Non – Stoichiometric Defects Online Class 12 Chemistry Notes
NCERT Chemistry Class 12 : When the ratio of cations & anions are not equal (required for ideal crystal), then compounds are called non-stoichiometric compounds & defect in these compounds are called non-stoichiometric defects. The number of cations and anions present in the compounds having non – stoichiometric defects are different from those expected from their chemical formulae. There is an excess of either positive charge or negative charge in the crystal. A crystal is always neutral, so if there is an excess of negative charge, it is balanced by the presence of extra positive charge. If the positive charge is in excess, it is balanced by the presence of extra electrons. Due to this, the crystal structure becomes irregular and defects arise. These defects are known as Non – Stoichiometric Defects. These defects exist in addition to Schottky and Frenkel defects.
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Non-Stoichiometric Defects are of two types:
- Metal Deficiency Defects when negative ions are in excess.
- Metal Excess Defects when positive ions are in excess.
Metal Deficiency Defects: –
These defects can occur in two possible ways as follows:
- In this way, one of the positive ions may be missing from its lattice point. The extra negative charge is balanced may be balanced by some nearby metal ion acquiring two charges instead of one. The metal must be in a position to acquire variable valency so, this defect is generally found in compounds of transition metals like FeO, FeS, NiO, etc.
- In another way, an extra negative ion may find an interstitial position. The extra negative charge may be balanced by one of the adjacent metal ion acquiring an extra charge. In this case, also, the metal ion should also be in a position to acquire variable valency. As negative ions are usually very large, it is expected that they would not be able to fit into the interstitial positions. The possibility of finding metal deficiency defect is only theoretical.
- Crystals with such defects may act as semiconductors. This phenomenon may arise from the movement of an electron from one ion to another. The ion A+ changes to A2+ and the movement of an electron is an apparent motion.
Metal deficiency due to a missing positive ion
Metal deficiency defect due to an extra negative ion
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Metal Excess Defect: –
These defects may arise in the following two ways:
- In the first case, a negative ion may be missing from its lattice site leaving a hole which may be occupied by an extra free electron to maintain electrical neutrality. Thus, there is an excess of metal or positive ions. This defect is similar to Schottky defect but differs in having only one hole and not a pair as in Schottky Those crystals which are likely to form Schottky defects show such defects. For example, if KCl is exposed to potassium vapor, a lilac-colored non- stoichiometric form of KCl is obtained, which contains an excess of potassium ion.
- In another way, an extra positive ion occupies an interstitial position in the lattice and extra electron also finds a place in the interstitial space to maintain electric neutrality. This type of defect is similar to Frenkel defect but differs in not having any holes and interstitial electrons. Those crystals which are likely to develop Frenkel defects show this type of defect. For example, ZnO crystal.
- These crystals contain free electrons, which can migrate in an electric field and conduct electricity to some extent. They conduct a very small amount of current as the number of electrons is very small. These crystals are called semiconductors.
- The crystals showing metal excess defects are generally colored, which may be due to the presence of free electrons. These electrons get excited easily to higher energy levels by absorption of visible white light and so the compounds appear colored.
- Thus, non-stoichiometric NaCl, KCl, ZnO appear yellow, lilac and yellow respectively.
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