Question

Explain the following terms
(i) Schottky defect (ii) Frenkel defect (iii) Interstitials (iv) F-centers

Answer 1

Hint: Crystalline solids have short range as well as long range order in the arrangement of their constituent particles, yet crystals are not perfect. The solids have a large number of small crystals. These crystals have defects in them. This happens when the crystallization process occurs at a fast or moderate rate.

Complete step by step answer:
Imperfections in the regular geometrical arrangement of atoms in a crystalline solid. There are four types of crystal defects. Among them, point defects are the irregularities from ideal arrangement around a point or an atom in a crystalline substance.
(i) Schottky defect
In ionic crystals, the defect forms when oppositely charged ions leave their lattice sites, creating vacancies. It is also known as a dislocation defect. In this both cation and anion leaves their lattice site and moves out of crystal and leaves a vacancy there. The vacancies are then free to move about as their own entities. These defects will lead to a decrease in the density of the crystal. E.g. ${\text{NaCl}},{\text{KCl}}$ etc.
(ii) Frenkel defect
If an ion leaves its lattice causing a vacancy and occupies the interstitial site, electrical neutrality as well as stoichiometry of the compound are maintained. This does not cause an effect on the density of the crystal as no ion leaves the crystal. E.g. ${\text{AgCl}},{\text{AgBr}}$ etc.
(iii) The regular positions of an array of atoms or ions occupied by other atoms or ions is called interstitial positions. When some constituent particles occupy vacant interstitial positions, the crystal is said to have interstitial defects.
(iv) Non-stoichiometric defects are of two types-metal excess defects and metal deficiency defects. Metal excess defects by anion vacancies occur if the number of missing anions are more than the number of missing cations. The location of the electron where it is trapped i.e. anion vacancies called F-centers.

Note:In metal excess defects, the anion may be missing from its lattice site leaving a hole which is occupied by an electron to maintain electrical neutrality. F-centers are responsible for imparting color to the crystal. At room temperature, ${\text{ZnO}}$ is white in color. When it is heated, some oxygen gets released in the form of oxygen and electrons are trapped at their location. Due to this, it appears yellow.