Pressure-dependent diffusion coefficients and haven ratios in cation-conducting glasses

The influence of hydrostatic pressure on diffusion and ionic conduction is providing deeper insights into the atomistic mechanisms of ionic motion in glasses. We have studied the tracer diffusion of Na-22 in a sodium borate glass and of Rb-86 in a rubidium borate glass as functions of hydrostatic pressures. The activation volumes of tracer diffusion are Delta V-D(Rb) = 33.5 cm(3) mol(-1) and Delta V-D(Na) = 6.1 cm(3) mol(-1). In comparison, the activation volumes of charge diffusion obtained recently from the pressure dependence of conductivity are smaller: Delta V-sigma(Rb) = 7.2 cm(3) mol(-1) and Delta V-sigma(Na) = 2.8 cm(3) mol(-1). These differences, where (Delta V-D - Delta V-sigma) > 0, imply that the Haven ratios decrease with pressure. This effect is particularly significant for the rubidium borate glass. Starting from basic equations of linear response theory for mass and charge transport, we develop a model that accounts for these experimental findings. The difference between the activation volumes, Delta V-D and Delta V-sigma, and the pressure-dependent Haven ratios are consequences of collective movements of ions in glass, implying a concerted motion of ions in a chain- or caterpillar-like fashion. In our treatment, it is a vacant site (with ions jumping into it successively) that moves along an extended pathway. Hence, we regard vacant sites as the carriers of charge and ions as the carriers of diffusing matter. The decrease of the Haven ratio with pressure is attributed to the influence of pressure on the topology of the conduction pathways, which are progressively straightened out with increasing pressure.