Shaillee Kaushal is teaching live on Unacademy Plus
XII -01 Solid State- Part 12
Defects in non-stoichiometric crystals: Those compounds in which the number of cations and anions are not same as depicted by their formulae are called non-stoichiometric crystals. For example FeO, VOx, ZnO etc. The defects which bring change in overall chemical composition are called non- stoichiometric defects .
Types ts Metal deficiency defect Metal excess defects: Here the metal ions or cations are in excess Here the metal ions or cations are in defeciency
Metal Excess Defect Anionic Vacancies (i) Sodium chloride with excess sodium appears yellow (ii) Potasium chloride with excess potasium appears violet. Lithium chloride with lithium vapours appears pink. The Zn2+ ions thus formed occupy interstitial sites and the electrons occupy the neighbouring interstitial sites. Such solids alos behave as semiconductors due to presence of free electrons Zn0 heated in Zn vapour Zn,0 >l) The excess cations occupy in The electrons (2c) released stay associated to the interstitial cation terstitial void:s Extra cations occupving interstial sites 0o0o0o
Metal deficiency defect Extra anions occupying interstitial sites Cationic Vacancies As the size of the anions is large therefore they occupy the interstitial ions with difficulty. So this type of defect is not very common Fe+ and Fe3+, Ni2and Ni3+
Impurity defects in ionic crystalline solids In this defect impurities are introduced from outside. If the impurity ions have different oxidation states than that of the host ions, vacancies are created. For example if we add CaCl2 or SrCl2 as impurity to NaCl then for every bivalent ion two Na* ions are removed in the crystal lattice to acquire electrical neutrality. So vacant site is developed This type of defect increases the electrical conductivity many times. For example is 0.1% SrCl2 is added to NaCl the electrical conductivity increases 10,000 times. Na Cl cant site Na CI CI CNa CINa Vacancy defect becasue of Srion