The hexagonal close-packed (hcp) structure is one of the most common and important crystal structures adopted by metals and other atomic solids. Despite its prominence in nature, the hcp structure has not been treated at the same level of geometric rigor as the common cubic structures. The illustrations of the hcp unit cell that are used in textbooks at all levels and also in crystallography and solid-state reference works are incomplete, in that they fail to include fractions of middle layer atomic spheres with centers lying outside of the unit cell. These fractions are necessary to properly determine the number of atoms enclosed in the unit cell. Though it seems unlikely that the middle layer fractions of atomic spheres have not been previously determined, we have found no evidence of this in our survey of textbooks and reference works. We have shown how these fractions may be determined using the solids-of-revolution method and have prepared animated 3D "cutaway" computer models and templates for paper and glue models to more accurately illustrate the hcp unit cell.
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