Berlin 2005 – wissenschaftliches Programm
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CPP: Chemische Physik und Polymerphysik
CPP 19: Dynamics I
CPP 19.5: Vortrag
Dienstag, 8. März 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 tm 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 tm is directly induced by ionic jumps between crystallographically
different Li sites, the decay step at longer tm 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).