AN EVALUATION OF THE IONIC-CONDUCTIVITY IN AGI-DOPED GLASSES - THE GRADED-PERCOLATION MODEL

Various percolation theories, Effective Medium Percolation Theory (EMPT), Site Percolation Theory (SPT) and Bond percolation Theory (BPT), have been applied to the ionic conductivity of a series of AgI-doped glasses in order to reexamine the proposed "AgI-microdomain" model for these glasses. In contrast to previous reports of the possibility that percolation effects do control the conductivity in these glasses, it is found here that SPT predicts a conductivity catastrophe at x almost-equal-to 0.3 at the percolation threshold and the EMPT model predicts too sharp a threshold as well. Both of these effects are not observed in any of the AgI-doped glass studied to date. Variation from the percolation theories and experimental data, however, has been used to support two new features of the ionic conduction in these glasses. At low AgI contents, sharp increases in conductivity are proposed to arise from a "co-connectivity" of the Ag+ cations and at higher AgI contents failure of the conductivity to reach the value of alpha-AgI is associated with the distortion of the I-polyhedra away from those in alpha-AgI.