"Quasi-crystal" is a common abbreviation for "quasi-periodic crystal"; In Chinese, it is further called "quasicrystal". It was first discovered in rapidly cooled Al-Mn alloy by Israeli, American and French scholars such as Chaitman, and was first reported as "metal phase with long-range orientation and no translational symmetry" on June 5438+0984+1October 9. The selected area electron diffraction pattern of the icosahedron is Ming Rui, which is regularly distributed with quintic symmetry, and the whole icosahedron has the symmetry of a regular triangle icosahedron (namely 6l510L315l215pc), so it is also called "icosahedral phase". The following spring, China scholar Zhang Ze and others reported that "a new icosahedral phase with symmetry" was found in Ti-V-Ni alloy based on their independent work.

Before the discovery of quasicrystals, the understanding of crystallography was that crystals are objects with long-range periodic three-dimensional translation order, so only crystals can make X-rays, neutrons and electron beams produce Ming Rui diffraction lines and form regular distribution diffraction patterns; At the same time, it is impossible to have five or more times the symmetry axis in the crystal. It is based on this * * * knowledge that when the report about quasicrystals came out, it caused a shock in academia. Some people say that this means "the disintegration of the laws of crystallography"; Some people think that the fifth symmetry of quasicrystals is the illusion caused by regularly connected crystals such as pentahedral crystals; Some authoritative scholars believe that the so-called quasicrystals are "nonsense".

However, further examination of the experimental results definitely ruled out the possibility of crystal growth; However, the diffraction pattern calculated according to the structural model of the pentasymmetrical regular triangle icosahedron is in good agreement with the experimental results in terms of spot position and intensity, thus confirming the certainty of the actual existence of quasicrystals. And only in the following five years, many scholars have discovered nearly 50 new icosahedral phases in intermetallic compounds with different compositions. At the same time, more than 30 kinds of two-dimensional quasicrystals with long-range orientation order but no translation order in a certain two-dimensional plane direction and long-range translation order in a third direction were found, and their symmetry axes were 8 times or 10 times or 12 times, which violated the crystal symmetry law perpendicular to this two-dimensional quasicrystal plane, so the corresponding two-dimensional quasicrystals were called octagonal phase and dodecagonal phase respectively. In addition, six kinds of one-dimensional quasicrystals have been found, that is, the structure with two-dimensional long-range translation order, and the third one is arranged according to the so-called Fibonacci sequence in mathematics, so it is also called Fibonacci phase.

The actual existence of quasicrystals completely proves that this is not nonsense, but the laws of crystallography have not collapsed. Instead, it broadens the field of crystallography and enriches its connotation. This can be seen from the following examples: 199 1 year, the Executive Committee of the International Federation of Crystallography approved the establishment of an aperiodic crystal committee to replace the original temporary committee on modulation structure, polycrystal and quasicrystal; In the report, the Executive Committee also pointed out that the "crystal" they said refers to "any solid with discrete diffraction patterns (meaning that the boundary of the diffraction spot itself is not connected with the diffraction spot)"; And "aperiodic crystal" refers to "any crystal that can be considered to have no periodicity of three-dimensional lattice". Obviously, this move by the International Federation of Crystallography is a signal, reflecting the discovery of quasicrystals and the results of their in-depth research. It is necessary for people to re-examine many known phenomena in crystal structure, such as modulation structure, polymorphism, incommensurate mismatch or composite crystal, from the perspective of quasi-periodicity (see section 10.3 for details), and incorporate them into a new and wider "aperiodic crystal" together with quasi-crystals.

But at the same time, it should be noted that the meaning of "crystal" proposed by the Committee on Aperiodic Crystals includes all aperiodic crystals such as quasicrystals, which is completely different from the existing concept of crystals that has lasted for hundreds of years. However, under the condition that the existing term "crystal" has not been abandoned, the term has been given a completely different connotation, which will inevitably lead to confusion, and actually has caused confusion. This is what readers should pay special attention to.

The known quasicrystals are intermetallic compounds, which are determined by their internal factors (see section 10.4); At that time, many scholars in many countries discovered quasicrystals independently, not by accident, but because the development of aerospace and other technologies urgently needed new materials with specific high-end properties (such as lighter, stronger and more high temperature resistant). Quasicrystals were discovered when developing new alloy materials by non-traditional processes, which has both important scientific significance and great practical value.