Mid-IR and far-IR investigation of AgI-doped silver diborate glasses

The structures of xAgI+(1-x)Ag2O . 2B(2)O(3) glasses, where 0.2 less than or equal to x less than or equal to 0.6, have been investigated using mid- and far-infrared spectroscopy. The mid-IR spectra revealed that in those glasses prepared using AgNO3 as the starting material for Ag2O, the BO4-/BO3 ratio is constant with increasing amounts of AgI as would be expected form the proposed behavior of AgI in these glasses. However, a survey of the literature revealed those glasses prepared from pure Ag2O show a strong linear dependence of the BO4-/BO3 ratio on AgI content. Most probably, in those glasses prepared with Ag2O the Ag2O/B2O3 ratio changes with AgI content due to the decomposition of Ag2O during melting. This different behavior is associated with AgNO3 decomposing to Ag2O with heating followed by incorporation into the glassy network. For Ag2O used directly, it is proposed that it decomposes to Ag metal and Oz(gas) with heating before it can be incorporated into the borate network. This latter behavior decreases with increasing AgI in the batch composition because AgI lowers the liquidus temperature of the melt considerably. The far-IR analysis of the AgI-doped silver diborate glasses suggests that there are three coordination environments for the Ag+ ions; one with iodide anions and the other two with oxygen ions. It is proposed that the separate oxygen coordination environments for the Ag+ ions arise from one with bridging oxygens of BO4- units, and the other with nonbridging oxygens on BO3- units. Furthermore, it is proposed that the Ag+ ions in the iodide-ion environments progressively agglomerate into disordered regions of AgI, but do not form structures similar to alpha-AgI. These results appear to support the conduction pathway or ''microdomain'' model for ionic conduction in xAgI+(1-x)Ag2O . 2B(2)O(3) glasses where the pathways are built up form disordered structures of AgI.