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Energy Levels of Uranium Ions in Calcium Fluoride Crystals

1970; American Physical Society; Volume: 2; Issue: 7 Linguagem: Inglês

10.1103/physrevb.2.2273

0556-2805

William A. Hargreaves,

Luminescence Properties of Advanced Materials

The crystal-field-split levels of divalent, trivalent, and tetravalent uranium ions have been studied at 20 \ifmmode^\circ\else\textdegree\fi{}K in selected crystals having single charge-compensation sites. Complete calculations have been made for the free-ion energy levels of ${\mathrm{U}}^{4+}$, with the pertinent constants determined as ${F}_{0}=13175$, ${F}_{2}=200$, ${F}_{4}=40$, ${F}_{6}=8.05$, ${\ensuremath{\zeta}}_{5f}=1660$, and $\ensuremath{\alpha}=26$. Also calculated crystal-field-split levels of ${\mathrm{U}}^{4+}$ in ${C}_{3v}$ symmetry are compared to experimental values, and the various "best fit" values of the crystal-field constants determined. ESR results on trigonal Ca${\mathrm{F}}_{2}$: ${\mathrm{U}}^{4+}$ are correlated with predicted values derived from optical measurements. Experimental studies of the ${\mathrm{U}}^{2+}$ ion in calcium fluoride are related to the probable ${\mathrm{U}}^{2+}$ configuration, ${f}^{3}s$. Crystal-field splittings of the lower levels of Ca${\mathrm{F}}_{2}$: ${\mathrm{U}}^{2+}$ are determined from measurements of absorption and fluorescence at 20\ifmmode^\circ\else\textdegree\fi{}K. The calculated value of ${\ensuremath{\zeta}}_{5f}$ for ${\mathrm{U}}^{2+}$, based on approximate $L\ensuremath{-}S$ coupling conditions for the lowest levels, is 1560 ${\mathrm{cm}}^{\ensuremath{-}1}$, and is compared with a similarly calculated value for ${\mathrm{U}}^{3+}$ of 1520 ${\mathrm{cm}}^{\ensuremath{-}1}$. Experimental crystal-field-split levels of tetragonal Ca${\mathrm{F}}_{2}$: ${\mathrm{U}}^{3+}$ are shown to fit symmetry requirements, and various laser wavelengths of Ca${\mathrm{F}}_{2}$: ${\mathrm{U}}^{3+}$ are related to the level system derived from absorption and fluorescence measurements. Temperature shifts of crystal-field-split levels of uranium ions are also reported.