Electric Field Gradient Calculations in Transition-Metal Sesquioxides

We present computations of the electric field gradient at the cation sites in the Cr, Fe, Ti, and V sesquioxides isomorphous to corundum. We include dipolar as well as monopolar contributions. Except for α-Fe₂O₃ we find that the dipolar and the monopolar field-gradient contributions are opposite in sign and roughly of the same order of magnitude. In so far as existing data permit, we have used these tabulations to determine the electric nuclear quadrupole moment (Q) of the odd isotopes. The Q values found here seem to be somewhat larger than the results of others. In the cases of Cr⁵³ and V⁵¹ existing data are such that only $|Q|$ can be determined. Our calculation for Al²⁷ has been given previously. In α-Fe₂O₃ the dipolar field-gradient term has the same sense as the monopolar, and is comparatively small. We find Q(Fe⁵⁷ᵐ) to be 0.41 b. A recent α-Fe₂O₃ field-gradient calculation which yielded a Q(Fe⁵⁷ᵐ) value of 0.277 b is believed to be in error.