Stoichiometric compounds are those where the numbers of the different types of atoms or ions present are exactly in the ratios indicated by their chemical formulae.They obey the law of constant composition that the same chemical compound always contains the same elements in the same composition by weight. Two types of defects may be observed in stoichiometric compounds, called Schottky and Frenkel defects respectively. At absolute zero, crystals tend to have a perfectly ordered arrangement. As the temperature increases the amount of thermal vibration of ions in their lattice sites increases and if the vibration of a particular ion becomes large enough, it may jump out of its lattice site. The higher the temperature, the greater the chance that lattice sites may be unoccupied. Since the number of defects depends on the temperature, these are sometimes called thermodynamic effect.

`text(Schottky Defects)` : A vacancy at a cation site is mostly accompanied by a vacancy at a nearby anion site. Such paired cation-anion vacancies are referred as Schottky defect. Such defect preserves the electrical neutrality of the crystal but the density of crystal decreases. The points which are unoccupied are called lattice vacancies. In `NaCl` the existence of two vacancies, one due to a missing `Na^(+)` ion and the other due to a missing `Cl^-` ion in a crystal of `NaCl`, is shown in fig.1. The crystal, as a whole remains neutral because the number of missing positive and negative ions is the same. Thus a Schottky defects consists of a pair of holes in the crystal lattice.

`text(Frenkel Defects)` : When an ion (cation or anion) leaves its lattice point and occupies some interstitial space, the defect is called Frenkel defect. This defect also preserves the electrical neutrality of the crystal and the density of the crystal also remains unaltered as shown in fig.2 for the crystal of `AgBr`. As can be seen, one of the `Ag^+` ions occupies a position in the interstitial space rather than its own appropriate site in the lattice. A vacancy is thus created in the lattice as shown. It may be noted again that the crystal remains neutral since the number of positive ions is the same as the number of negative ions. The presence of `Ag^+` ions in the interstitial space of `AgBr` crystal is responsible for the formation of a photographic image on exposure of `AgBr` crystals (i.e., photographic plate) to light. `ZnS` is another crystal in which Frenkel defects appear. `Zn^(2+)` ions are entrapped in the interstitial space leaving vacancies in the lattice. Frenkel defects appear in crystals in which the negative ions are much larger than the positive ions. Like Schottky defects, the Frenkel defects are also responsible for the conduction of electricity in crystals and also for the phenomenon of diffusion in solids.

The defects discussed so far do not disturb the stoichiometry of the crystalline substance. However, a large number of non stoichiometric inorganic solids are known which contain the constituent elements in non-stoichiometric ratio due to defects in their crystal structures. ln non-stoichiometric or Berthollide compounds the ratio of the number of atoms of one kind to the number of atoms of the other kind does not correspond exactly to the ideal whole number ratio implied by the formula. Such compounds do not obey the law of constant composition. There are many examples of these compounds particularly in the oxides and sulphides of the transition elements. Thus in `FeO`, `FeS` or `CuS` the ratio of `Fe : O`, `Fe : S` or `Cu : S` differs from that indicated by the ideal chemical formula. If the ratio of atoms is not exactly `1 : 1` as in the above cases, there must be either an excess of metal ions or a deficiency of metal ions. Electrical neutrality is maintained either by having extra electrons in the structure or changing the charge on some of the metal ions. This makes the structure irregular in some way i.e., it contains defects. These defects are of two types :

(i) metal excess defect and

(ii) metal deficiency defect.

(i) `text(Metal Excess defect)` : This may occur in two different ways

F-Centres : A negative ion may be absent from its lattice site leaving a hole which is occupied by an electron, thereby maintaining the electrical balance. This type of defect is formed by crystals which would be expected to form Schottky defects. When compounds such as `NaCl`, `KCl` are heated with excess of their constituent metal vapours, or treated with high energy radiation, they become deficient in the negative ions and their formulae may be represented by `AX _(1- d)`, where `d` is a small fraction. The crystal lattice has vacant anion sites which are occupied by electrons. Anion sites occupied by electrons in this way are called `F` centres (`F` is an abbreviation Farbe, the German word for colour).

Interstitial ions and electrons : Metal excess defects also occur when an extra positive ion occupies an interstitial position in the lattice and electrical neutrality is maintained by the inclusion of an interstitial electron. Their composition may be represented by general formula of `A_(1+d) X`. This kind of metal excess defect is more common than the first and is formed in crystals which would be expected to form Frenkel defects. Examples include `ZnO`, `CdO`, `Fe_2O_3`.

Crystals with either type of metal excess defect contain free electrons, and if these migrate they conduct an electric current. These free electrons may be excited to higher energy levels, giving absorption spectra and in consequence their compounds are often coloured e.g. non-stoichiometric `NaCl` is yellow, nonstoichiometric `KCl` is lilac.

(ii) `text(Metal Deficiency Defects)` : In certain cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring two charges instead of one. There is evidently, a deficiency of the metal ions although the crystal as a whole is neutral. This type of defect is generally found amongst the compounds of transition metals which can exhibit variable valency. Crystals of `FeO`, `FeS` and `NiO` show this type of defects. The existence of metal deficiency defects in the crystal of `FeO` is illustrated.

It is evident from the above discussion that this defect results in the creation of vacancies or 'holes' in the lattice of the crystals. The presence of holes lowers the density as well as the lattice energy or the stability of the crystals. The presence of too many holes may cause a partial collapse of the lattice.