Surface of glassy GeS[subscript 2]: A model based on a first-principles approach

Abstract

First-principles calculations within the framework of the density functional theory are used to construct realistic models for the surface of glassy GeS[subscript 2](g−GeS[subscript 2]). Both calculations at T = 0 K and at finite temperature (T = 300 K) are considered. This allows for a comparison between the structural and electronic properties of surface and bulk g−GeS[subscript 2]. Although the g−GeS[subscript 2] surface recovers the main tetrahedral structural motif of bulk g−GeS[subscript 2], the number of fourfold coordinated Ge atoms and twofold coordinated S atoms is smaller than in the bulk. On the contrary, the surface system features a larger content of overcoordinated S atoms and threefold coordinated Ge atoms. This effect is more important for the g−GeS[subscript 2] surface relaxed at 0 K. Maximally localized Wannier functions (WF) are used to inspect the nature of the chemical bonds of the structural units present at the g−GeS[subscript 2] surface. We compare the ability of several charge derivation methods to capture the atomic charge variations induced by a coordination change. Our estimate for the charges allows exploiting the first-principles results as a data base to construct a reliable interatomic force field.