Understanding 6Li MAS NMR spectra of Li2MSiO4 materials (M=Mn, Fe, Zn)

Analysis of Li-6 MAS NMR spectra of several lithium transition-metal orthosilicates (Li2MSiO4, 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 Li-6 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 Li-6 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 Li-6 MAS NMR spectra. Pure zinc-lithium orthosilicate exhibits a narrow Li-6 MAS NMR isotropic signal and no spinning-sideband powder pattern. (C) 2011 Elsevier Inc. All rights reserved.