Formation and cation distribution in supported manganese ferrite nanoparticles: an X-ray absorption study

Abstract

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe₂O₄nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900 °C). Up to 450 °C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900 °C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe₂O₄ (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900 °C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn²⁺ to Mn³⁺ in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.