Quasicrystalline

Quasicrystalline describes materials whose atomic arrangement is ordered but nonperiodic. They exhibit long-range order, as shown by sharp Bragg peaks in diffraction, but they lack translational symmetry of ordinary crystals. A hallmark is symmetry that is forbidden in periodic crystals, such as fivefold, tenfold, or icosahedral symmetry.

Discovered by Dan Shechtman in 1982 in a rapidly cooled aluminum-manganese alloy, quasicrystals challenged the conventional definition of a crystal. The concept is that the structure is quasiperiodic: it can be described by a projection from a higher-dimensional periodic lattice or by nonrepeatable tilings such as Penrose tilings. Quasicrystals are often categorized by their rotational symmetry, including icosahedral and decagonal types, and can have multiple atomic species arranged in complex motifs.

In terms of properties, quasicrystalline alloys are typically metallic and can show high hardness and low friction, along with unusual electronic and thermal behavior, such as higher electrical resistivity and low thermal conductivity compared to simpler metals. They are generally more brittle than many conventional metals and can pose processing challenges. The structure supports approximants—periodic crystals whose motifs locally resemble the quasicrystal.

Production methods include rapid solidification, vapor deposition, and specific alloy compositions that stabilize the quasicrystalline phase. Notable early examples include Al-Mn and later Al-Cu-Fe, Al-Pd-Mn, among others. Research continues into understanding their formation, fundamental order, and potential applications in coatings, catalysis, photonics, and beyond, with attention to improving mechanical properties and manufacturability.