A perfect crystal is an idealization. Real crystals contain point defects localized around one or a few lattice sites.

The simplest are:

• a vacancy, where an atom or ion is missing from a normally occupied site;
• an interstitial, where an extra atom occupies a space between regular lattice sites;
• a substitutional solute, where a different species occupies a normal lattice site.

In ionic crystals, defects must also respect overall charge neutrality. A missing ion or a substitution with a different charge can therefore be accompanied by other defects that compensate the charge imbalance.

Defects exist at nonzero temperature

Creating a defect costs energy, but defect-containing crystals also have many more possible microscopic arrangements than one perfectly ordered crystal. The competition gives a finite equilibrium defect population at nonzero temperature.

For vacancies, a common dilute approximation is

$$\frac{N_v}{N}\approx \exp!\left(-\frac{E_f}{k_B T}\right),$$

where $E_f$ is vacancy-formation energy, $k_B$ is Boltzmann's constant and $T$ is absolute temperature. The exponential dependence means that equilibrium vacancy populations can increase rapidly as a solid is heated.

Point defects matter because they change local composition and strain, scatter electrons and lattice vibrations, and provide the sites through which atoms can migrate. They are therefore a bridge from ideal crystal structure to transport, strengthening and nonstoichiometric materials.