=================================================================================

Face-centered cubic (fcc) crystals are common among metals at low temperatures because the atoms pack well. Steels are body-centered cubic at low temperatures and face-centered cubic at high temperatures. For FCC and HCP systems, the coordination numbers are 12, while for BCC it’s 8. Assuming a hard sphere model, atomic packing factor is defined as the ratio of atomic sphere volume to unit cell volume, which is 74% for both FCC and HCP and 68% for BCC. In general, ~90% elemental metals crystallize into three crystal structures which are BCC, FCC, and HCP.

Table 2872a. Some normalized surface energies of low-index surfaces of fcc.

Table 2872b lists the characteristics of the three cubic Bravais lattices.

Table 2872b. The only three cubic Bravais lattices.

Lattice	Number of lattice points per unit cell	Number of atoms per unit cell	Nearest distance between lattice points	Maximum packing
				Maximum packing condition	Density (or fraction of packing, Vatom/Vcell)
Simple cubic	1	1		When the adjacent atoms touch each other along the edge of the cube	52.4%
BCC	2	2		When the adjacent atoms touch each other along the body diagonal of the cubic cell	68.0%
FCC	4	4		When the adjacent atoms touch each other along the face diagonal of the cubic cell	74.0%
Diamond	4	8			34 %

For FCC and BCC structures, the lattice constants are given by, (For FCC, you can download the excel file for your own calculations)

          ----------------- [2872]

Table 2872c. Other characteristics of FCC structures.

Figure 2872a. Atomic structure of FCC.