Critical Assessment of the Thermodynamics of Vacancy Formation in Fe<sub>2</sub>O<sub>3</sub> Using Hybrid Density Functional Theory
Amitava Banerjee, Aaron A. Kohnert, Edward F. Holby, Blas P. Uberuaga
Abstract
Fe2O3 hematite is a technologically important material with applications in energy storage as well as being the key phase formed in the rust of iron-based materials. Despite this central importance, there is much that is still unknown regarding the properties of defects in Fe2O3 and consequently oxide growth. Here, using screened hybrid density functional theory (HSE06), we consider the thermodynamics of vacancies in Fe2O3, considering the effects of ionic and electronic chemical potentials on both iron and oxygen vacancy formation. We find that, in the oxygen-rich limit, iron vacancies are easier to form, though the difference in formation energy between the two vacancies is only about 1 eV. In contrast, in the Fe-rich limit, oxygen vacancies have an extremely low formation energy, only 0.07 eV at mid-gap, and would spontaneously form as the Fermi level is reduced, while Fe vacancies require over 5 eV to form. Consistent with experiment, this indicates that Fe2O3 is relatively easily reduced but not oxidized. However, the theoretical picture is very different when considering other exchange–correlation functionals (GGA + U or SCAN), emphasizing the critical role of the exchange–correlation functional in describing this system accurately.