Understanding (6)Li MAS NMR spectra of Li(2)MSiO(4) materials (M = Mn, Fe, Zn).

Analysis of (6)Li MAS NMR spectra of several lithium transition-metal orthosilicates (Li(2)MSiO(4), M = Mn, Fe, Zn) improved the understanding of the relation between the spectral parameters and the structural characteristics of the materials. It was shown that for manganese- and iron-containing materials the width of the (6)Li spinning-sideband powder patterns can be roughly related to the arrangement of the transition-metal cations within the first cation coordination sphere around lithium. In mixed zinc-manganese lithium orthosilicates the (6)Li isotropic shift depends on the number of Li-O-Mn bonds, in which a particular lithium site is involved. Each bond contributes a small negative Fermi-contact hyperfine shift of about -20 to -40 ppm. The precise values of the contributions cannot be easily related to the geometry of the bonds. In iron-containing materials the isotropic shifts are composed of two contributions, the hyperfine shift and the pseudo-contact shift. The latter depends on the anisotropy of the magnetic susceptibility of the material. The magnetic properties of the iron-containing lithium orthosilicates are responsible also for very broad lines within their (6)Li MAS NMR spectra. Pure zinc-lithium orthosilicate exhibits a narrow (6)Li MAS NMR isotropic signal and no spinning-sideband powder pattern.