Non-Stoichiometric Crystals Defects
Non-stoichiometric defects unsettle the stoichiometry of solids. The ionic sites occupied by unpaired electrons are called F-centers (Farbenzenter or colour centre). They impart colour to alkali metal halide crystals. The colours are produced by the excitation of these electrons when they absorb energy from visible light falling on the crystal. Excess of Na turns NaCl crystals yellow, excess of Li makes LICI pink, and excess of potassium turns KCl crystals purple.
- Metal Excess Defect
- Metal Deficiency Defect
Metal Excess Defect
If in the lattice structure, an anion gets displaced the number of metal cations gets larger than the number of anions causing the access of the metal cations and resulting in the metal access defect.
Metal excess defect due to anionic vacancies is shown by alkali halides like NaCl and KCl. In such defects, a negative ion is missing from its lattice site leaving a hole which is occupied by an electron to maintain electrical neutrality.
The excess of metal due to the presence of additional cations at the interstitial sites is shown by zinc oxide on heating. On heating, zinc oxide (white) loses its oxygen and turns yellow.
ZnO + Heating → Zn²+ + O₂ +2e–
Metal Deficiency Defect
Metal deficiency defects arose due to two factors
- Cation Vacancies
- Extra anions occupying Interstitial sites.
In cation vacancies, the cation of the crystal lattice gets dispaced from the lattice structure which causes the meatal deficiency defects.
These defects can also arise if the interstitial space in the crystal lattice is filled by the extra anions.
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Point Defects
Solids are made up of a lot of small crystals. However, the defect in solids occurs during the crystallization process due to the fast or moderate rate of crystal formation. Defects are described as irregularities in the arrangement of constituent particles in general. The defect could be a point or a line defect based on the irregular arrangement. A deviation point defect occurs in an ideal crystal when there are anomalies in the arrangement around any point or atom in the crystal. Similarly, a line defect occurs when there is a variation in the arrangement of the entire row of lattice points in crystals.
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