Page 36 - Instant notes

A5
CRYSTALLINE SOLIDS

Key Notes
Solids may be broadly grouped into two categories, amorphous
and crystalline. Crystalline materials are characterized by highly
ordered packing of molecules, atoms or ions. This order allows
relatively easy structural studies. Seven crystal systems exist in
three-dimensional crystals, from which all possible crystal
morphologies may be generated. The deviation from these crystal
systems which real crystals exhibit is primarily due to the
different growth rates of each crystal face.
A crystalline material is composed of an array of identical units.
The smallest unit which possesses all of the properties of the
crystal is the unit cell. From a unit cell, the entire crystal may be
built up by allowing a simple translation operation parallel to any
of the three unit cell axes. In principle, there are an almost
infinite number of possible unit cells, but it is customary to
choose a unit cell which exhibits the symmetry properties of the
entire lattice, within the minimum volume, and with angles as
close as possible to 90°. In three-dimensional crystals, the 14
Bravais lattices are sufficient to account for all possible unit cells.
In addition to the planes which are parallel to the cell axes, an
ordered array also contains an infinite number of sets of parallel
planes containing the basic motif. The interplanar distances are of
primary importance in diffraction studies, and the Miller indices
provide the most useful method for discussing the physical
attributes of particular sets of lattice planes. Most notably, Miller
indices allow interplanar distances to be readily calculated, which
ultimately allows convenient analysis of X-ray and neutron
diffraction measurements.
Related topics Diffraction by solids (A6)

Crystalline solids

Solids may be loosely categorized into two groups. Amorphous solids have no long-
range order in their molecular or atomic structure. By their nature they are not easily
studied, since the powerful analytical methods which are described in Topic A6, for
example, are not applicable to such disordered structures. In contrast crystalline solids
which consist of ordered three-dimensional arrays of a structural motif, such as an atom,
molecule or ion. This internal order is reflected in the familiar macroscopic structure of