No real crystal is perfect. Irregularities from the ideal arrangement are called crystal defects or imperfections; when they involve only individual lattice points they are point defects. These defects control colour, conductivity and many useful properties.
In stoichiometric defects the ratio of cations to anions (and hence the chemical formula) stays unchanged. The two main types are:
- Schottky defect — equal numbers of cations and anions are missing (a vacancy defect). It lowers density and appears in ionic solids with high coordination number and similar-sized ions (NaCl, KCl, CsCl, AgBr).
- Frenkel defect — a smaller ion (usually the cation) shifts to an interstitial site (a dislocation defect). Density is unchanged; it occurs with a large ion-size difference (ZnS, AgCl, AgBr, AgI). AgBr shows both.
Non-stoichiometric defects change the cation:anion ratio. A metal excess defect arises when anion sites are vacant and balancing electrons are trapped there, forming F-centres that impart colour (NaCl heated in Na vapour turns yellow; LiCl turns pink). A metal deficiency defect occurs when some cations are missing and the charge is balanced by an ion in a higher oxidation state (e.g. $\text{Fe}_{0.95}\text{O}$, where some $\text{Fe}^{2+}$ becomes $\text{Fe}^{3+}$). Impurity defects arise when foreign ions occupy lattice sites; e.g. $\text{SrCl}_2$ added to NaCl introduces $\text{Sr}^{2+}$ replacing two $\text{Na}^+$, creating cation vacancies.
Electrical properties. Solids span an enormous conductivity range. Conductors (metals) have conductivity $\sim10^7\ \Omega^{-1}\text{m}^{-1}$ because the conduction band overlaps the valence band. Insulators have a very large band gap. Semiconductors have a small band gap, so conductivity rises with temperature. Doping a Group-14 element (Si, Ge) produces:
- n-type semiconductor — doped with a Group-15 element (P, As); the extra electron carries the current (negative carriers).
- p-type semiconductor — doped with a Group-13 element (B, Al, Ga); an electron-deficient hole acts as a positive carrier.
Magnetic properties depend on unpaired electrons. Diamagnetic (all electrons paired) are weakly repelled (NaCl, $\text{H}_2\text{O}$); paramagnetic (unpaired electrons) are weakly attracted and lose magnetism when the field is removed ($\text{O}_2$, $\text{Cu}^{2+}$). Ferromagnetic solids (Fe, Co, Ni, $\text{CrO}_2$) are strongly attracted and retain magnetism — their domains align in the same direction. Ferrimagnetic solids ($\text{Fe}_3\text{O}_4$) have unequal opposite domain alignment (small net moment), while antiferromagnetic solids (MnO) have equal opposite alignment that cancels to zero net moment.