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16.1 |
Introduction |
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Solids may be classified into different types such as crystalline, amorphous, glassy and so on. At low temperatures and high pressures, most substances condense into a solid state. The formation of a solid is a consequence of a variety of intermolecular forces such as ionic, covalent as well as non-covalent (such as van der Waals forces). In this lecture we will classify crystalline solids into various types of lattice structures and give examples of each type. We will also estimate lattice energies of ionic crystals. |
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The variety and beauty of patterns in crystals is due to the presence of repeating units. The repeating units extend periodically in three dimensional space giving a space filling structure. The structurally repeating unit may be a group of one or more atoms, molecules or ions. If each one of these units is represented by a point, a space filling pattern can be obtained by regularly repeating this unit in three dimensions. This space filling pattern is called a space lattice or a crystal lattice or a Bravais lattice. Bravais showed in 1875 that there can be only 14 distinct lattices in three dimensions. Each point of a Bravais lattice can be associated with a unit cell, which is an imaginary parallelopiped (i.e., a figure with parallel sides) that contains one unit of the translationally repeating pattern. |
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