Li-6 and Li-7 MAS NMR studies on fast ionic conducting spinel-type Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4

摘要

Li-6 and Li-7 MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4 have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions (Li2-xXxMgCl4)-Mg-1 (M-1 = Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Li-tet of inverse spinel-type Li2MgCl4 and the high-field signal to Li-oct as proposed by Nagel et al. (2000). In contrast to spinel-type Li2-2xMg1+xCl4 solid solutions with clustering of the vacancies and Mg2+ ions, the Cu+ and Na+ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are E-a = 6.6-6.9 and DeltaG* > 79 kJ mol(-1) (in addition to 23, 29, and 75 kJ mol(-1) obtained by other techniques), respectively. The largest activation energy of >79 kJ mol(-1) corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6-6.9 kJ mol(-1), which was derived from the temperature dependence of both the spin-lattice relaxation times T-1 and the correlation times tau(C) of Li-tet, reveals a dynamic process for the Li-tet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site. (C) 2002 Elsevier Science (USA). [References: 27]