Solid State Background
This chapter provides a background to solid state chemistry, particularly in the field of solid state ionics. There are a number of ways of classifying solids, but perhaps the most useful division is into crystalline and amorphous solids. Crystalline solids have a regular repeating array of atoms characterized by a repeat unit known as the unit cell. Solids which possess a regular repeating crystal structure form the majority of solid materials in common use. Metallic solids are generally metallic elements or alloys. These solids are usually formed between electropositive elements. Ionic solids are formed between elements which have a large difference in electronegativity. Substitutional solid solutions involve direct substitution of ions, or atoms by different, but isovalent ions or neutral atoms, respectively. In ionic solids where substitution is by an aliovalent ion, electroneutrality is maintained either by formation of vacancies or by introduction of interstitials.