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

Li-6 and Li-7 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(2)O .(1-x)P2O5 glasses, where 0.05 less than or equal to x less than or equal to 0.55. Both the Li-6 and Li-7 show chemical shift variations with changes in the Li2O concentration, but the observed Li-6 NMR chemical shifts closely approximate the true isotropic chemical shift and can provide a measure of the lithium bonding environment. The Li-6 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 LiPO3. An increase in the Li-6 NMR chemical shift with increasing Li2O 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 Li-6 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 Li2O concentration is increased, and such abrupt changes can not be used to explain the T-g minimum. (C) 1999 Elsevier Science B.V. All rights reserved.