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Comparing with other crystal structures, body-centered cubic (bcc) packings of atoms tend to be most stable near the melting point of materials because the atoms have room to vibrate (important at high temperatures). 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.

Figure 2873 shows the effects of structural symmetry on XRD patterns. These effects include the difference of the reflection numbers and the peak splitting for the tetragonal and orthorhombic structures, for instance, shown in the red dotted box.

Figure 2873. The effects of structural symmetry on XRD patterns: (a) body-centered-cubic (bcc), (b) body-centered-tetragonal (bct), and (c) body-centered-orthorhombic (bco).

Table 2873a. Example of compounds with similar structures to BCC.

Table 2873b. Some normalized surface energies of low-index surfaces of bcc.

Table 2873c. Some symmetrical diffraction patterns of cubic crystals.

Zone axis	[100]	[110]	[111]
Symmetry	Square	Rectangular	Hexagonal
Aspect Ratio	1:1	1: for BCC, SC (almost hexagonal for FCC)	Equilateral

Table 2873d lists the characteristics of the three cubic Bravais lattices.

Table 2873d. 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 BCC, you can download the excel file for your own calculations)

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Table 2873e. Other characteristics of BCC structures.