Introduction

A solid has a definite volume and shape, neither of which changes appreciably with changes in temperature or pressure. A study of the solid state is principally a study of crystalline materials, since almost all solids form crystals. Certain solids are described as amorphous: in some cases they are microcrystalline, but other examples, such as glass or many polymers, do not have the regularity in structure that is associated with true crystals. The term amorphous is best restricted to those solids in which order extends over only a few atomic dimensions. In crystals, the atoms or molecules are arranged on, or in a fixed relation to, the points of a Bravais lattice, where they vibrate about their mean positions. The mean positions are, normally, invariant with time, and the vibrational energy is a major contributory factor to the heat capacity of the solid.

The atomic vibrations are anharmonic (see also Section 3.6.5.1), so that an increase in temperature causes an increase in the distance between the mean positions of the atoms and the material expands. Even if the vibrations were harmonic, the increase in free energy with temperature would lead to an increase in volume, although the expansion from this source is, normally, a second-order effect.

The time-invariance of mean atomic positions may be invalidated by disorder. In some solids, atoms may exhibit free rotation (dynamic disorder) in the solid state, or they may show static disorder.