Quadrupole Coupling Constant,eqQh, ofFe3+in Several Rare-Earth Iron Garnets

Using the Mössbauer effect, values of $\frac{\mathrm{eqQ}}{h}$ of ${\mathrm{Fe}}^{3+}$ have been obtained at each of two sites in several rare-earth iron garnets. The measured values of $\frac{\mathrm{eqQ}}{h}$ were compared with values obtained using a point-charge lattice sum calculation. For the octahedral ($a$) site there was reasonable agreement between the measured and calculated values of $\frac{\mathrm{eqQ}}{h}$ indicating that an ionic model is reasonably valid. However, the measured values of $\frac{\mathrm{eqQ}}{h}$ at tetrahedral ($d$) sites were larger than those estimated. This difference is attributed to a small amount of covalent bonding which is also evident in the values of the ($d$) site internal field and isomer shift.