Zero-Field Splittings of Am2+ and Cm3+ in Cubic Symmetry Sites in CaF2

The g-value anisotropies of the ground crystal-field states of the nominally S7/28 ions, Am2+ and Cm3+, have been measured in cubic symmetry in CaF2 at 35 Gc/sec and magnetic fields of approximately 5500 G by EPR spectroscopy. From the magnitude of these anisotropies the Γ6 – Γ8 zero-field splittings of 13.4 ± 0.5 and 18.6 ± 0.5 cm−1 have been deduced for Cm3+ and Am2+, respectively. These splittings are about 250 times greater than for the corresponding lanthanides, Gd3+ and Eu2+. It is shown that the large intermediate coupling effects in the actinide ions can account qualitatively for these differences.