Lithium ion distribution and motion in β-alumina: An NMR and Raman investigation

NMR and Raman evidence is presented to show that, above room temperature, a change occurs in the distribution of lithium ions in Li+-containing β-aluminas. We interpret the results in terms of a model in which, below room temperature, lithium ions reside predominantly in positions off the center of the conduction plane (''out-of-plane'' positions). As the temperature increases, they spend an increasing fraction of their time in the middle of the conduction plane (''in-plane'' positions). It is then possible, from the experimental data, to follow temperature changes of the in-plane ion fraction and to estimate the average energy difference between in-plane and out-of-plane Li+ configurations. A discussion of the Li+ dynamics is then presented, which is based upon the 7Li NMR spin-lattice relaxation and spectral data, as well as upon Raman data