Atomistic simulation and ab initio study of the defect structure of spinel-related Li0.5−0.5xMgxFe2.5−0.5xO4

The position of magnesium ions in Mg2+-doped lithium ferrite of the composition Li0.5−0.5xMgxFe2.5−0.5xO4, 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 Mg2+ ions evenly replacing Li+ and Fe3+ ion on octahedral sites. This gives a decrease in magnetisation for the Mg2+-doped ferrite relative to the un-doped lithium ferrite. The results suggest that some experimental observations of increased magnetisation of spinel lithium ferrite on Mg2+-doping could be due to substitution of Mg2+ 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.

Graphical abstractUnit cell of Li05−0.5xMgxFe2.5−0.5xO4, showing the lowest energy structure obtained using interatomic potential and DFT ab initio calculations. Large white spheres O2−; small light grey spheres Mg2+ (evenly substituting of Li+ and Fe3+ at octahedral sites); small dark grey spheres Fe3+; small black spheres Li+.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Defect structure of Li0.5−0.5xMgxFe2.5−xO4 is studied with atomistic and DFT methods. ► 19 possible defect structure models with ∼60 defect configurations are investigated. ► The most favourable model found is when Mg2+ ions evenly replace Li+ and octahedral Fe3+. ► This defect structure decreases the magnetisation relative to that of Li0.5Fe2.5O4. ► Experimentally-deduced models, at variance with the one obtained here, are discussed.