Effect of Pressure on the Quadrupole Interaction in Iron-Fluorine Compounds

The interaction between the nuclear quadrupole moment and the electric field gradient at the nucleus for Fe(III) in ${\mathrm{K}}_3$Fe${\mathrm{F}}_6$, ${\mathrm{Na}}_3$Fe${\mathrm{F}}_6$, and ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_3$Fe${\mathrm{F}}_6$ has been measured to 170-kbar pressure. The quadrupole splitting increases markedly with increasing pressure. Calculations are presented for both a point-charge and a covalent model of ${\mathrm{K}}_3$Fe${\mathrm{F}}_6$. In both cases a relatively small change in local symmetry with presure will account for the results. The amount of covalency appears to be appreciable. Similar calculations are made for Fe${\mathrm{F}}_2$ using previously published quadrupole-splitting data plus x-ray measurements of the $c$ and $a$ axes as a function of pressure. The observed decrease in quadrupole splitting can be accounted for qualitatively. The effect of covalency is negligible.