When Density Functional Approximations Meet Iron Oxides

Meng, Yu [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Liu, Xing-Wu [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Huo, Chun-Fang [National Energy Center for Coal to Clean Fuels, Synfules China Co, Beijing, China] Guo, Wen-Ping [National Energy Center for Coal to Clean Fuels, Synfules China Co, Beijing, China] Cao, Dong-Bo [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Peng, Qing [Department of Mechanical, Aerospace and Nuclear Engineering, New York, United States] Dearden, Albert [Department of Physics, Berea College, Kentucky, United States] Gonze, Xavier [UCL] Yang, Yonh [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Wang, Jianguo [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Jiao, Haijun [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyuan, China] Li, Yongwang [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] Wen, Xiao-Dong [State Key Laboratory of Coal Conversion, Chinese Academy of Sciences, Taiyan, China] et al.[show all]

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd−Scuseria−Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-Fe2O3, Fe3O4, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% “screened” Hartree−Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.