Solid State
8.0 Imperfection in Solids
8.0 Imperfection in Solids
An ionic crystal which has the same unit cell containing the same lattice points throughout the crystal is known as ideal crystal but crystals tend to have a perfectly ordered arrangement at only absolute zero. This arrangement corresponds to state of lowest energy. And as the temperature increases, the crystals start deviating from the perfectly ordered arrangement. This defect may appear at a point, along a line or over a surface.
A point defect could arise due to the absence of a particle (vacancy), presence of some foreign particle at a lattice site, presence of a foreign particle at the interstitial site or displacement of a particle to the interstitial site.
Two main defects in crystals which are discussed as follows are Schottky and Frenkel defects.
$1.$ Schottky Defect:
This defect is caused when some of the lattice points are unoccupied and those points are called vacancies or holes as shown in figure.
The number of missing positive and negative ions is the same so that the crystal remains neutral in all.
Schottky defects are more common in ionic compounds with high co-ordination number, and where the sizes of positive and negative ions are almost equal for example, $NaCl$, $KCl$, $CsCl$ and $KBr$.
The number of defects increases with increase in temperature. The number of defects increases to one in $106$ sites at $775\ K$ and one in $104$ sites at $1075\ K$. The presence of large number of Schottky defects in crystal results in significant decrease in its density.
$2.$ Frenkel Defect:
This defect is caused when some of the ions leave their lattice sites to occupy an interstitial site as shown in figure. Frenkel defects are more common in ionic compounds with low co-ordination number and where there is large difference in size between positive and negative ions for example, $ZnS$, $AgCl$, $AgBr$ and $AgI$.
In pure alkali metal halides, these defects are not very common because the ions cannot get into interstitial positions due to their large sizes.
