LITHIUM-RICH BOROSULFATE GLASSES - ANALYSIS OF BI-11 AND LI-7 NMR, GLASS-TRANSITION AND CONDUCTIVITY DATA

The structure of the lithium borate-lithium sulphate glasses is investigated with 11B NMR MAS (Magic Angle Spinning) spectroscopy. Over a wide range of boron contents, it is found that addition of the sulphate to a lithium borate glass promotes the conversion of three coordinated borons (B3) into fourfold coordinated borons (B4) up to a rate [B4]/[B3] = 1, and that the sulphate is coordinated to B3 units. We propose the existence of a "sulfo-borate" group (SB), which accounts for the observed solubility limit of the sulphate (≈ 15% molar of Li1SO4), and allows to qualitatively explain the dependence of the glass transition temperature upon composition. No clear patterm emerges from analysis of a collection of conductivity data (from the literature and our laboratories), and this fact points to the occurrece, in lithium-rich borates, of poorly understood crystallization and demixing phenomena. On the other hand, short range diffusion of lithium ions, as probed by the narrowing of the 7Li NMR line, is always enhanced by addition of the sulphate. © 1990.