You can introduce defects into a perfect crystal by adding an impurity. The addition of the dopant causes point defects to occur in the crystal. These defects are classed as extrinsic defects (mentioned earlier).

Take NaCl heated in sodium vapour. Sodium is taken into the crystal and changes the compostion from NaCl to Na1+xCl. The sodium atoms taken into the crystal occupy cation sites leaving an equivalent number of unoccupied anion sites. The atoms then ionise such that Na+ ions occupy the cation sites as expected and the electrons now occupy the anion vacancies. This solid is now a non-stoichiometric compound as the ratio of atoms is no longer the simple integer you expect. It is quite common for the ratios to be non-integral, eg vanadium oxide varies from VO0.79 to VO1.29. There are many other examples such as TiOx, NixO, UOx and LixWO3.

Covalent compounds are held to together by very strong covalent bonds which are difficult to break, so normally


these compounds do not show a wide range of composition. Ionic compounds do not show a wide range either because a large amount of energy is required to remove / add ions.

However, non-stoichiometry can be caused by doping as already mentioned. There is also a way of making ionic crystals non-stoichiometric: If the crystal has a multi-valent element in it, changes in the number of ions can be compensated for by changes in the charge on the ions, therefore maintaining charge balance but changing the stoichiometry.

Basically, non-stoichiometric compounds have formulae with non-integer ratios and can exhibit a range of compositions. They are formed by introducing an impurity (doping) or by the ability of an element to show multi-valent character.

The electronic, optical, magnetic and mechanical properties of non-stoichiometric compounds can be controlled by varying their composition.

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