Investigation of the mixed alkali effect in boro-tellurite glasses - the role of NBO-BO switching in ion transport

Abstract

Glasses of the composition 30(Li,Na)₂O,xB₂0₃,(70-x) Te0₂ (20 ≤ x ≤ 50) have been prepared using the melt­ quenching method. Properties of these glasses have been measured and examined for the presence of mixed alkali effect (MAE). Infrared, Raman and ¹¹B HR MAS NMR spectroscopies have been used to investigate the structure of these glasses. DC and a.c. conductivities and dielectric properties have been measured over a wide range of frequencies and temperatures. Densities, molar volumes, glass transition temperatures, thermo­ dynamic fragilities, infrared absorption frequencies, d.c. conductivities and activations barriers have been found to exhibit MAE. However, MAE does not manifest in the a.c. transport and dielectric relaxation parameters. Structure and bonding considerations account for the observed MAE in several properties. But the presence of MAE in d.c. conductivities and its absence in a.c. transport and dielectric phenomena is notable and its understanding requires a critical review of currently used conceptual basis of ion transport in oxide glasses. A new approach is therefore proposed in which the primary event in ion transport is the motion of negative charge through the non-bridging oxygen (NBO)-bridging oxygen (BO) switching. NBO-BO switching initiates the jump of alkali ions as the latter become energetically unstable in their positions. The present mechanism leaves unaffected the Arrhenius behaviour of d.c. conductivity, but with a new meaning of the parameters in it.