Electric Field Gradients at 57Fe in ZnFe2O4 and CdFe2O4

The nuclear quadrupole coupling constants and isomer shifts of ⁵⁷Fe in the spinels ZnFe₂O₄ and CdFe₂O₄ were measured using the Mössbauer effect. The signs of the quadrupole coupling constants were determined from spectra which were taken in an applied magnetic field. The sign is negative in both spinels. The isomer shifts and Fe–O distances indicate that Fe³⁺ in ZnFe₂O₄ is somewhat more covalently bonded than in CdFe₂O₄. The external field gradients at the Fe³⁺ positions can be interpreted in terms of the ionic point‐multipole model modified by some charge transfer between oxygen and the cations. The point charge contribution to the field gradient is positive in case of ZnFe₂O₄ and nearly zero in case of CdFe₂O₄; the predominant contribution is due to the electric dipole moments of the oxygen ions and is negative. The dipole polarizability of the oxygen ion which gave the best fit is ${\alpha }_D{\text{= 0.8\hspace{0.17em}Å}}^3$ . The effect of charge transfer on the ionic field gradient is small.