Structural evolution of Li/Fe³⁺-doped forsterite (Mg2−2xLixFe³⁺xSiO₄) and the crystal-chemical viability of the LiFeSiO₄ olivine

The structural evolution of forsterite (Mg₂SiO₄) upon heterovalent coupled substitution with Li⁺ and Fe³⁺, forming the solid solution Mg (2-2x) Li x Fe³⁺ x SiO₄, has been investigated up to ≈ 0.30. Single crystals were synthesized via high-temperature flux growth and characterized by single-crystal X-ray diffraction. Crystal structure refinements confirm a 2Mg²⁺ = Li⁺ + Fe³⁺ substitution, where Li⁺ occupies the smaller, less distorted M 1 octahedron, while Fe³⁺ occupies the larger M 2 octahedron, in agreement with charge-balance and bond-valence constraints. This substitution induces a linear contraction of the unit-cell volume, driven primarily by a decrease in the c -parameter, while concurrently causing a systematic expansion of the M 1 octahedron and a contraction of the M 2 octahedron. All measured structural parameters— such as mean bond distances, polyhedral volumes, and distortion indices—vary linearly with composition, indicating a continuous virtual solid solution up to x = 1.00. Extrapolation of the trends and subsequent Distance Least-Squares modelling predict a structurally viable atomic arrangement for the hypothetical LiFe³⁺SiO₄ endmember, which is crystal-chemically feasible as well as comparable to the known LiScSiO₄ olivine-type structure. These findings address conflicting literature reports by demonstrating that the olivine structure can accommodate the complete Li⁺ + Fe³⁺ ↔ 2Mg²⁺ substitution, thereby supporting the potential of LiFeSiO₄-type compounds as candidate cathode materials for lithium-ion batteries.