simplest form of these cells are the primitive
cells, denoted as P, which have lattice points
at each corner. The non-primitive cells have
lattice points, not only at the corners, but also
at additional locations. A body-centered unit
cell, denoted by I, has a lattice point at the
center of the cell, a side-centered cell, denoted
by C, has two lattice points on opposite
faces, and a face-centered cell, denoted by F,
has lattice points at the center of each of the
six faces.
CHAPTER 15 X-RAY DIFFRACTION AND EXAFS 321
Figure 15.5Crystals are formed by
repeating units termed unit cells.
Rhombohedral Hexagonal
Orthorhombic systems
Cubic
PIC
P
P
P
PPC
P
CIF
I
Monoclinic Triclinic
Tetragonal
c
b
0 a
Table 15.1
The seven Bravais crystal systems.
System Symmetry*
Triclinic None
Monoclinic One C 2 axis
Orthorhombic Three perpendicular C 2 axes
Rhombohedral One C 3 axis
Tetragonal One C 4 axis
Hexagonal One C 6 axis
Cubic Four C 4 axes in tetrahedral arrangement
*The rotation axes are denoted by Cn, where 360°/nis the rotational symmetry.
Figure 15.6The Bravais lattices showing primitive
(P), body-centered (I), face-centered (F), and side-
centered (C) lattices.