In the next chapters, four groups of materials will be introduced and discussed. These will embrace metals, brittle solids, polymers and energetic materials. Some of these will be pure elements in various microstructures, others will be composites of several in different conformations. A lot of what follows has been described and studied by materials science and much terminology and commonplace understanding will be borrowed from there. Appendix A at the end of the book summarises some key concepts for those trained in other disciplines. At the most basic level, materials can be classified as metals or non-metals according to their ability to conduct electricity. The metals consist of cations in a delocalised electron cloud with structure determined by electrostatic bonds formed between the ions and the electron cloud. As pressure increases this bonding changes nature and above the finis extremis localisation of the electron density away from the nucleus occurs leading to new states.
Metals are the most common class of elements in the periodic table (Figure 5.1). Atomic stacking rules define a lattice of ions surrounded by a delocalised cloud of electrons, but from the point of view of the electronic states, one may equally consider them as materials where conduction and valence bands overlap. This definition opens the descriptor to metallic polymers and other organic metals and, considering the context within this book, one must consider the behaviour of materials that change their characteristics under high pressures and cause them to achieve metallic states (to conduct) at pressures below the finis extremis. A diagonal line drawn from aluminium (Al) to polonium (Po) separates the metals from the non-metals, and within that region the elements order themselves into subgroups defined by their electronic structures.