Nonstoichiometric defects

In a nonstoichiometric defect the ratio of the atoms -- or of the cations to the anions -- genuinely becomes different from the ratio indicated by the compound's chemical formula, though the overall crystal structure is preserved. There are two types.

i. Metal deficiency defect : This defect occurs only in compounds of metals that show variable oxidation states. Some cations are missing from their regular lattice sites, and the resulting deficit of positive charge is made up by an equal extra positive charge carried on other cations of the same metal in a higher oxidation state. In nickel oxide, for every missing $Ni^{2+}$ ion, two other $Ni^{2+}$ ions are oxidised to $Ni^{3+}$, restoring the charge balance -- the crystal then has the nonstoichiometric formula $Ni_{0.97}O_{1.0}$ (Fig. 1.20).

ii. Metal excess defect : A crystal can come to contain excess metal in two ways.

• A neutral atom or an extra positive ion occupies interstitial position : In the first case, an electrically neutral metal atom is present in an interstitial space -- in ZnO, an excess neutral Zn atom occupies an interstitial site (Fig. 1.21a), giving the formula $Zn_{1+x}O_{1.0}$.

In the second case, ZnO loses oxygen on heating:

The $Zn^{2+}$ ions so formed, together with the released electrons, occupy interstitial spaces (Fig. 1.21b); the crystal again has the formula $Zn_{1+x}O_{1.0}$.

• By anion vacancies (Colour or F-centres) : When NaCl crystals are heated in sodium vapour, Na atoms are deposited on the crystal surface, and $Cl^{-}$ ions diffuse out to the surface to combine with them:

$Na + Cl^{-} \longrightarrow NaCl + e^{-}$ …