Chemical structure and the mixed mobile ion effect in silver-for-sodium ion exchange in silicate glasses

The relationship between chemical structure and ion transport in silver-for-sodium ion exchange in the endpoints of a series of sodium aluminosilicate glasses is examined. It is shown that the non-bridging oxygen (NBO) content of the glass has a major impact on both the local environments of the mobile cations and the mixed mobile ion effect (MMIE). X-ray absorption fine structure studies of the local environments of the mobile cations are used to help formulate a structural picture of ion exchange, in which interactions between the mobile cations affect ion transport rates. Studies of the diffusion coefficient using energy dispersive microanalysis and the modified quasi-chemical (MQC) diffusion coefficient are used to obtain quantitative values for MMIE. Parameter fits of the MQC diffusion coefficient to experimentally obtained values are used to extract self-diffusion coefficients, excess interaction energies and cation-cation coordination numbers. It is found that the NBO-rich glass has the highest excess interaction energy and exhibits the greatest MMIE, consistent with the structural model.