A complete ab initio thermodynamic and kinetic catalogue of the defect chemistry of hematite α-Fe2O3, its cation diffusion, and sample donor dopants

This paper studies comprehensively the defect chemistry of and cation diffusion in alpha-Fe2O3. Defect formation energies and migration barriers are calculated using density functional theory with a theoretically calibrated Hubbard U correction. The established model shows a good agreement with experimental off-stoichiometry and cation diffusivities available in the literature. At any temperature, and are the predominant ionic defects in hematite at the two extremes of oxygen partial pressure (pO(2)) range, reducing and oxidizing, respectively. Between these two extremes, an intrinsic electronic regime exists where small polaronic electrons and holes are the dominant charge carriers. The calculated migration barriers show that Fe ions favor the diffusion along the 111 direction in the primitive cell through an interstitial crowdion-like mechanism. Our model suggests that cation diffusion in hematite is mainly controlled by the migration of , while may contribute to cation diffusion at extremely low pO(2). Our analysis in the presence of two sample donor dopants Ti and Sn indicates that high temperature annealing at T > 1100 K is needed to prepare n-type hematite at ambient pO(2), consistently with prior experimental findings. Alternatively, annealing at lower temperatures requires much lower pO(2) to avoid compensating the donors with Fe vacancies. A synergistic comparison of our theoretical model and the experimental results on Ti-doped hematite led us to propose that free electrons and small polarons coexist and both contribute to n-type conductivity. Our validated model of defective hematite is a foundation to study hematite in applications such as corrosion and water splitting.

Automated summary

This paper comprehensively studies the defect chemistry and cation diffusion in alpha-Fe2O3, with a model showing good agreement with experimental results. Fe ions prefer diffusion along the 111 direction through a specific mechanism, indicating that cation diffusion in hematite is mainly controlled by migration.