Electron-paramagnetic-resonance investigations ofCm3+243andCm3+244in LuPO4single crystals

Abstract

The actinide isotopes ${}_{}{}^{243}{}_{}{}^{}\mathrm{Cm}$ and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Cm}$ have been incorporated as dilute trivalent impurities in single crystals of lutetium orthophosphate. Electron-paramagnetic-resonance (EPR) spectra of both isotopes were observed at ∼9 GHz and $T=4.2\mathrm{}$ K, and a positive identification of the ${\mathrm{Cm}}^{3+}$ resonance was made from the six-line hyperfine pattern characteristic of the $I=\frac{5}{2}$ nuclear spin of ${}_{}{}^{243}{}_{}{}^{}\mathrm{Cm}$. The host LuP${\mathrm{O}}_4$ crystal is a tetragonal-symmetry system and the axial ${\mathrm{Cm}}^{3+}$ $g$ values were determined to be $g_{\parallel }=7.98\mathrm{}\left(1\right)\mathrm{}$ and $g_{\perp }=4.096\mathrm{}\left(4\right)\mathrm{}$. These values could not be fitted to a doublet wave function of the form $\alpha |\pm \frac{7}{2}〉+\beta |\mp \frac{1}{2}〉$ to give a consistent value for the Landé $g$ factor. They could, however, be fitted to an excited-state doublet wave function of the form $\alpha |\pm \frac{5}{2}〉+\beta |\mp \frac{3}{2}〉$ to yield a consistent Landé factor of 1.921. The identification of the observed spectra as arising from an excited state was confirmed by the observation that lowering the temperature reduced the intensity of the EPR signals. These results show that the sign of the dominant axial crystal-field term for the $5f^7$ ${\mathrm{Cm}}^{3+}$ ion in LuP${\mathrm{O}}_4$ is negative and that a predominantly $|\pm \frac{7}{2}〉$ state is the ground state.