Theoretical study of electronic, magnetic, and structural properties of α-${\mathrm{Fe}}_2$${\mathrm{O}}_3$ (hematite)

Antiferromagnetic rhombohedral α-${\mathrm{Fe}}_2$${\mathrm{O}}_3$ has been studied by calculations of the ground-state spin-polarized wave function and total energy, using the ab initio periodic unrestricted Hartree-Fock approach. All-electron basis sets of contracted Gaussian-type functions are employed to represent the O and Fe atoms (18 and 27 orbitals, respectively); Fe is alternatively described by a large-core pseudopotential plus 18 valence-shell orbitals. Computations have been performed for both the antiferromagnetic (AF) and ferromagnetic (FM) structures; the correct relative stability is reproduced, with ΔE(AF-FM)=-0.0027 hartree per formula unit (including a correction for correlation energy). The dependence of ΔE(AF-FM) on variations of the Fe-O bond lengths and Fe-O-Fe’ angles involved in superexchange is analyzed, finding that for some configurations the FM structure becomes more stable than the AF one. The athermal equation of state, equilibrium crystal structure, elastic bulk modulus, and binding energy have been computed and compared to experimental quantities. An analysis of the density of electronic states show that the band gap is of p-d rather than d-d type, confirming the charge-transfer-insulator nature of hematite as inferred from photoelectron spectra. The overall shape of the valence band is also fully consistent with spectroscopic results. Mulliken electron population data indicate a charge back transfer of 0.29‖e‖ from ${\mathrm{O}}^{2\mathrm{-}}$ to the d shell of ${\mathrm{Fe}}^{3+}$, causing a partial spin pairing with a magnetic moment of 4.7${\mathrm{\mu }}_{\mathit{B}}$.