Stabilization of hercynite structure at elevated temperatures by Mg substitution

Hercynite, FeAl2O4, is a spinel applied in various fields, including refractory materials or H2 production. FeAl2O4 is unstable at elevated temperatures of 1000°C in air, decomposing into different forms of alumina and iron oxides. In this work, we show that it is possible to inhibit this decomposition by the substitution of Fex+ in tetrahedral sites of hercynite by Mg2+. Spinel series, Fe1-xMgxAl2O4 (x=0,0.3,0.5,0.7), were prepared using arc plasma melting technique to obtain high-purity and dense materials. The composition Fe0.7Mg0.3Al2O4 was subjected to further in-depth high-temperature in situ investigation of stability using High-Temperature X-Ray Diffractometry up to 1200°C in air. Also, isothermal oxidation was conducted in the same conditions on both powdered and bulk samples, followed by subjecting the oxidized samples to structural analysis by X-Ray Diffractometry, Mössbauer Spectroscopy and X-Ray Absorption Spectroscopy using synchrotron radiation. The microstructure of oxidized bulk samples was analyzed by SEM/EDS. Mg substitution in hercynite was found to sluggish the spinel oxidation and decomposition. Instead, magnesioferrite inverse spinel forms, stabilizing the structure up to 1200°C in air.