6Li, 7Li nuclear magnetic resonance investigation of lithium coordination in binary phosphate glasses

Abstract

6Li and 7Li solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local coordination environment of lithium in a series of xLi 2O·(1− x)P 2O 5 glasses, where 0.05⩽ x⩽0.55. Both the 6Li and 7Li show chemical shift variations with changes in the Li 2O concentration, but the observed 6Li NMR chemical shifts closely approximate the true isotropic chemical shift and can provide a measure of the lithium bonding environment. The 6Li NMR results indicate that, in this series of lithium phosphate glasses, the Li atoms have an average coordination between four and five. The results for the metaphosphate glass agree with the coordination number and range of chemical shifts observed for crystalline LiPO 3. An increase in the 6Li NMR chemical shift with increasing Li 2O content was observed for the entire concentration range investigated, correlating with increased cross-linking of the phosphate tetrahedral network by O–Li–O bridges. The repolymerization of the glass structure occurs with the sharing of edges, faces and vertices of Li–O polyhedra. The 6Li chemical shifts were also observed to vary monotonically through the anomalous glass transition temperature ( T g) minimum. This continuous chemical shift variation shows that abrupt changes in the Li coordination environment do not occur as the Li 2O concentration is increased, and such abrupt changes can not be used to explain the T g minimum.