Amorphous and Crystalline Solids

Solids are classified as crystalline or amorphous depending on the nature of the order in which their constituent particles are arranged.

Crystalline solids — long range order

A crystalline solid is usually made up of a very large number of small crystals, each with a definite characteristic geometrical shape. Inside a crystal the particles (atoms, ions or molecules) are arranged in an ordered, repetitive pattern in three dimensions. If you know the arrangement in one small region, you can predict exactly where the particles sit anywhere else in the crystal, however far away. This regular pattern repeating periodically over the whole crystal is called long range order. Sodium chloride and quartz are typical examples.

Amorphous solids — short range order

Substances like glass, rubber and many plastics do not crystallise when their melts cool; they are amorphous solids. The word comes from the Greek amorphos, meaning "no form". Here the arrangement has only short range order — a regular pattern appears over short distances, but these ordered patches are scattered and the arrangement between them is disordered.

As Fig. 1.1 shows, quartz (a) and quartz glass (b) are built from the very same SiO$_2$ units joined the same way, yet in the crystalline quartz the rings repeat in a regular lattice while in the amorphous quartz glass the rings are distorted with no repeating pattern. The structure of an amorphous solid is, in fact, very much like that of a liquid.

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How their properties differ

Melting behaviour. A crystalline solid has a sharp melting point — at one characteristic temperature it melts abruptly to a liquid. An amorphous solid instead softens and starts to flow over a range of temperature, which is why it can be moulded and blown into shapes. Because amorphous solids resemble liquids and even tend to flow very slowly, they are regarded as extremely viscous liquids, and are sometimes called pseudo solids or super cooled liquids. Over long periods they may even partly crystallise — some ancient glass objects turn milky for exactly this reason.

Isotropy vs anisotropy. Amorphous solids are isotropic: properties such as mechanical strength, refractive index and electrical conductivity are the same in all directions, because there is no long range order to make any direction special. Crystalline solids are anisotropic: physical properties like electrical resistance or refractive index can have different values along different directions. As illustrated in Fig. 1.2, in a two-dimensional pattern of two kinds of atoms, shearing along the CD direction displaces a row containing both types of atoms, while shearing along the AB direction moves rows made of a single type — so the resistance differs with direction.

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Distinction between crystalline and amorphous solids (Table 1.1)