Ionicity dependence of defect reactions and negative-U states in glasses, I

Abstract

The effect of ionicity on the nature of defects in glasses is investigated using a simple tight-binding Hamiltonian, with particular reference to defects with negative effective correlation energies (U). It is noted that in heteropolar sytems there are two types of U. The first arises from amphoteric reactions of a neutral defect, D°, and negative values of U cause Fermi level pinning when the electron density changes. The second type of U involves defects associated with different elements and occurs in optical and thermal transitions at constant electron density. When the latter U is negative, neutral paramagnetic defects are cleared from the gap. Increasing ionicity reduces the stability of dative bonds in which the anion is the Lewis base, and they become unstable above a polarity of (1-S ²)/(1 + S² ), where S is the overlap integral. The cation-originating defect D_c ⁺ returns from the overcoordinated anion A_n-1 ⁻ to the undercoordmated C_n-1 ⁻ configuration (C = cation, A = anion). The negative defects D⁻ remain undercoordinated, and the D_c ⁻ can often increase its stability by bond rearrangements, replacing the C_n-1 ⁻ by a C-C bond and an A_n-1 ⁻ site. The configuration of neutral defects, D°, depends strongly on S in our model and is undercoordinated for S > ⅓ in homopolar systems and only for high polarities does D_a° become C₃°.