1. Crystalline medium

1.1 General

The properties of a material are conditioned by its structure, i.e. by the way atoms, ions and molecules are arranged within it. On a macroscopic scale, the properties of a solid, homogeneous chemical compound are identical to themselves at every point. On the microscopic scale, its structure is characterized by the three-dimensional repetition of an identical pattern of atoms, ions or molecules. If this repetition is triply periodic, the medium is said to be crystalline, in the classical sense of the term. The medium can also be ordered, endowed with rotational symmetry but not translational symmetry, a situation encountered in quasi-crystals, of which certain metal alloys discovered in 1981 are the most characteristic representatives.

The importance...