Absorption Spectrum ofU4+in Zircon (ZrSiO4)

The electronic energy levels of ${\mathrm{U}}^{4+}$ in ZrSi${\mathrm{O}}_4$ have been obtained from the absorption spectra of oriented single crystals. This host was chosen because of the relatively high ($D_{2d}$) symmetry of the ${\mathrm{Zr}}^{4+}$ site occupied by the ${\mathrm{U}}^{4+}$ ion. Spectra were obtained at a number of temperatures between 4 and 300°K. By means of polarization selection rules, electronic energy levels were identified according to their irreducible representations; then the Slater integrals $F_k$, the spin-orbit parameter ${\zeta }_{5f}$, and the crystal-field parameters were determined by a least-squares fit. Because of the large crystal field, it was necessary to diagonalize all these interactions within the $5f^2$ configuration. In all, 30 levels up to 24 000 ${\mathrm{cm}}^{-1\mathrm{}}$ have been fitted to an average deviation of 110 ${\mathrm{cm}}^{-1\mathrm{}}$ by adjusting nine parameters. The results in ${\mathrm{cm}}^{-1\mathrm{}}$, are $F_2=185.3\mathrm{}$, $F_4=37.1\mathrm{}$, $F_6=3.67\mathrm{}$, ${\zeta }_{5f}=1780\mathrm{}$, $A_2^{}{}_{}{}^0〈r^2〉=-349\mathrm{}$, $A_4^{}{}_{}{}^0〈r^4〉=400\mathrm{}$, $A_4^{}{}_{}{}^4〈r^4〉=770\mathrm{}$, $A_6^{}{}_{}{}^0〈r_6〉=-435\mathrm{}$, and $A_6^{}{}_{}{}^4〈r_6〉=5.7\mathrm{}$. The first four "free-ion" parameters are in good agreement with those determined for U$\mathrm{Cl}_6^{}{}_{}{}^{=}$ complexes. The ratios of the crystal-field parameters show large deviations from those calculated on a point-charge model, presumably because of the contribution of the polarization of the ligands.