Schematic diagram of hcp structure. (a) Numbered balls represent atoms which belong to successive atomic layers, perpendicular to [11-20] crystallographic direction. The open circles refer to the layers with odd numbers, while the filled circles represent those with even numbers. Edges of hexagonal prism which belong to  and [1-100] directions in the second layer are denoted with black line, while those in the fourth layer are shown by gray lines. (b) Array showing an atomic arrangement in the successive layers of the hcp structure (a top view). The open circles refer to the atoms in the odd layers, while those in the even layers are represented by the filled circles. Two crystallographic directions are denoted.
Huang et al. Nanoscale Research Letters 2012 7:189 doi:10.1186/1556-276X-7-189