Non-stoichiometry refers to the deviation of the actual composition of a compound from its ideal or expected composition based on simple whole number ratios. This can occur due to factors such as vacancies in crystal lattices or the presence of impurities in the compound. Non-stoichiometry is commonly observed in materials science and can affect the properties of the material.
The chemical composition of nonstoichiometric compounds do not respect the law of definite proportions.
P. Kofstad has written: 'Nonstoichiometry, diffusion, and electrical conductivity in binary metal oxides'
ZnO has an n-type character because of presence of nonstoichiometry that is absence of elemental composition that can be represented by a ratio of well-defined natural numbers.This n-type behaviour is due to point defects like vacancies and interstitials in the ionic crystal. An alternate reason presented , based on theoretical calculations is the unintentional substitutional hydrogen impurities in the compound.