No abstract

No abstract

No abstract

Ion exchange, which is commonly used to alter the optical and mechanical properties of glasses usually involves a one-for-one substitution of an alkali cation (Na+1) from the glass by a monovalent dopant cation (Ag+1). The structural chemistry of this process, however, is far from understood. In this study we have used XAFS spectroscopy to determine the environments of sodium in a tetrasilicate and aluminosilicate glass and also ion exchange the sodium for silver. We find that the coordination number of sodium falls in the presence of aluminium, but that this is also accompanied by an increase in site disorder. By contrast, the environment of ion exchanged silver is less disordered, the nearest neighbour coordination resembling that of Ag2O. The presence of aluminium in the glass is clearly effective in removing some of the steric constraints of mobile cations, defining preferred routes for ion exchange.

Ion exchange is often used to alter the index of refraction of a glass to fabricate waveguide and micro-optics devices, usually resulting in a gradient index. However, in order to efficiently design such devices it is necessary to be able to accurately predict and control the ion exchange. One important complication that must be accounted for is the Mixed Mobile Ion Effect (MMIE), also known as the Mixed Alkali Effect, in which many of the glass properties vary from additivity when two species of monovalent cations are present. This effect, although extensively documented, is poorly understood. As a result, parameters and conditions for ion exchanges are usually determined through a tedious trial-and-error process. We have found that insight into ion-exchange processes, including the MMIE, can be gained from studies of glass structure. We have studied silver-for-sodium exchange in a series of aluminosilicate glasses, commercially important for ion-exchanged waveguides. In this work we relate the results of our previous structural studies to MMIE and the concentration dependence of the diffusion coefficient.