Li NMR of Crystalline Solids

A selection of Li and Li NMR shifts for lithium compounds are shown in Table 10.1. 

Lithium aluminate, LiAlsOg with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. Li and Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that Li relaxes most significantly through interactions with paramagnetic impurities, whereas Li relaxes much more strongly through dipole-dipole interactions. 

This trend, which is in the same direction as for A1, Si, Na and Mg, appears to be related to the increase in Li-O bond length and Li ionicity with increasing coordination number. A similar trend in the Li isotropic shift with ionicity has also been reported in sulphide-based glasses (Eckert et at. 1990). 

This simple relationship between the Li shifts and Li coordination number has been found not to hold for crystalline lithium phosphates, indicating that other factors must be taken into account (Alam et al. 1999). By analogy with Na NMR (Chapter 7), Alam et al. (1999) sought a relationship between the Li shift and the average degree of polymerisation in the phosphate tetrahedra as reflected by the number of non-bridging 

By contrast with the results for Na (Chapter 7), both these lines have a positive slope. This unexplained descrepancy suggests that the formalism for defining the parameter A requires refinement, although the treatment is potentially useful within limited groups of similar compounds. 

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