Crystal Structure and Li–Fe Order in Synthetic Mg_{(2–2<i>x</i>)}Li_<i>x</i>Fe³⁺_<i>x</i>(SiO₄) Olivine Structure

Olivines are naturally occurring silicates consisting of isolated (SiO₄)^4- tetrahedra linked through <i>M</i>1O₆ and <i>M</i>2O₆ octahedra. In this study, we report the structural and crystal-chemical characterization of synthetic olivine crystals containing up to 25% Li-Fe³⁺ synthesized using the flux growth technique. Based on site scattering, <<i>M</i>1-O> and <<i>M</i>2-O> mean bond lengths, and charge neutrality of the chemical formula, we found a perfect ordering of Li and Fe³⁺ at the two distinct <i>M</i>1 and <i>M</i>2 sites. Unrestrained linear extrapolation to a hypothetical isostructural LiFe³⁺(SiO₄) composition aligns well with the tabulated ionic radii of Li and Fe³⁺. Comparison made with the isostructural LiSc(SiO₄) reveals that the Li-centered <i>M</i>2O₆ octahedron has a significant capacity to distort in order to accommodate structural stresses, due to the relatively weak Li-O bond, while still achieving a bond valence sum that closely matches the formal charge of Li⁺. This behavior suggests the potential feasibility of an extended Li + Fe³⁺ for 2 Mg coupled substitution within the olivine structure. The reported structure of the LiFe³⁺(SiO₄) endmember in the literature, despite its apparent matching of cell dimensions and space group with olivine, exhibits extremely unconventional crystal chemical features, raising questions about its validity. Given the importance of the suitability of Li-insertion in LiFeSiO₄ as electrodes in rechargeable Li-ion batteries, further studies are needed to investigate its crystal structure and crystal chemistry.