Symmetry Breaking in Chemical Interactions

We develop a unified description of the chemical bond, in which distinct types of bonding can be thought of as mediated by electrons with differing degrees of localization. Rooted in the venerable density-functional theory, the present method shows that localized and delocalized electrons can be treated similarly to distinct phases of a single-component substance. Transitions between these phases are accompanied by changes in the translational symmetry and can be either discontinuous or continuous, the control parameters being the density and electronegativity variation. In the present picture, electron-delocalized and localized regimes correspond with the metallic and ionic bond respectively. Coexistence between the two types of bonding corresponds with the multicenter bond. At sufficiently low densities, the multicenter bond undergoes a further symmetry lowering transition resulting in a coexistence of the covalent and closed-shell, secondary interactions. We discover that symmetry-breaking in the electronic wave function may take place even when the underlying atomic lattice remains symmetric. Such transitions delineate regimes in which the bond-order is well-defined in that it is largely insensitive to bond deformation. We find that if the localized-molecular orbitals cover all nearest-neighbor bonds in a putative structure, the latter is either stable or, at worst, metastable. This finding suggests a high-throughput, automated procedure for screening candidate compounds and structures with regard to stability. Despite the intrinsically quantum-mechanical nature of electronic bands in solids, the interplay between delocalized and localized electrons is similar to that arising during the classical liquid-to-solid transition. In contrast with classical systems, the metal-insulator transition can be continuous, as we show using both direct modelling and a coarse-grained, Landau-Ginzburg description. In a yet more dramatic departure from classical systems, quantum-mechanical treatment of degenerate charge-density-wave (CDW) states shows that interfaces between distinct CDW states must host special midgap electronic states. These states are of topological origin and are robust with respect to elastic deformation. Such midgap states are intrinsic to structural glasses and quantitatively account for several puzzling light-induced phenomena in amorphous chalcogenide alloys; the chalcogenides are of interest in applications such as phase-change computer memory.