Four-time 7Li stimulated-echo spectroscopy for the study of dynamic heterogeneities: Application to lithium borate glass

摘要

Graphical abstract Display Omitted Highlights ? Pulse sequence for 4-time stimulated echoes of nonselectively excited spin I =3/2 nuclei. ? Density matrix calculations show use of octupolar spin order in multidimensional NMR. ? Applicability is demonstrated for 7Li in lithium borate glasses. ? Subensemble selection reveals dynamic heterogeneities and dynamic exchange. ? Complementary experiments using 6Li support results. Abstract To study the nature of the nonexponential ionic hopping in solids a pulse sequence was developed that yields four-time stimulated-echo functions of previously inaccessible spin-3/2-nuclei such as 7Li. It exploits combined Zeeman and octupolar order as longitudinal carrier state. Higher-order correlation functions were successfully generated for natural-abundance and isotopically-enriched lithium diborate glasses. Four-time 7Li measurements are presented and compared with two-time correlation functions. The results are discussed with reference to approaches devised to quantify the degree of nonexponentiality in glass forming systems and evidence for the occurrence of dynamic heterogeneities and dynamic exchange were found. Additional experiments using the 6Li species illustrate the challenge posed by subensemble selection when the dipolar interactions are not very much smaller than the quadrupolar ones. ]]>