Title: Metallic Phase with Long-Range Orientational Order and No Translational Symmetry
Authors: D. Shechtman, I. Blech1, D. Gratias2, and J. W. Cahn3
Journal: Physical Review Letters
Affiliations: 1Department of Materials Engineering, Israel Institute of Technology Technion, 3200 Haifa, Israel. 2Centre d’Etudes de Chimie Metallurgique, Centre National de la Recherche Scientiftque, F 94400-Vitry, France. 3Center for Materials Science, National Bureau of Standards, Gaithersburg, Maryland 20760
This year’s Nobel Prize in Chemistry was awarded to Professor Dan Shechtman of the Israel Institute of Technology (Technion) for his discovery of quasicrystals. His 1984 paper was the first to report an Al-Mn alloy crystal that showed sharp X-ray diffraction peaks, despite its 5-fold rotational symmetry. Prior to this discovery, crystals were defined as molecularly uniform structures when translated in all directions in a crystal lattice. This required that the crystal have 2, 4, or 6-fold rotational symmetry because these geometric shapes could fit together in a repeating pattern without leaving gaps between neighboring units (see below). Therefore, it was thought that only a crystal of an allowed symmetry would produce an x-ray diffraction pattern and could be called a crystal.
In their paper, Shechtman et. al. synthesized a non-conducting alumnium crystal with 10-14% manganese. The resulting mixture was crystallized by rapid cooling and showed a diffraction pattern that had six fivefold, ten three-fold and fifteen twofold axes, exhibiting icosahedral (Ih) symmetry. His crystals, however, did not have a uniform structure or periodic structure. When a fivefold symmetric structure is repeated in all directions there is a slight discrepancy in translational distance and the new set of lattice points is offset from the first so that the new set of axes does not produce a periodic crystal arrangement (see below).
The paper mentions that this unusual pattern could be explained by twinning. Twinned crystals share similar lattice properties because they grow into one another. Shechtman lists several reasons why the crystals do not exist as twins; twinned arrangements can be translated in all directions, while Shechtman’s crystals could not. Shechtman’s discovery resulted in the redefinition of a crystal as any structure that yields a sharp diffraction pattern.