Electronic Structure in the Actinides–Three Cases Studies [Thesis]

The zero-field splittings (ZFS) of Cm³⁺ incorporated into single crystals of LuP0₄ have been determined by electron paramagnetic resonance (EPR) spectroscopy to be 0.0, 2.4, 8.0 and 9.3 cm^–1 at 298 K; they show a fair agreement with those measured by excitation and fluorescence experiments. The total splitting of = 10 cm'1 is significantly smaller than the only previously known ZFS of the half-filled 5f shell of Cm³⁺ (20.2 cm^–1 in Cm³⁺:SrCl₂). This unexpectedly small crystal-field interaction is probably responsible for the first room-temperature observation of an EPR signal of an actinide. The resulting spin-Hamiltonian parameters cannot be used to predict accurately the ground electronic states of the neighboring trivalent Kramers ions Pu³⁺ and Cf³⁺ diluted into LuP0₄. The relatively low excited 6d configuration in the Th³⁺ free ion is found to be stabilized relative to the 5f by trigonal crystal-fields with no axial ligands to the extent that it becomes the ground configuration in tris(bis-trimethylsilylcyclopentadienyl) thorium (III) (Cp$^{''}_\text{3}$Th) and trisindenyl thorium (III). The 5f crystal-field states in Cp$^{''}_\text{3}$ Th are found to be over 15,000 cm^–1 above ground. This represents the first instance where the ground electronic configuration in an actinide is different than 5f". Theoretical consideration has reached the same conclusion. A similar crystal-field around Ce³⁺ was shown to result in a 5d excited state that is the lowest in energy ever observed in trivalent cerium.