Atomistic simulation and ab initio study of the defect structure of spinel-related Li _0.5-0.5xMg _xFe _2.5-0.5xO ₄

The position of magnesium ions in Mg ²⁺-doped lithium ferrite of the composition Li _0.5-0.5xMg _xFe _2.5-0.5xO ₄, which has been a matter of uncertainty among some experimentalists, is investigated using interatomic potential and ab initio DFT calculations. Among possible 19 defect structure models, some of which have been reported experimentally to be the most favorable, the lowest energy is found for Mg ²⁺ ions evenly replacing Li ⁺ and Fe ³⁺ ion on octahedral sites. This gives a decrease in magnetisation for the Mg ²⁺-doped ferrite relative to the un-doped lithium ferrite. The results suggest that some experimental observations of increased magnetisation of spinel lithium ferrite on Mg ²⁺-doping could be due to substitution of Mg ²⁺ or Li ⁺ on tetrahedral sites at the high temperatures used in preparation of the solid and/or the presence of undetected defects in the initial precursors.