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Module 4 : Solid State Chemistry

Lecture 17 : Closed Packed Structures

17.4

Defects in Solids:
At non-zero temperatures, there is hardly a perfect solid and imperfections or defects in solids not only exist, but also play a crucial role in the dynamics of atoms in crystals as well as catalysis. The defects include vacancies, substitutional impurities, interstitial impurities, self interstitial dislocation, edge dislocations and grain boundaries. Some of these examples are shown in Fig 17.7. If E is the energy required to create one impurity or vacancy, then the fraction of vacancies or impurities ( f ) is given by

f = N_v / N = e ^{- E / RT}	(17.11)
Among point defects we have vacancies ( atom or ion missing from the site ), substitutional impurities (one site in the lattice substituted by a "foreign" site, which is not a part of the crystal), self interstitial site (a particle moving to a site which is not the usual site of particle in the lattice ) and an interstitial impurity (a foreign particle sitting in an interstitial position). All these defects are shown in Fig 17.7(a). In ionic lattices, the above imperfections can be reclassified as a Frenkel defect (an ion moving from a regular site to an alternative site), a Schottky defect ( a missing anion - cation pair ) and an F - center ( an electron in place of a negative charge ). These are shown in Fig 17.7(b). Other more complex defects are edge dislocations ( rows of atoms missing from a layer) and grain boundaries ( boundaries along which there are a collection of defects ). When a crystal is placed under a shearing stress, the motion is around the dislocations. When a lattice dissolves, the "dissolution" starts near dislocations or grain boundaries. Defects play major role in conductivity, reactivity and catalysis by permitting greater mobility and reactivity at these active sites.