What is the Difference Between Primitive Hexagonal Unit Cell and Hexagonal Closed Packing?

The main difference between a primitive hexagonal unit cell and hexagonal closed packing (HCP) lies in the arrangement of atoms within the unit cell and the type of lattice structure they form.

• Primitive Hexagonal Unit Cell: This is a unit cell that has a hexagonal base with a single atom at the center of the hexagon. It is a part of the hexagonal crystal family, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). The primitive hexagonal unit cell has a coordination number of 6, meaning that each atom in the unit cell is in contact with six neighboring atoms.
• Hexagonal Closed Packing (HCP): HCP is a close-packed structure where each sphere is in contact with six neighbored spheres, forming a hexagonal arrangement. In HCP, the unit cell consists of three primitive unit cells, forming a hexagonal prism containing six atoms. The HCP structure has a coordination number of 12, which is the highest coordination number among all possible lattice structures. However, HCP is not a Bravais lattice because the single lattice sites (lattice points) are not completely equivalent.

In summary, the primitive hexagonal unit cell is a simple repeating unit of a hexagonal crystal structure, while hexagonal closed packing is a more complex arrangement of atoms forming a close-packed structure with a higher coordination number.

Comparative Table: Primitive Hexagonal Unit Cell vs Hexagonal Closed Packing

The key difference between primitive hexagonal unit cell and hexagonal closed packing lies in their structure and packing efficiency. Here is a table comparing the two:

In summary, the primitive hexagonal unit cell is a repeating unit of a hexagonal crystal lattice, while hexagonal closed packing is an arrangement of spheres in layers. The hexagonal unit cell has a high atomic density due to its hexagonal shape, whereas hexagonal close packing is a more efficient way of packing spheres.