Imperfections in Crystal Structure: Non-Stoichiometric Defects

Non-stoichiometric defects in crystal structures refer to deviations from the ideal chemical composition of a compound.

Metal excess defects arise when a compound has more metal ions than the stoichiometric ratio.

For example, when sodium chloride crystals are heated in sodium vapor, they turn yellow because a few sodium atoms enter the crystal and ionize into Na⁺ ions and electrons.

These electrons fill sites meant for chloride ions, creating excess Na⁺ ions.

When light is absorbed, these delocalized electrons transition to an excited state, giving the crystal color. These sites are termed color centers or F-centers.

In metal deficiency defects, a missing positive ion creates an extra negative charge, which is compensated by a nearby metal ion gaining an extra positive charge.

These are common in transition metal compounds like metal oxides and sulfides with varying oxidation states.

For instance, Iron oxide, found in meteorites and oceanic basalt, is deficient in Fe²⁺, which is balanced by having Fe³⁺ ions in interstitial sites between Fe²⁺ vacancies, forming Fe₃O₄-type clusters.