The Role of Sublattices in Chemical Bonding in Essentially Ionic Crystals

Abstract

A technique for calculating the self-consistent electron density of sublattice and crystal as a whole has been developed based on the local electron density functional theory (DFT). Difference density, obtained by subtraction of the densities of the individual sublattices from the total electron density, is introduced as a quantity defining the relationship between the sublattices. Self-consistent calculations have been performed for series of compounds with a NaCl lattice: alkali and noble metal halides and alkali metal oxides and sulfides with an antifluorite lattice. As a result of these calculations, we formulated the key principle of chemical bonding based on sublattices. In the anion sublattice, alike atoms form a covalent type of bond with each other by transferring some portion of the charge to the interstice. The same mechanism of bonding is realized in sublattices of metal atoms containing the levels of the d-states in the valence region. In sublattices of simple metals, the valence density behaves as “metallic,” changing slightly within the unit cell, with electrons again moving from the metal sites into the interstice. Addition of the valence densities of sublattices leads to two essentially different situations: in one case, the metal sites coincide with the density maxima at the interstice of the anion sublattice, and in the other, they do not. In the first case, the superposition of the sublattice densities at the lattice sites leads to anion-cation hybridization, forming an essentially ionic chemical bond with a possible admixture of the covalent bond. In the second case, the anion sublattice is a covalently bonded framework incorporating the metal sublattice. This takes place in alkali metal oxides and sulfides.