Rhombohedral (Trigonal)

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1. A Rhombohedral Crystal is defined as a crystal structure with a rhombohedral shape and a specific arrangement of atoms, such as Bi2Se3, which exhibits a layered structure with a hexagonal lattice within each layer. (ScienceDirect, AI generated definition based on: Encyclopedia of Interfacial Chemistry, 2018). 2. The rhombohedral system (or trigonal) is a crystal system which can be described in two ways. Either we describe its unit cell as a rhombohedron whose 6 faces are equal rhombuses, with therefore a = b = c and alpha = beta = gamma, all different from 90°, or we describe it as the hexagonal system (a = b ≠ c) and alpha = beta = 90°, with gamma = 120°). This second way is now preferred for the presentation of crystallographic parameters of minerals. The characteristic element of the system is an axis of symmetry of order 3. This system comprises 5 crystal classes. (https://www.le-comptoir-geologique.com/rhombohedral-crystal-system-glossary.html). 3. Components of crystals in the trigonal system, like those of the hexagonal system, are located by reference to four axes—three of equal length with 120° intersections and one perpendicular to the plane of the other three. (Britannica).

https://purls.helmholtz-metadaten.de/skosmos/sdv/Rhombohedral

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