Characterization of the Mn–Li ferrite system Li1−0.5xFe1.5x+1Mn1−xO4 (0.2 ≤ x ≤ 1)

Abstract

The Mn–Li ferrite system Li_1−0.5xFe_1.5x+1Mn_1−xO₄ (x = 1, 0.8, 0.6, 0.4, 0.2) has been studied by means of X-ray diffraction, Mössbauer spectroscopy and Mn K- and Fe K-edge XANES/EXAFS. All the samples show the spinel-related structure with the lattice constant a gradually decreasing between a = 0.8310 nm (x = 1) and a = 0.8301 nm (x = 0.2). Mössbauer and Fe K-edge XANES/EXAFS results have shown that the oxidation state of Fe is +3 and that the fractions of Fe³⁺ ions occupying the octahedral and tetrahedral sublattices do not change significantly along the series. Mössbauer data recorded under applied magnetic field indicate the existence in the Mn-containing samples of canted spin structures for the Fe³⁺ ions on both the tetrahedral and octahedral sites, the canting angle being larger for the octahedral sublattice. Mn K-edge XANES and EXAFS have shown Mn to be present as Mn³⁺ and Mn⁴⁺ and to occupy only octahedral sites. The XANES/EXAFS results have also shown that the Mn⁴⁺/Mn³⁺ ratio increases with increasing Li/Mn content. The results indicate that as the extent of Li/Mn insertion increases the new inserted Li⁺ ions occupy tetrahedral sites and that some of the Li⁺ ions initially occupying octahedral sites are also driven into tetrahedral positions.