High temperature X-ray powder diffraction as a tool for monitoring of thermally induced transformation of β-Fe₂O₃ in various atmospheres

J. Kašlík, O. Malina, I. Medřík, J. Filip, R. Zbořil

Regional Centre of Advanced technologies and Materials, Departments of Experimental Physics and Physical Chemistry, Faculty of Science, Palacký University Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic
josef.kaslik@upol.cz

Iron oxide nanomaterials became one of the most studied materials up to date due to their significantly different properties comparing to their bulk counterparts. Generally, four crystalline form of iron(III) oxide exist exhibiting different crystallographic and magnetic properties. We present thermally induced solid state transformations of one of the rare iron(III) oxide polymorph, i.e., β-Fe₂O₃, by view of high temperature X-ray powder diffraction. All transformation experiments were performed in very similar conditions (i.e., temperature increment, gas pressure) with only difference in exchange of reaction gases. Gases were chosen to represent oxidative, inert, and reductive atmospheres (i.e., synthetic air, CO₂, N₂, H₂). Transformation in the oxidative atmosphere of synthetic air led directly to creation of the most stable iron(III) oxide polymorph, i.e., hematite, in temperature range 680 – 760 °C. Transformation performed in carbon dioxide atmosphere led to creation of magnetite via hematite at temperature range 475 – 700 °C. Transformation scheme of experiment performed in nitrogen atmosphere, which is considered as inert, was more complicated and transformation via two intermediates (i.e., hematite and magnetite) led to final product identified as wustite, which was created between 800 and 900 °C. In these experiments, all intermediates and final products were investigated at room temperature by view of X-ray powder diffraction and Mössbauer spectroscopy to confirm the phase composition and iron ions state. Reductive atmosphere of hydrogen led to creation of metallic iron as expected due to the nature of reducing gas.

The authors gratefully acknowledge the financial support by Internal IGA grant of Palacký University Olomouc, Czech Republic (IGA_PrF_2015_017).