Structure and ion dynamics of silver borate glasses: A 19Ag NMR study

An analysis of the 109Ag isotropic chemical shift δ in the glasses of the AgI:Ag2O:B2O3 system is presented. In these glasses, δ covers a range of ∼500 ppm but does not depend linearly upon the fraction of I− anions X=I/Ag. This means that, when AgI is added to the borate network, structures are preferentially formed in which silver is coordinated to both I− and negatively charged borate groups. We discuss also the 109Ag chemical shift range covered by the different forms of AgI and the dependence of δ upon the BO4 units and non-bridging-oxygen (NBO’s) concentrations. We analyze the relationship between spatial compositional fluctuations and NMR–MAS width of the 109Ag line in fast silver conducting glasses. The spin-lattice relaxation of 109Ag and 11B, as well as some of the 109Ag line narrowing results, are quantitatively interpreted by assuming a distribution of Ag+ jump times and by discussing how the different nuclear interactions are affected by the diffusion of silver. We conclude that the NMR evidence does not support the microscopic picture of cation diffusion in vitreous electrolytes suggested by the weak electrolyte theory.