Impurity: also aliovalent

Nonstoichiometric Compounds Intrinsic defects are stoichiometric defects (i.e., they do not involve any change in overall composition). Defects can also be nonstoichiometric. In the case of extrinsic defects where the host crystal is doped with aliovalent impurities, the solid so formed is a nonstoichiometric compound because the ratio of the atomic components is no longer the simple integer. There is also... 

Point defects are an important part of the work in this paper. There are many reasons for the formation of point defects in minerals and their presence can exert important perturbations on the properties of the material (4). Point defects are formed because of the thermally driven intrinsic disorder in a lattice, the addition of aliovalent impurities or dopants, the presence of metal-nonmetal nonstoichiometry, and the creation of nonideal cation ratios. The first three source of defects are well-known from binary compounds but the last is unique to ternary compounds. Ternary compounds are much more complex than the binary compounds but they also have gained a great deal of attention because of the variety of important behavior they exhibit including now the presence of superconductivity at high temperatures. The point defects can be measured by introducing probe ions into the lattice. 

Hence aliovalent impurity concentrations affect the concentrations of interstitials and vacancies. Dopands can also act as sensitizers (they improve the formation of the latent image) or as efficient traps for defects Pb g for e Cu g for h and Cu g for 7 ,. 

Despite this possibility, also defect-chemical treatments most commonly assume that the concentration of dopants and impurities is constant, often (tacitly) because they are assumed to be fully soluble (case 2a)). In reality, this may have been tme at the high temperatures used during synthesis and fabrication, while during characterisation or use at lower temperatures the concentrations remain constant rather because they are frozen in (cases 1)). We shall start our treatment with cases of invariable concentrations of aliovalent foreign substituents, disregarding whether this is a result of full solubility or freezing-in. 

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