Surface Structures of CaFe₂O₄ (001), (100), (110) and (111): A Density Functional Theory Study

Abstract

Calcium ferrite is the main binder phase in sinter, which affects the physical and metallurgical properties of sinter. It was also widely applied to photocatalysts, oxidation catalysts, photocathodes and gas sensors etc. The electronic, magnetic and chemical properties of it were investigated since its crystal structure was first reported. However, the right terminations, structures and relative stabilities of CaFe₂O₄ surface have not been systemically studied. Hence, the surface structures of CaFe₂O₄ (001), (100), (110) and (111) were calculated by using a generalized gradient approximation considering on-site Coulomb interaction of iron 3d electrons (GGA + U) in the paper. With U = 4.70 eV, the band gap for up-spin and down-spin energy of CaFe₂O₄ was calculated to be 1.91 and 1.81 eV, closed to the experimental value (1.90 eV). For the CaFe₂O₄ (001) surface, the O1 terminations were the most stable and the surface energy was 1.307 J·m⁻². In the case of CaFe₂O₄ (100) surface, the surface energies at Ca and O1 terminations were 1.278 and 1.568 J·m⁻², respectively. There were also two most stable CaFe₂O₄ (110) surfaces in close surface energies and terminated with the exposed Fe2 and O3 atoms. The surface energies of them were 1.489 and 1.570 J·m⁻², respectively. Among the fifteen CaFe₂O₄ (111) terminations, the surface energies at O2 (l) and O4 (f) terminations were the lowest and they were 1.421 and 1.455 J·m⁻². The calculated surface energies indicate that (100) was better than (001), (110) and (111) in thermodynamic, which agrees well with the experimental results.