A crystalline liquid or solid has a crystal structure composed of a repeating unit cell, which is a particular arrangement of atoms. This repeating unit cell is also called the crystal lattice.
There are several types of crystal structures, or lattices. The simple cubic, or sc, lattice is the simplest repeating unit cell. A cube is the repeating unit cell, and there is one atom at each corner of the cube. This crystal lattice is not often found in nature because it is an inefficient use of space.
A body centered cubic, or bcc, structure has one atom in the middle of the cube and one atom at each corner of the cube. Lithium naturally has a bcc structure. A face centered cubic, or fcc, structure is more common that the bcc or sc structure. There is an atom at each corner of the cube, in the middle of the cube and in the center of each face of the cube. Gold and platinum are examples of materials that naturally have an fcc lattice.
The hexagonal close packed, or hcp, structure, does not have a cube as the repeating unit cell. There are three layers of atoms in the hcp structure. In the bottom layer there are six atoms, five at the vertices of a hexagon and one in the center. The second layer has three atoms in a triangle formation that fit into the empty spaces of the layer below it. The top layer is exactly the same as the bottom layer. Titanium and beryllium are examples of materials that naturally have an hcp lattice.