Li2FeSiO4 Polymorphs Probed by 6Li MAS NMR and 57Fe Mossbauer Spectroscopy

Several samples of pure Li2FeSiO4 polymorphs, prepared by hydrothermal synthesis and then quenched from either 400 degrees C (Pmn2(1)), 700 degrees C (P2(1)/n), or 900 degrees C (Pmnb), were thoroughly investigated by Mossbauer and NMR spectroscopy. Fe-57 Mossbauer spectroscopy showed that (i) iron-in all the polymorphs existed in the divalent oxidation state, and (ii) chemical shifts and quadrupolar splittings correlated well with Fe-O bond lengths. Li-6 MAS NMR spectra of the three different polymorphs exhibit substantially different spinning sideband patterns and slightly different isotropic shifts. The sideband patterns stemming from the anisotropic electron nucleus dipole dipole interaction reflect the arrangement of paramagnetic iron ions around lithium nuclei and allowed unambiguous assignments of Li-6 MAS NMR signals to crystallographically nonequivalent Li sites. Analysis of isotropic NMR shifts of Li-6 nuclei revealed that they comprised two contributions of comparable magnitude, a hyperfine (contact) shift and a pseudo-contact shift. Based on the structural models of Li2FeSiO4, both contributions could be successfully predicted by first-principles calculations. The contact shift was obtained from the hyperfine-coupling constant on Li-6 nuclei, which was calculated within a DFT/PAW approach, and the pseudo-contact shift was derived from the electron nucleus dipolar matrix and the g-tensor, which was computed within the DFT/GIPAW frame.