Quantitative assessment of the structure of Ge20Te73I7 chalcohalide glass by first-principles molecular dynamics

Abstract

First-principles molecular dynamics, within the density functional theory framework, is employed to assess the structural properties of the chalcohalide glass Ge20Te73I7 (gGTI). The calculated total x-ray and neutron structure factors are in quantitative agreement with the experimental counterparts. Glassy gGTI features a predominantly Ge-centered tetrahedral network moderately altered by iodine atoms. Compared to glassy Ge20Te80, gGTI shows a substantial decrease in the number of Ge-Te bonds as the presence of I leads to a breaking of the Ge-Te connections and favors the formation of Ge-I terminations where iodine atoms are onefold. Furthermore, we remark a concomitant increase of mixed GeTe4?xIx tetrahedra and GeTe3?xIx trigonal pyramidal units. A minor yet not-negligible fraction of iodine atoms forms Te-I bonds. Breaking of Ge-Te connections and formation of terminal Ge(Te)-I bonds results in a reduced average coordination number associated with an increase of undercoordinated Ge at the expense of fourfold coordinated Ge atoms. The analysis of chains and rings statistics reveals the presence of both linear and cross-linked chains made of Te and five-members rings containing at least one Te-Te or one Ge-Ge homopolar bond.