Ĭalculating is a bit trickier, but it can be done with trigonometry. Since is the distance between two touching atoms, a must be. Is the distance between any two nearest atoms, which is also the length of each side of the hexagon. The HCP cell is defined by two lattice constants a and c, which correspond to two side lengths in the primitive cell (there is also a 3rd side length, b, but that’s the exact same as a). You can also visualize the primitive HCP cell, which has an atom at each of 8 corners and another one near the center. Using the hard sphere model, which imagines each atom as a discrete sphere, the HCP crystal has each atom touch along the top and bottom of the prism. The hexagonal close-packed lattice is a hexagonal prism with an atom on each vertex and three in center. Each of these NNs contributes a bond, giving the crystal structure very high stability. Hexagonal Close-Packed Coordination NumberĬoordination Number (CN) is the number of nearest neighbors that each atom has.Īs a close-packed structure, the HCP crystal has the number of nearest-neighbors (NN): 12. HCP is common because of its high coordination number. This list is not comprehensive HCP can also be found in high temperature/pressure phases, or in alloys. Since HCP is one of the most common crystal structures, there are many materials to choose from! Scandium, titanium, cobalt, zinc, yttrium, zirconium, technetium, ruthenium, cadmium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, lutetium, hafnium, rhenium, osmium, and thallium all have an HCP structure at standard pressure and temperature.
It can also be imagined as stacking 3 close-packed hexagonal layers such that the top layer and bottom layer line up.
The Hexagonal Close-Packed (HCP) unit cell can be imagined as a hexagonal prism with an atom on each vertex, and 3 atoms in the center. The hexagonal close-packed cell belongs to space group #194 or P6 3/mmc, Strukturbericht A3, and Pearson symbol hP2. HCP is one of the most stable crystal structures and has the highest packing density. The HCP crystal structure is based on the Bravais lattice of the same name, with 1 atom per lattice point at each corner of the hexagonal prism, and 3 inside the prism. The Hexagonal Close-Packed (HCP) crystal structure is one of the most common ways for atoms to arrange themselves in metals.