Electron-Paramagnetic-Resonance Investigations of $5 f^{5}$ Configuration Ions in Cubic Single Crystals: ${Pu}^{3 +}$ in Th $O_{2}$ and Sr ${Cl}_{2}$ , and ${Am}^{4 +}$ in Th $O_{2}$

Electron-paramagnetic-resonance spectra of the isoelectronic ions

{Pu}^{3 +}

and

{Am}^{4 +}

in cubic sites of fluorite-structure single crystals have been investigated at 1.5 and 4.2 K. The observed isotropic spectra and associated

g

values identify the ground states as

Γ_{7}

doublets. This ground state is produced by intermediate-coupling effects which are much larger for

5 f^{5}

configuration ions than for the analogous

4 f^{5}

ions. Spin-Hamiltonian parameters were determined to be:

g = 1. 3124 \pm 0. 0005

A = (65. 4 \pm 0. 2) \times 10^{- 4}

{cm}^{- 1}

for

{}^{239}{Pu}^{3 +}

in Th

O_{2}

;

g = 1. 1208 \pm 0. 0005

A = (127. 9 \pm 0. 4) \times 10^{- 4}

{cm}^{- 1}

for

{}^{239}{Pu}^{3 +}

in Sr

{Cl}_{2}

; and

g = 1. 2862 \pm 0. 0005

A = (45. 7 \pm 0. 1) \times 10^{- 4}

{cm}^{- 1}

for

{}^{241}{Am}^{4 +}

and

A = (45. 3 \pm 0. 1) \times 10^{- 4}

{cm}^{- 1}

for

{}^{243}{Am}^{4 +}

in Th

O_{2}

. Spectra observed for both

{Pu}^{3 +}

and

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Electron-Paramagnetic-Resonance Investigations of 5f5 Configuration Ions in Cubic Single Crystals: Pu3+ in ThO2 and SrCl2, and Am4+ in ThO2