The SOLID STATE

INTRODUCTION 

Solids are characterised by their rigidity, hardness, incompressibility and characteristic geometry. They have a definite volume and shape

Types of solid:

 Solid substances are classified into two forms

(a) crystalline (alen called true solidat,

(b) Amorphous.

A crystalline solid exista as small crystals, each crystal possesses a characteristic geometrical shape. In a crystal, the atoms, molecules or ions are arranged in regular, repeating three-dimensional pattern called the Crystal Inttice. They possess sharp melting points. Sugar and common salt are crystalline solids. An amorphous solid (Gr amorphous no form) has atoms, molecules or ions arranged at random and lacks the ordered crystalline lattice. Unlike crystalline solids, they do not exhibit sharp melting points.

Examples are rubber, plasties, glass and amorphous sulphur.

CRYSTAL LATTICE AND UNIT CELL

All crystals consist of regularly, repeating three dimensional orderly arrangement of constituent particles (atoms, ions or molecules). The positions of particles in a crystal, relative to one another in space, are designated by points signifying only the position of the centres of the particles but not their actual sizes. The over all arrangement of particles in a crystal is called the crystal lattice, space lattice or simply lattice. The positions occupied by the particles in the crystal lattice are called lattice sites or lattice points. Conceptually, a space lattice is defined as a regular three dimensional array of similar points (representing an atom, ion or a molecule) in space, arranged in such a way that a straight line passing through any two points will puss, at equal interval, through a series of similar points. The most important fundamental characteristic of a space lattice is that each point (in space lattice) has an identical surroundings throughout.

 The smallest geometrical portion of the crystal which can be used as repetitive unit to build up the whole crystal is called a unit cell. In other words, "the smallest volume of a crystal showing all the characteristics of its lattice is called a unit cell.

and c with a (alpha) as shown in Fig.4.1. A unit cell has a definite shape, has constant values for three unit cell lengths and for the angles between them. The unit cell lengths are the distances parallel to the three major axes. It is customary to represent the unit cell length parallel to x-axis with the letter 'a', parallel to y-axis with b, and parallel to z-axis with 'c'. The angle between a and b is denoted by y (gamma), between a and e with ẞ (beta) and between b and c with a.