Spin–lattice relaxation in the candidate solid-state electrolyte Li2OHCl is studied using both conventional and triple-quantum filtered 7Li NMR experiments over a range of temperatures above the orthorhombic-to-cubic phase transition. The use of the recently introduced triple-quantum filtered saturation-recovery 7Li NMR method allows the inherently biexponential spin–lattice relaxation behavior of the I = 3/2 7Li nucleus to be readily observed and quantified. The resulting data are compared with a modified version of the basic quadrupolar relaxation theory that incorporates the additional possibility of magnetic relaxation. The multiple-quantum 7Li NMR experiments reveal the magnetic contribution to be significant, especially in the lower part of the temperature range, while the modified theory shows that this explains the near-absence of biexponential behavior in the conventional 7Li spin–lattice relaxation data. At temperatures just above the phase transition to a cubic form, there is unwanted direct excitation of 7Li triple-quantum coherences by the radiofrequency pulses used in the NMR experiments, indicating the presence of quadrupolar splittings in the 7Li NMR spectrum. The limitations of standard relaxation theory in this limit are discussed. At higher temperatures, the lithium ions are more mobile, and in this more liquid-like regime, there is good qualitative agreement between the modified relaxation theory and the experimental data.
Wimperis et al. (Tue,) studied this question.