hexagonal close-packed structure

The hexagonal close-packed (hcp) structure is a way that atoms can be arranged in a crystalline solid. It's one of the most compact ways to pack spheres, filling 74 percent of the available volume.1

In this structure, atoms are arranged in layers. The first layer has atoms packed in a triangular lattice. The second layer's atoms sit in the holes of the first layer. The third layer is placed directly above the first layer, creating an alternating sequence of layers (ABABAB...). This arrangement is characteristic of the hexagonal system in crystallography, which uses four axes for locating components: three of equal length at 120° to each other, and a fourth perpendicular to the plane of the others. Metals like cadmium and zinc crystallize with the hcp structure.12