Berlin 2005 – scientific programme

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CPP: Chemische Physik und Polymerphysik

CPP 19: Dynamics I

CPP 19.5: Talk

Tuesday, March 8, 2005, 10:45–11:00, TU C230

Slow Cation Exchange Processes in Polycrystalline Lithium Orthosilicate Probed by Two-Time ⁷Li Stimulated-Echo NMR Spectroscopy — •Martin Wilkening and Paul Heitjans — Universität Hannover, Institut für Physikalische Chemie und Elektrochemie

Li diffusion in the low-temperature modification of Li₄SiO₄ is investigated by two-time ⁷Li stimulated-echo NMR spectroscopy. Slow Li motions are directly monitored between 300 K and 433 K by recording spin-alignment echoes as a function of mixing time t_m varying over six decades from 10⁻⁵ s to 10 s. In this temperature range the decay curves show biexponential behaviour: Whereas the decay step at short t_m is directly induced by ionic jumps between crystallographically different Li sites, the decay step at longer t_m indicates spin-alignment decay simply due to (quadrupolar) longitudinal relaxation. The frequency independent decay rates τ⁻¹ (50 s⁻¹ ≤ τ⁻¹ ≤ 7800 s⁻¹) show Arrhenius behaviour with an activation energy of 0.52(1) eV and a preexponential factor τ₀⁻¹=1.5(1) · 10¹⁰ s⁻¹. Extracted jump rates and activation energies for Li diffusion are in good agreement with those obtained by one- and two-dimensional ⁶Li exchange (rotor synchronized) MAS-NMR [1].
[1] Z. Xu and J.F. Stebbins, Science 270, 1332 (1995).