C hapter 4 Material Classes, structure, and properties
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that we know what a unit cell is, we can introduce another idea:
the notion of a crystal lattice. This is the structure that arises from
the repetition of a particular pattern or unit cell in a 3-D space. In
nature, there are 14 possible unit cells, also called the Bravais lattices (see Figure 4.12). These 14 types of unit cells are grouped into
seven possible crystal systems: cubic, tetragonal, orthorhombic,
hexagonal, rhombohedral, monoclinic, and triclinic.
The simplest crystal structures typically belong to pure metals. The
most common are the face-centered cubic (FCC), the body-centered
cubic (BCC), and the hexagonal closed packed (HCP) crystal structures
(see Figure 4.13). The FCC structure is composed of atoms located
at each corner and at the center of each face. Some examples of
elements that have this structure are the metals copper, silver, gold,
nickel, and aluminum. The BCC unit cell has a configuration in
which the atoms are located at each corner of the cube, with one
atom at the center of the cube. Some examples of this crystal structure are iron, tungsten, and chromium. The HCP crystal structure
has a configuration in which atoms are located in the corners of a
Figure 4.12
The 14 Bravais lattices.
Simple
cubic
Facecentered
cubic
Bodycentered
cubic
Simple
tetragonal
Bodycentered
tetragonal
Hexagonal
Simple
orthorhombric
Bodycentered
orthorhombric
Basecentered
or thorhombric
Facecentered
or thorhombric
Simple
monoclinic
Rhombohedral
Basecentered
monoclinic
Triclinic
Figure 4.13
(a) Face-centered cubic unit cell, (b) body-centered
cubic unit cell and (c) hexagonal closed packed
unit cell.
a
c
b
a
b
c
b
a
c
(b)
(a)
(c)