Positive and Negative Mixed Glass Former Effects in Sodium Borosilicate and Borophosphate Glasses Studied by 23Na NMR

Glasses with varying compositions of constituent network formers but constant mobile ion content can display minima or maxima in their ion transport which are known as the negative or the positive mixed glass former effect, MGFE, respectively. Various nuclear magnetic resonance (NMR) techniques are used to probe the ion hopping dynamics via the Na-23 nucleus on the microscopic level, and the results are compared with those from conductivity spectroscopy, which are more sensitive to the macroscopic charge carrier mobility. In this way, the current work examines two series of sodium borosilicate and sodium borophosphate glasses that display positive and negative MGFEs, respectively, in the composition dependence of their Na+ ion conductivities at intermediate compositions of boron oxide substitution for silicon oxide and phosphorus oxide, respectively. A coherent theoretical analysis is performed for these glasses which jointly captures the results from measurements of spin relaxation and central transition line shapes. On this basis and including new information from B-11 magic angle spinning NMR regarding the speciation in the sodium borosilicate glasses, a comparison is carried out with predictions from theoretical approaches, notably from the network unit trap model. This comparison yields detailed insights into how a variation of the boron oxide content and thus of either the population of silicon or phosphorus containing network-forming units with different charge-trapping capabilities leads to nonlinear changes of the microscopic transport properties.