CBSE Class 12 Chemistry Notes – Unit 1 DEFECTS IN CRYSTALS
An ideal crystal is a crystal which has the same unit cell containing the same lattice points throughout the whole of the crystal. At absolute zero, most of the ionic crystals show no defects. With the increase of temperature, chances of one or more of the lattice sites remaining unoccupied by the ions increase. This constitutes a defect. The number (n) of such defects per cm3 is given by the equation-
n = N x e-w/2RT
Where, N= total number of sites per cm3, w= work or energy required to produce a defect, T= absolute temperature, R = gas constant and e is base of natural logarithm.
- What is absolute zero temperature?
Absolute zero is the lowest possible temperature where nothing could be colder and no heat energy remains in the substance. By international agreement, absolute zero is defined as precisely, 0 Kelvin on Kelvin scale and -273.15 degrees Celsius on the Celsius scale.
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Two types of defects namely Schottky defect and Frenkel defect are more common.
- Schottky Defect: If some of the lattice points are unoccupied, then Schottky defect arises. The points which are unoccupied are known as holes or lattice vacancies. There are two holes in the lattice, one due to a missing anion and other due to a missing cation. The crystal as a whole remains neutral because the number of positive and negative ions is same. Generally, Schottky defect appears in strongly ionic compounds in which the radius ratio r+/r– is not far below unity and have a high coordination number. Thus NaCl, CsCl having coordination numbers 6 and 8 respectively shows Schottky defect.
- Frenkel Defect: This defect arises when an ion occupies an interstitial position between the lattice points. In it, one of the cations occupies a position in the interstitial space instead of its own fixed site. A hole is thus produced in the lattice. The crystal as a whole is neutral, as the number of positive ions and negative ions is the same. The positive ions being smaller are more likely to occupy interstitial position than the negative ions. Generally, the Frenkel defect arises in compounds in which the radius ratio r+/r– are low and the coordination number is also small. For example, in AgBr, ZnS, Ag+ ions and Zn2+ ions, respectively occupy a position in between the other ions in the lattice.
Consequences of Schottky and Frenkel Defects
- Due to both types of defects, the crystal is able to conduct electricity to a small extent by an ionic mechanism.
- The closeness of the similar charges brought by Frenkel defect tends to increase the dielectric constant of the crystals.
- The presence of holes lowers the lattice energy or the stability of the crystals. Too many holes may cause a partial collapse of the lattice.
- The presence of holes lowers the density of the crystal.
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