Electron-nuclear double resonance ofFe3+in guanidinium aluminum sulfate hexahydrate

Electron-nuclear double resonance (ENDOR) measurements of the $S$-state ion ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_{}^{3+}{}_{}{}^{}$ have been performed in guanidinium aluminum sulfate hexahydrate [C${\left(\mathrm{N}{\mathrm{H}}_2\right)}_3$Al${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·6${\mathrm{H}}_2$O] single crystal at liquid-helium temperature. ENDOR frequencies for the two ${\mathrm{Fe}}^{3+}$ sites in the unit cell were measured for various orientations of the external magnetic field. All the frequencies observed for each site were simultaneously fitted in a rigorous least-squares fitting procedure to evaluate the hyperfine parameters $A$, $B$, $U_{\parallel }$, $U_{\perp }$, and $g_n$ for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_{}^{3+}{}_{}{}^{}$. It is found that these parameters, as evaluated from the data for the two sites, agree within experimental errors. Further, the ratio of $\left|\frac{\mathrm{}(A-B)\mathrm{}}{A}\right|$ to $\frac{\mathrm{eqQ}}{\hslash }$ (as found by Mössbauer spectroscopy for the first excited state of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$) in C${\left(\mathrm{N}{\mathrm{H}}_2\right)}_3$Al${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·6${\mathrm{H}}_2$O is in agreement with those found for the hosts rutile (Ti${\mathrm{O}}_2$) and octaethylhemin. This supports the analysis of Schlaak [Ber. Bunsenges. Phys. Chem. 79, 1016 (1975)] predicting a linear dependence of $\left|\frac{\mathrm{}(A-B)\mathrm{}}{A}\right|$ on $Q^{\prime } (=\frac{1}{2}\frac{\mathrm{eqQ}}{\hslash })$, the coefficient of the term [$I_z^2-\frac{I(I+1\mathrm{})\mathrm{}}{3}$] in the spin Hamiltonian.