ELECTRON-SPIN RESONANCE INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF HOPPING CENTERS AND THE POLARONIC CONDUCTION IN IRON-CONTAINING PHOSPHATE-GLASSES

We have previously shown that the electronic conductivity of copper-containing glasses sensitively depends upon the microscopic phase structure as well as the extent of clustering phenomena. In this context, an accurate description of the transition-metal ion wavefunction appeared to be necessary. Further confirmation has been obtained from the investigation of glasses of general composition xZnO·yP2O5·zFe2O3. ESR analyses showed two types of iron(III) centres, the concentration of which apparently depends both on the total iron content and the matrix structure. A dramatic improvement of the agreement between theoretical predictions and experimental data has been achieved for the onset of the transport properties by a proper description of the electronic structure of the hopping centres. This conclusively demonstrates the general need of a realistic description of the localized states in order to account for the hopping processes in such systems, stressing in turn the primary role which can be played in their investigation by techniques that enable a direct observation of the environment of the metal sites.