Extremely slow cation exchange processes in Li₄SiO₄ probed directly by two-time ⁷Li stimulated-echo nuclear magnetic resonance spectroscopy

Lithium self-diffusion in the low-temperature modification of polycrystalline lithium ortho-silicate Li₄SiO₄ is investigated by ⁷Li two-time stimulated echo NMR spectroscopy. Extremely slow Li exchange processes were directly monitored between 300 and 433K by recording spin-alignment echoes as a function of mixing time varying over six decades from 10^-5 to 10s. In the investigated temperature range the hopping correlation functions show biexponential behaviour. Whereas the first decay step reflects directly Li jumps between electrically different sites, the second one is simply induced by the decay of alignment order due to quadrupolar relaxation. The echo decay rates τ^-1(10 ¹s^-1≤τ^-1≤10⁴s ^-1), which can be identified with Li jump rates, show Arrhenius behaviour with an activation energy of 0.53(1)eV. The directly measured jump rates are in good agreement with those obtained recently by one-and two-dimensional ⁶Li exchange MAS NMR reported in the literature.