Influence of glass network ionicity on the mixed‐alkali effect

Abstract

AbstractMost studies of the mixed‐alkali effect (MAE) have focused on relating the differences between the cations to the strength of the MAE; here we examine the effect of the glass former by comparing the MAE in aluminofluorophosphate (FP) and aluminosulfofluorophosphate (FPS) glasses. The sulfate anion in the FPS series does not bond directly to the aluminophosphate network, decreasing connectivity and increasing the ionicity of the FPS glasses. The increased degrees of freedom imparted by the sulfate are evident in the single‐alkali FPS glasses, which have lower Ea, hardness, shear moduli, and Young's moduli, and higher Tg than the corresponding single‐alkali FP glasses (without sulfate). Conversely, for the mixed‐alkali FPS compositions, the sulfate's mobility is reduced by the mixed cations, resulting in a magnified MAE for the FPS series. For dynamic properties, this phenomenon is explained by reduced plasticity and slippage along ion channels, while understanding Tg and elastic properties requires examination of the shape of the sulfate's potential energy well and the resulting regions of compressive and tensile stress. We posit that glass compositions with higher plasticity, that is, less covalent bonding, will exhibit a larger MAE, but, simultaneously, sufficient conventional glass network must be present to constrain the mobile ions.