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

Li-6 and Li-7 solid state magic angle spinning (MAS) nuclear magnetic reson ance (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, b ut the observed Li-6 NMR chemical shifts closely approximate the true isotr opic chemical shift and can provide a measure of the lithium bonding enviro nment. The Li-6 NMR results indicate that, in this series of lithium phosph ate glasses, the Li atoms have an average coordination between four and fiv e. The results for the metaphosphate glass agree with the coordination numb er and range of chemical shifts observed for crystalline LiPO3. An increase in the Li-6 NMR chemical shift with increasing Li2O content was observed f or the entire concentration range investigated, correlating with increased cross-linking of the phosphate tetrahedral network by O-Li-O bridges. The r epolymerization of the glass structure occurs with the sharing of edges, fa ces and vertices of Li-O polyhedra. The Li-6 chemical shifts were also obse rved to vary monotonically through the anomalous glass transition temperatu re (T-g) minimum. This continuous chemical shift Variation shows that abrup t changes in the Li coordination environment do not occur as the Li2O conce ntration is increased, and such abrupt changes can not be used to explain t he T-g minimum. (C) 1999 Elsevier Science B.V. All rights reserved.