Surface of glassyGeS2: 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 $\mathrm{GeS}{}_{2}\phantom{\rule{0.3em}{0ex}}(g\text{\ensuremath{-}}\mathrm{GeS}{}_{2})$. 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 $g\text{\ensuremath{-}}\mathrm{GeS}{}_{2}$. Although the $g\text{\ensuremath{-}}\mathrm{GeS}{}_{2}$ surface recovers the main tetrahedral structural motif of bulk $g\text{\ensuremath{-}}\mathrm{GeS}{}_{2}$, 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 $g\text{\ensuremath{-}}\mathrm{GeS}{}_{2}$ 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 $g\text{\ensuremath{-}}\mathrm{GeS}{}_{2}$ 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.