Name: AB Type Ionic Crystals
Uploaded: 2019-07-19
Duration: 7 min 12 s
Description: Ionic crystals are crystalline structures that grow from ionic bonds and are held together by electrostatic attraction. Let's explore some of the common ionic crystals.

Question

What do you understand by doping? How are the impurity defects introduced into covalent solids and ionic solids? In what way is the electrical conductivity of a crystal affected by doping?

Accepted Answer

The introduction of defects by the addition of a small amount of a foreign substance in the crystal of a pure substance is called doping. The crystal containing a foreign substance as impurity is known as a doped crystal.

Impurity defects in covalent solids: Germanium and silicon belong to Group- $14$ of the periodic table have four valence electrons, and they form four covalent bonds with the neighbouring atoms. The Group- $13$ elements such as gallium and aluminium and the Group- $15$ elements such as phosphorous and arsenic can be introduced substitutional into silicon and germanium crystals. Group- $14$ elements have tendency to form four covalent bonds. The Group- $15$ elements possess one excess valence electron as compared to silicon and germanium, and therefore after forming four covalent bonds, one electron is left free.This excess electron gives rise to the electronic conduction.

Similarly, the Group- $13$ elements possess one valence electron less as compared to Group- $14$ elements. Therefore, a Group- $13$ element forms an electron deficient bond or a hole. Such hole can move in the crystal and can give rise to electrical conductivity. Hence, impurity-doped silicon and germanium act as semiconductors.

Impurity defects in ionic solids: A defect in an ionic solid can be introduced by adding foreign ions. If the impurity ions have a different oxidation state than that of the host ions, vacancies are created.

Example: Addition of ${MgCl}_{2}$ to sodium chloride gives a solid solution in which the divalent ${Mg}^{2 +}$ ions occupy the ${Na}^{+}$ ion sites. For every ${Mg}^{2 +}$ ions introduced into the lattice, two ${Na}^{+}$ ions are removed in order to maintain electrical neutrality. Thus, one of these lattice sites is occupied by ${Mg}^{2 +}$ ion, and the other site remains vacant. Thus, cation vacancies are created in the sodium chloride crystal lattice by the doping of ionic solids, which increases the electrical conductivity. This defect is called impurity defect in ionic solids.

What do you understand by doping? How are the impurity defects introduced into covalent solids and ionic solids? In what way is the electrical conductivity of a crystal affected by doping?

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