Non-Stoichiometric Defects
- Nonstoichiometric inorganic solids contain the constituent elements in a non-stoichiometric ratio due to defects in their crystal structures.
- These defects are of two types: (i) metal excess defect and (ii) metal deficiency defect.
(i) Metal Excess Defect.
- Metal excess defect due to anionic vacancies:
- This type of defect is exhibited by alkali halides like NaCl and KCl.
- Application of heat to NaCl in an atmosphere of sodium vapour results in deposition of the sodium atoms on the surface of the crystal.
- The sodium atoms lose electron to form Na+
- The Cl– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl.
- The released electrons diffuse into the crystal and occupy anionic sites.
- The anionic sites occupied by unpaired electrons are called F-centres that imparts yellow colour to the crystals of NaCl due to the excitation of the electrons on absorption of energy from the visible light falling on the crystals.
- Excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet.
- Metal excess defect due to the presence of extra cations at interstitial sites:
- Heating Zinc oxide that exists in white colour at room temperature loses oxygen and turns yellow.
- This results in excess of zinc in the crystal and its formula becomes Zn1+xO.
- The excess Zn2+ ions move to interstitial sites and the electrons to neighbouring interstitial sites.
- Metal Deficiency Defect:
- Many solids possess less amount of the metal as compared to the stoichiometric proportion.
- For example, FeO is mostly found with a composition ranging from Fe 93O to Fe 0.96O.
- In crystals of FeO some Fe2+ cations are missing and the loss of positive charge is made up by the presence of required number of Fe3+
