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.