Unit Cell

The unit cell is the basic building block of a crystal, repeated infinitely in three dimensions. It is characterised by:

three vectors (a, b, c) that form the edges of a parallelepiped;
the angles between the vectors (alpha, the angle between b and c; beta, the angle between a and c; gamma, the angle between a and b).

Depending on the crystal system, there are sometimes restrictions on the values that unit cell parameters can take:

Triclinic/Anorthic	no restrictions
Monoclinic	alpha = gamma = 90
Orthorhombic	alpha = beta = gamma = 90
Tetragonal	a = b alpha = beta = gamma = 90
Trigonal/Hexagonal	a = b alpha = beta = 90, gamma = 120
Rhombohedral	a = b = c alpha = beta = gamma
Cubic	a = b = c alpha = beta = gamma = 90

In the above table, alpha=gamma=90 for monoclinic corresponds to the b-axis unique setting. Two other settings are possible:

a-axis unique: beta = gamma = 90

c-axis unique: alpha = beta = 90

Relevant Program Options:

Viewing along unit cell axes (see Section 5.5)
Creating Miller planes (see Section 7.3)
Displaying unit cell axes (see Section 9.1)
Customising the display of unit cell axis (see Section 6.2.6)
Colouring conventions for unit cell axes (see Section 9.2)
Displaying the contents of a unit cell (see Section 9.3.2)
Displaying the contents of several unit cells or fractions of a unit cell (see Section 9.3.3)
Turning packing on and off (see Section 9.3.4)
Displaying a slice through a crystal (see Section 9.4.2)

Related Topics

Crystal System
Fractional Coordinates
Miller Plane
Reciprocal Cell Axes
Space Group
Z (Number of Molecules in Unit Cell)

Cambridge Crystallographic Data Centre Web: http://www.ccdc.cam.ac.uk Support Email: support@ccdc.cam.ac.uk Support Phone: +44 1223 336022