The reaserch team from AGH University in Kraków used beamline PIRX to study the sructural properties of spinel compounds, including hercynite, FeAl₂O₄, which belongs to wide & mixabe oxide spinel group with dense packing and cubic AB₂ O₄ structure. This work was conducted in a team led by PhD Eng. Ilona Jastrzębska from the Faculty of Materials Science and Ceramics.

FeAl₂O₄ is applied in various fields, including refractory materials or H₂ production. This is thanks to its high melting point of 1800^oC and low thermal expansion coefficient of 0.9% (up to 1000^oC) as well as the activity of ions in hercynite in air atmosphere and at elevated temperatures. This activity lowers brittleness, thus, improving flexibility of refractory oxide materials.

In the refractory materials field, hercynite is used as Cr-free alternative compound substituting chromite spinel (Mg,Fe)(Cr,Al,Fe)₂O₄ in the production of materials for thermal linings of industrial furnaces. In H₂ production hercynite is applied for the thermal water splitting process, in which FeAl₂0₄ was found even 12 times more effective than cerium oxide. Despite the more common and significant applications of hercynite, its properties have been still poorly recognized at elevated temperatures. Precise recognition of the structural stability of metal-containing oxide spinels is essential to designing and applying new high-technology materials, environmentally friendly refractories or catalytic processes. In our previous research on the stability of pure hercynite (DOI: 10.1127/ejm/2017/0029-2579) - for the first time - we revealed the decomposition mechanism of this spinel at elevated temperature. In the current work, we develop the previous research to show that hercynite structure is sensitive to substitutions by different ions and it is possible to stabilize it. 

Here, we synthesized pure and dense hercynite using the innovative technique of Arc Plasma Melting in our special-design and user-friendly furnace (SpekoArc300). Also, we obtained numerous solid solutions – hercynites, by substituting Fe with other metallic ions, including Mg and Mn in different amounts. We further subjected hercynites to in-depth high-temperature in-situ investigation of stability using High-Temperature X-Ray Diffractometry up to 1200^oC in air. Also, we conducted isothermal oxidation of hercynites in air at 1200^oC. After that, we investigated changes which occurred in the structure of spinels by using XRD, Mössbauer Spectroscopy, X-Ray Absorption Spectroscopy (XAS), DTA/TG and SEM-EDS. In this work, using HTXRD we discovered that even a small substitution of Fe by Mg (0.3 mole) improves the stability of spinel, thus, inhibiting hercynite decomposition linked with the release of Fe₂O₃ which is not beneficial as it forms low melting phases after reaction with components of refractory material, e.g. common impurity CaO. Instead, after longer heating in air magnesioferrite forms (XRD), arresting Fe in spinel structure. The application of X-ray Absorption Spectroscopy allowed us to prove the local cation environment using a beamline PEEM/XAS and confirm the stability of hercynite structure after oxidation at 1200^oC. XAS spectrum of oxidized Fe_0.7Mg_0.3Al₂O₃ was confirmed to cover with spectrum of original run product FeAl₂O₄.

Picture 1. Arc plasma in argon flow (Ar) generated in an arc furnace SpekoArc300

Picture 2.  I. Jastrzębska 

Picture 3. From the left: E. Partyka-Jankowska, E. Knapik, I. Jastrzębska

This research was supported by the LIDER grant (LIDER/14/0086/L-12/20/NCBR/2021), funded by the National Center for Research and Development.

Author: Ilona Jastrzębska

Link to the publication: I. Jastrzębska, J. Stępień, J. Żukrowski, Stabilization of hercynite structure at elevated temperatures by Mg substitution, Materials & Design, 235, 112449(2023) doi: 10.1016/j.matdes.2023.112449