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## 1.7 Crystals Many solid substances, including the metals, are crystalline in structure. That is, their basic particles are arranged in definite three-dimensional patterns of rigid geometrical form which are repeated many times. Allowed to grow freely, crystals take on a distinctive geometric for...

## 1.7 Crystals Many solid substances, including the metals, are crystalline in structure. That is, their basic particles are arranged in definite three-dimensional patterns of rigid geometrical form which are repeated many times. Allowed to grow freely, crystals take on a distinctive geometric form as shown in Figure 1.4. Non-crystalline "solids" such as pitch, glass and many "plastic" materials are said to be amorphous (without shape) and are best understood if they are considered as being extremely viscous liquids. The structure of crystals may be understood if their constituent atoms are considered to be spherical in shape. Figure 1.5 (a) shows a simple cubic crystal built up from eight spherical particles. The dotted lines joining the centres of the spheres represent the unit cell of the simple crystal. The unit cell is the geometric figure which illustrates the fundamental grouping of the particles in the solid. To form the crystal this unit cell is repeated many times to form the space lattice as shown in Figure 1.5 (b). It is this regular, repetitive pattern of particles which characterises crystalline materials. All crystal structures can be analyzed into fourteen basic lattices. These are called the Bravais space lattices. For simplicity only the unit cells of each lattice is shown in Figure 1.6. * **P type** These are shown in Figure 1.6 (a) and are classified as primitive. * **C-type** These are shown in Figure 1.6 (b) and are classified as centred on the "ab" face.