Ions in crystals: The topology of the electron density in ionic materials .1. Fundamentals

The topological theory of atoms in molecules is applied to periodic crystalline ionic systems. A systematic investigation of the fundamental properties of the topology of the charge density in crystals is undertaken, and several basic facts, peculiar to the solid state and not previously explored, to our knowledge, are put forward. We also show how the theory allows us to define unambiguously very important concepts of solid-state theory, like the coordination index or the coordination polyhedron of an ion in a solid. We particularize our results by means of the detailed study of an example crystal, the rocksalt phase of LiI. It is shown that this crystal is best described as made up of 18-fold-coordinated iodides and sixfold-coordinated lithiums, contrary to the usual six-six description.