The chemical-bonding origins of over-coordinated defects in chalcogenide glasses

Point defects in (sulfide, selenide) chalcogenide glasses have traditionally been described by the venerable ‘valence-alternation-pair’ (VAP) model of coordination defects, due to Mott, Davis and Street, and Kastner, Adler and Fritzsche. For the case of pure chalcogens (e.g. Se), for example, electron-spin pairing is assumed to occur, producing diamagnetic defects from paramagnetic broken (dangling) bonds. This can be achieved by the energy reduction arising from dative bonding, involving p -like lone-pair (LP) electrons, forming positively-charged, (three-fold) over-coordinated chalcogen sites. This bond formation can be sufficient to overcome the Hubbard correlation energy cost incurred in placing an extra electron on a (one-fold) under-coordinated site to create a conjugate negatively-charged defect. Similar considerations apply to pnictogens (e.g. As). However, quantum-accurate molecular-dynamics (MD) simulations have revealed two other types of over-coordinated defects in chalcogen(ide) glasses: namely, a negatively-charged, four-coordinated pnictogen (As) defect found in amorphous As 2 S 3 ; and an electrically neutral, four-coordinated chalcogen (Se) defect found in MD simulations of amorphous Se at high applied electric fields. The formation of these two special defects cannot be understood in terms of the LP dative-bonding mechanism for VAP formation. Instead, their occurrence can be understood if the over-coordination arises instead via ‘hyperbond’ formation, involving a donor-acceptor pair interaction between a p -like LP and an antibonding orbital, as is believed to be the origin of the chemical bonding in crystalline and glassy phase-change-memory telluride materials.