Structure Relaxation and Liquidlike Enhanced Cu Diffusion at the Surface of β-Cu₂S Chalcocite

The hitherto unexplored surface structural and dynamical properties of the thermoelectric material β-Cu₂S chalcocite, are uncovered using ab initio molecular dynamics simulations at 450 K. The material exhibits a hybrid crystalline-liquid behavior, with the liquidlike dynamics of the Cu atoms and the crystalline order of the sulfur sublattice. The topmost nanoscale region of the material is predicted to undergo significant structural relaxation, resulting in a ∼10% increase in the distance between the topmost S-layers accompanied by an increased Cu density. Cu diffusion in the interlayer regions of the surface S-sublattice is enhanced (doubled) compared to the bulk value, and an underlying microscopic mechanism, entailing marked emergent surface-induced softening of the S-sublattice vibrational dynamics, is described.