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Insights from Local Network Structures and Localized Diffusion on the Ease of Lithium Ion Transport in Two Mixed Glass-Former Systems

The conductivity enhancement observed in the two glass systems, A: 40Li2O-(60-x)-P2O5-xGeO2 and B: 40Li2O-10B2O3-(50-x)P2O5-xGeO2, as germanate content is increased from 0 to 25 mol% confirms the positive mixed glass-former effect (MGFE) in these systems. In this study, we further employ state-of-art NMR techniques along with Raman spectra to probe the local network structures. We use the MIGRATION concept to model the experimental conductivity and permittivity spectra obtained from impedance spectroscopy to understand scaling features of spectra and to calculate the value of the spatial extent of the localized diffusion of the lithium ion. As x increases from 0 to 25 mol%, for system A, a continuous increase of the POGe cross linkages as well as a continuous and strong modification of the phosphate network is observed. In system B, deeper analysis of the 31P MAS-NMR experiments done using 2D 11B/11B homo-nuclear and 11B/31P hetero-nuclear NMR is used to determine the nature of the BOB linkages as well as that of POB linkages. Modelling of the conductivity spectra shows that the shape parameter of the spectra remains the same for both systems and for all compositions, a feature typical for MGFE. Furthermore, correlating the trends of the spatial extent of localized diffusion with that gleamed from the local structures, we infer that as relative germanate content increases, in the ternary glass (system A) the ease of mobility of the Li+ ion is enhanced while in the quaternary glass (system B, x > 0) it is somewhat hindered.

Germanophosphate glasses, Mixed glass formers, Local network structures

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