The electric-field gradient at the ${\mathrm{Fe}}^{3+}$ site in hematite ($\alpha$-${\mathrm{Fe}}_2$${\mathrm{O}}_3$) has been recomputed by use of the ionic model, including contributions of electronic dipole and quadrupole moments of the oxygen ion, as well as apparent quadrupolar effects due to thermal vibration of oxygen. The calculations have been performed self-consistently for various parameter sets of dipole and quadrupole polarizabilities, ${\alpha }_D$ and ${\alpha }_Q$. As in corundum ($\alpha$-${\mathrm{Al}}_2$${\mathrm{O}}_3$), the field gradient is insensitive to ${\alpha }_D$ because of the (accidentally) nearly vanishing field at the oxygen position. ${\alpha }_Q$, however, is a sensitive parameter. The vibrational contribution is small, being ∼5% of the total field gradient. Assuming a dipole polarizability ${\alpha }_D\left({\mathrm{O}}^{2-}\right)=2.91\mathrm{}$ ${\mathrm{\AA }}^3$, we find a range of values for the nuclear quadrupole moment of the 14.4-keV excited state of ${}_{}{}^{57m}{}_{}{}^{}\mathrm{Fe}$, depending on ${\alpha }_Q\left({\mathrm{O}}^{2-}\right)$, which is not well known. For ${\alpha }_Q\left({\mathrm{O}}^{2-}\right)=0\mathrm{}$, $Q\left({}_{}{}^{57m}{}_{}{}^{}\mathrm{Fe}\right)=0.29\mathrm{}$ b. For ${\alpha }_Q=0.3\mathrm{}$, $Q=0.16\mathrm{}$ b.

• Received 20 December 1968

DOI:https://doi.org/10.1103/PhysRevB.1.979