Generally we can say that the compound which contains the combining elements in a ratio different from that required by their stoichiometric formulae, are non-stoichiometric defects.
Metal – Excess Defect: The solids with metal excess defect contain metal in excess to the stoichiometric ratio. Such defect is caused due to either of the following reasons.
- Metal – Excess Defect due to Anionic Vacancies:
This kind of defect is shown by alkali halides such as, sodium chloride NaCl and Potassium Chloride KCl.When crystal of an alkali halide, say sodium chloride NaCl is heated in an atmosphere of alkali metal vapour.The metal atoms get deposited on the surface of the crystal. The halide ions diffuse to the surface of the crystal and combine with alkali metal to form alkali halide. The electron released during this reaction diffuses into the crystal and occupies the anionic (halide ion) site.
As a result of this reaction, the crystal will have an excess of alkali metal. The ionic site occupied by an unpaired electrons is called F – centre, From the German word Farbenzenter meaning color Centre. If the concentration of F-centres in any crystal is high, then the otherwise colourless crystals develop some colour. For example, F-centres impart violet colour to KCl, and pink colour to LiCl crystals.
- Metal – Excess Defect due to Presence of extra acations athe the interestial sites:
Zinc oxide is white at room temperature, but on heating it loses oxygen and turns yellow. This reaction is reversible. The Zn2+ ions so formed enter the vacant voids (interstices) in the crystal .The released electrons get traps in its neighbouring interstitial sites .This results into a slight excess of zinc in the crystal. Such a sample of zinc oxide can be described the formula Zn 1+x O. The non-stoichiometric ZnO shows increased conductivity due to these trapped electrons.
In to the Metal-deficiency defect certain solids by virtue of their chemical limitations contain metal in the lesser proportion than the stoichiometric ratio. Iron2 oxide is a typical compound of this type. In iron2 oxide, the actual composition corresponds to the range Fe0.93O to Fe0.96O.
- define stoichiometry