Structural, electronic, magnetic and hyperfine properties of Fe2AlO4 and FeAl2O4: a DFT study

Abstract

In this work, the structural, electronic, magnetic, and hyperfine properties of two Fe-Al spinel oxides, namely FeAl2O4 (hercynite) and Fe2AlO4 (Al-ferrite) were studied by means of Density Functional Theory (DFT)-based first principles calculations. To determine the structural and magnetic equilibrium structures of both oxides, different cationic inversion degrees, magnetic configurations and distributions were considered for Fe and Al ions in the octahedral and tetrahedral sites of the spinel structures. Calculations confirmed the preference of the Al ions to occupy the octahedral cationic sites and predicted that both Fe-Al spinel oxides present a semiconductor nature. They also enabled the determination that the lowest energy structure of FeAl2O4 corresponds to an antiferromagnetic normal spinel, in which Fe2+ ions populate the tetrahedral sites and Al+3 ions occupy the octahedral B sites. Partial inversion cases are also discussed for this system. The lowest energy structure of Fe2AlO4 corresponds to a system with a net magnetic moment in which eight Fe+2 ions populate the A sites and eight Fe3+ and eight Al3+ ions populate the B sites. It was also shown that FeAl2O4 presents a lower formation energy than Fe2AlO4. By comparing the predictions for the hyperfine parameters at the Fe sites with the experimental results obtained in the Mössbauer experiments, the validity of the proposed structural and magnetic structure of FeAl2O4 was confirmed. Finally, a discussion is made to compare the results of this study with the Mössbauer results reported in the literature for Fe2AlO4.