Bombardment induced ion transport--part II. Experimental potassium ion conductivities in borosilicate glass.

A new experimental approach for measuring the ionic conductivity of solid materials is proposed. The experiment is based on bombarding an ion conducting sample with an alkali ion beam. This generates a well defined surface potential which in turn causes ion transport in the material. The ion transport is measured at the back side of the sample. The viability of the concept is demonstrated by measuring the temperature dependence of the potassium ion conductivity of a potassium borosilicate glass. The activation energy for the potassium transport is 1.04 eV ± 0.06 eV. For comparison, conductivity data obtained by impedance spectroscopy are presented, which support the bombardment induced data.

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 22Na in a sodium borate glass and of 86Rb in a rubidium borate glass as functions of hydrostatic pressures. The activation volumes of tracer diffusion are DeltaVD(Rb) = 33.5 cm3 mol-1 and DeltaVD(Na) = 6.1 cm3 mol-1. In comparison, the activation volumes of charge diffusion obtained recently from the pressure dependence of conductivity are smaller: DeltaVsigma(Rb) = 7.2 cm3 mol(-1) and DeltaVsigma(Na) = 2.8 cm3 mol(-1). These differences, where (DeltaVD - DeltaVsigma) > 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, DeltaVD and DeltaVsigma, 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.