Dictionary of Chemistry [6th Ed.]

152

c

CRYSTAL DEFECTS

A crystal *lattice is formed by a repeated arrangement of atoms, ions, or

molecules. Within one cubic centimetre of material one can expect to find up

to 10^22 atoms and it is extremely unlikely that all of these will be arranged in

perfect order. Some atoms will not be exactly in the right place with the result

that the lattice will contain *defects. The presence of defects within the crystal

structure has profound consequences for certain bulk properties of the solid,

such as the electrical resistance and the mechanical strength.

Point defects

Local crystal defects called point defects, appear as either impurity atoms or

gaps in the lattice. Impurity atoms can occur in the lattice either at interstitial

sites(between atoms in a non-lattice site) or at substitutional sites(replacing an

atom in the host lattice). Lattice gaps are called vacanciesand arise when an

atom is missing from its site in the lattice. Vacancies are sometimes called

Schottky defects. A vacancy in which the missing atom has moved to an

interstitial position is known as a Frenkel defect.

Colour centres

In ionic crystals, the ions and vacancies always arrange themselves so that

there is no build-up of one type of charge in any small volume of the crystal. If

ions or charges are introduced into or removed from the lattice, there will, in

general, be an accompanying rearrangement of the ions and their outer

valence electrons. This rearrangement is called charge compensationand is

most dramatically observed in colour centres. If certain crystals are irradiated

with X-rays, gamma rays, neutrons, or electrons a colour change is observed.

For example, diamond may be coloured blue by electron bombardment and

quartz may be coloured brown by irradiation with neutrons. The high-energy

radiation produces defects in the lattice and, in an attempt to maintain charge

neutrality, the crystal undergoes some measure of charge compensation. Just

as electrons around an atom have a series of discrete permitted energy levels,

so charges residing at point defects exhibit sets of discrete levels, which are

separated from one another by energies corresponding to wavelengths in the

visible region of the spectrum. Thus light of certain wavelengths can be

absorbed at the defect sites, and the material appears to be coloured. Heating

the irradiated crystal can, in many cases, repair the irradiation damage and the

crystal loses its coloration.

Dislocations

Non-local defects may involve entire planes of atoms. The most important of

Formation of a Schottky defect Formation of a Frenkel defect

Point defects in a two-dimensional crystal