Computer Modeling Study of the Lithium Ion Distribution in Quaternary Li-Mn-Fe-O Spinels

Computational and experimental techniques have been used to investigate the structural properties, especially the cation distribution, of spinel solid solutions formed between LiMn2O4 and LiFe5O8. The series of solid solutions of composition (1-x) LiMn2O4-xLi0.5Fe2.5O4 are single-phase products with the cubic spinel structure, with the lattice parameter increasing from 8.21 to 8.33 A as x varies from 0 and 1. The variation of the unit cell parameter with x shows a distinct departure from Vegardʹs law, which is most pronounced for samples with the Fe:Mn molar ratio close to 1:1. With higher Fe content, the ordering of lithium ions in the octahedral spinel sites leads to the reduction of the crystal symmetry and to a strongly marked preference of Li+ ions to occupy the octahedral positions. Our lattice energy minimization results make clear predictions as to the Li+ ion content and distribution in these materials and thereby resolve confusions in the experimental data.