Structural properties of glassy Ge2Se3from first-principles molecular dynamics

Abstract

The structural properties of glassy Ge${}_{2}$Se${}_{3}$ were studied in the framework of first-principles molecular dynamics by using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our results for the total neutron structure factor and the total pair distribution function are in very good agreement with the experimental results. When compared to the structural description obtained for liquid Ge${}_{2}$Se${}_{3}$, glassy Ge${}_{2}$Se${}_{3}$ is found to be characterized by a larger percentage of fourfold coordinated Ge atoms and a lower number of miscoordinations. However, Ge$\ensuremath{-}$Ge homopolar bonds inevitably occur due to the lack of Se atoms available, at this concentration, to form GeSe${}_{4}$ tetrahedra. Focusing on the family of glasses Ge${}_{x}$Se${}_{1\ensuremath{-}x}$, the present results allow a comparison to be carried out in reciprocal and real space among three prototypical glassy structures. The first was obtained at the stoichiometric composition (glassy GeSe${}_{2}$), the second at a Se-rich composition (glassy GeSe${}_{4}$) and the third at a Ge-rich composition (glassy Ge${}_{2}$Se${}_{3}$). All networks are consistent with the ``$8\ensuremath{-}N$'' rule, in particular, glassy GeSe${}_{4}$, which exhibits the highest degree of chemical order. The electronic structure of glassy Ge${}_{2}$Se${}_{3}$ has been characterized by using the Wannier localized orbital formalism. The analysis of the Ge environment shows the presence of dangling, ionocovalent Ge$\ensuremath{-}$Se, and covalent bonds, the latter related to Ge$\ensuremath{-}$Ge connections.