Ionic conductivity of alkali oxide glasses at microwave frequencies

The use of glass as additive in multilayered dielectric structures, as substrate in microwave integrated chips for communication devices etc. has all been a motivation to study the microwave dielectric properties of glass. Early studies of high frequency conductivity, particularly in the microwave region conducted at one or two frequencies and at room temperature were suggested to be due to five independent sources of dielectric loss mechanisms based on migration, vibration, and/or deformation associated with ion movement in glasses and glass-ceramics. However, these theories are either inconsistent with experimental data or lack satisfactory microscopic description.; The microwave conductivity, σ_MW, of oxide glasses is shown to increase linearly with frequency but with weak thermal activation. This is similar to the low temperature - low frequency conductivity, where the mechanism of conduction is established to originate from jellyfish-like localized fluctuations of atoms in a&barbelow;symmetric d&barbelow;ouble w&barbelow;ell p&barbelow;otential (ADWP) configurations. In other words, the high frequency - high temperature dielectric loss in oxide glasses can be understood in terms of the jellyfish conduction. Computer simulations of the ADWP formulation explain the observed linear frequency dependence and power law temperature dependence of microwave conductivity well. Thus the conduction mechanism at microwave frequencies can be described very well by the localized fluctuations of jellyfish structures. Careful examination of the ADWP parameters especially the pre-factor to relaxation time, the asymmetry energy, and the concentration of ADWP structures in the two regions, viz. low f - low T and high f - high T, reveal that different kinds of jellyfish structures are responsible for conduction in the two regions. From the composition dependence of σ_MW and ADWP concentration, we conclude that not all alkali ions participate in conduction, but only a fraction that are co-ordinated to the network in some special configuration contribute to the jellyfish mechanism at MW frequencies. The effective region of the jellyfish, which consists of the modifier as well as the network atoms, is estimated to be approximately ∼25 Å or less.