Ionicity and coordination in glasses

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Abstract
A simple tight-binding Hamiltonian is introduced to describe the bonding of cations into a glassy network, and this enables a demonstation of how the stability of the various covalent coordinations and ionic sites depends on the covalent, ionic and metallic gaps. Restricting the discussion to the coordination transition between four fold (sp3) sites and ionic sites, it is found empirically that the ionic sites are stable above a certain critical ionicity in a similar fashion to the transition between the zinc-blende and rock-salt families of crystals. Formal charge, an integer, is introduced which describes the number of electrons lost by a site in dative or ionic bonding. Over-coordination covalent sites of electron-deficient atoms are negative and ionic sites are positive, as expected. When electronically active, metal additive sites donate carriers having a sign opposite to that of their formal charge.