From polyanions to infinite chains: chemical bonding evolution in AX3 polyhalides under pressure

Abstract

Polyhalides are molecular systems that defy conventional views of chemical bonding, with infinite linear halide chains being the most challenging systems. By studying CsI₃ under compression, we show how I₃⁻ polyanions, with electron-rich multicenter bonds, undergo progressive pressure-induced polymerization giving rise to infinite linear iodine chains, I_∞, and demonstrate that these chains, and, by extension, infinite linear halide chains, feature electron-deficient multicenter bonds that are in good agreement with the recently published unified theory of multicenter bonding. This result is in sharp contrast with previous assumptions that considered electron-deficient multicenter bonds to be impossible in valence electron-rich elements such as halogens. The pressure-induced formation of these unconventional bonds explains the decrease of the bandgap and the increase in electrical and photoelectrical conductivity in AX₃ polyhalides under compression.