⁵⁷Fe Mössbauer and Mo K-EXAFS investigations of Mo_xFe_3−xO₄, an interesting mixed-valent oxide system

Abstract

Ferrites of the formula Mo_xFe_3−xO₄, prepared by a soft-chemistry route, show mixed valence states of both iron and molybdenum cations. Mössbauer studies show that Fe²⁺ and Fe³⁺ ions are present on both the A and B sites, giving Fe an average oxidation state between 2+ and 3+. Molybdenum is present in the 3+ and the 4+ states on the B sites. The presence of Mo in the 3+ state has been established by determining the Mo³⁺-O distance (2.2 Å), for the first time, by Mo K-EXAFS. The mixed valence of Fe on both the A and B sites and of Mo on the B sites is responsible for the fast electron transfer between the cations. All the Mössbauer parameters including the line width show a marked change at a composition (x≈0.3) above which the concentration of Fe²⁺_A increases rapidly.