BioPHYSICAL chemistry

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.