Connection between the microwave and far infrared conductivity of oxide glasses

It was demonstrated for a lithium silicate glass that the low temperature (<150 K)-low frequency (Hz-kHz) and high temperature (>200 K)-high frequency (GHz) conductivity have the same origin. Presumably, either when the temperature is too low or the time scale is too short to observe a single ion hopping, conductivity arises from the localized motion of a group of atoms like the 'wiggling of a jellyfish'. Mathematically, this complex motion has been described at low temperatures in terms of the thermally activated motion of atoms over a distribution of asymmetric double well potential (ADWP). Empirical analysis of experimental data reveals that microwave conductivity, sigma(MW), cannot be explained simply by the combined contribution of low frequency single ion hopping and far infrared (FIR) single ion vibrations. Computer simulations using the ADWP model explain the linear frequency dependence of microwave conductivity reasonably well and predict a plateau at high frequencies. Finally, the results of simulations are compared with the data to establish the relative contributions from ADWP excitations and from single ion vibrations to the experimentally observed conductivity at 10(8)-10(12) Hz frequency range. (C) 2000 Elsevier Science B.V. All rights reserved.