ) peaks typical of crystals , but on the whole, the icosahedron had a point symmetry , that is, in particular, it had a fifth-order symmetry axis, which was impossible in a three-dimensional periodic lattice. The diffraction experiment initially allowed an explanation for an unusual phenomenon by diffraction on multiple crystal twins, fused into grains with icosahedral symmetry. Soon, however, more subtle experiments proved that the symmetry of quasicrystals is present on all scales, up to the atomic scale, and unusual substances are indeed a new structure for the organization of matter.

Later it turned out that physicists had been confronted with quasicrystals long before their official discovery, in particular, during the study of the de-patterns obtained by the Debye-Scherer method from intermetallic grains in aluminum alloys in the 1940s. However, at that time, icosahedral quasicrystals were mistakenly identified as cubic crystals with a large lattice constant . Predictions about the existence of an icosahedral structure in quasicrystals were made in 1981 by Kleinert and Maki ^[4] .

At present, there are hundreds of types of quasicrystals with point symmetry of the icosahedron, as well as ten-, eight- and twelve-angles.

Deterministic and entropy-stabilized quasicrystals

There are two hypotheses about why quasicrystals are (meta-) stable phases. According to one hypothesis, stability is caused by the fact that the internal energy of quasicrystals is minimal compared to other phases, as a result, quasicrystals must be stable even at an absolute zero temperature. In this approach, it makes sense to talk about certain positions of atoms in an ideal quasicrystal structure, that is, we are dealing with a deterministic quasicrystal. Another hypothesis suggests the determining contribution of entropy to stability. Entropy stabilized quasicrystals at low temperatures are fundamentally unstable. Now there is no reason to believe that real quasicrystals are stabilized solely due to entropy.

Multidimensional Description

The deterministic description of the structure of quasicrystals requires the position of each atom to be indicated, and the corresponding model of the structure should reproduce the experimentally observed diffraction pattern. The generally accepted way of describing such structures uses the fact that the point symmetry forbidden for the crystal lattice in three-dimensional space can be resolved in a space of higher dimension D. According to these structure models, atoms in a quasicrystal are located at the intersection of some (symmetric) three-dimensional subspace R ^D (called the physical subspace) with periodically located manifolds with the edge of dimension D-3, transversal to the physical subspace.

Build Rules

The multidimensional description does not answer the question of how local interatomic interactions can stabilize a quasicrystal. Quasicrystals have a paradoxical from the point of view of classical crystallography structure, predicted from theoretical considerations ( Penrose mosaics ). The Penrose mosaic theory allowed to deviate from the usual ideas about the Fedorov crystallographic groups (based on periodic space fillings).

Obtaining quasicrystals is hampered by the fact that they are either metastable or are formed from a melt whose composition differs from that of the solid phase ( incongruence ).

Rocks with natural Fe – Cu – Al – quasicrystals were found on the Koryak Highland in 1979. However, only in 2009, scientists from Princeton established this fact. In 2011, they published an article ^[5] in which they said that this quasicrystal (icosahedrite) is of extraterrestrial origin ^[6] . In the summer of the same 2011, during the expedition to Russia, mineralogists found new samples of natural quasicrystals. ^[7]

• Initially, experimenters managed to get into a very narrow "temperature gap" and get quasicrystalline materials with unusual new properties. However, later quasicrystals were found in the Al-Cu-Li system and other systems, which can be stable up to the melting point and grow practically under equilibrium conditions , just like ordinary crystals.
• The electrical resistance in quasicrystals, unlike metals , is abnormally high at low temperatures, and decreases with increasing temperature. In layered quasicrystals, along the packing axis, the electrical resistivity behaves as in a normal metal, and in quasicrystalline layers in the manner described above.
• Magnetic properties. Most of the quasicrystalline alloys are diamagnetic , but alloys with manganese are paramagnetic .
• Mechanical properties . The elastic properties of quasicrystals are closer to the elastic properties of amorphous substances than crystalline ones. They are characterized by lower values ​​of elastic moduli compared with crystals. However, quasicrystals are less plastic than crystals of similar composition and are likely to play the role of hardeners in metal alloys.

• AP Tsai "Topical review - Icosahedral clusters, icosaheral ordering and stability of quasicrystals - a view of metallurgy" Sci. Technol. Adv. Mater. 9 No 1 (2008) 013008 download for free
• Yu. Kh. Vekilov, M. A. Chernikov. Quasicrystals (rus.) // UFN . - 2010. - T. 180 . - pp . 561-586 .

• A talk about quasicrystals with a doctor of technical sciences, materials scientist Valentin Kraposhin
• Nobel Prize in Chemistry awarded for the discovery of quasicrystals
• Detailed description and history of discovery