Electron-Paramagnetic-Resonance Investigations of $5f^5$ Configuration Ions in Cubic Single Crystals: ${\mathrm{Pu}}^{3+}$ in Th${\mathrm{O}}_2$ and Sr${\mathrm{Cl}}_2$, and ${\mathrm{Am}}^{4+}$ in Th${\mathrm{O}}_2$

Electron-paramagnetic-resonance spectra of the isoelectronic ions ${\mathrm{Pu}}^{3+}$ and ${\mathrm{Am}}^{4+}$ 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 ${\Gamma }_7$ doublets. This ground state is produced by intermediate-coupling effects which are much larger for $5f^5$ configuration ions than for the analogous $4f^5$ ions. Spin-Hamiltonian parameters were determined to be: $g=1.\mathrm{}3124\pm 0.0005$, $A=(65.\mathrm{}4\pm 0.2)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}_{}^{3+}{}_{}{}^{}$ in Th${\mathrm{O}}_2$; $g=1.\mathrm{}1208\pm 0.0005$, $A=(127.\mathrm{}9\pm 0.4)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}_{}^{3+}{}_{}{}^{}$ in Sr${\mathrm{Cl}}_2$; and $g=1.\mathrm{}2862\pm 0.0005$, $A=(45.\mathrm{}7\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${}_{}{}^{241}{}_{}{}^{}\mathrm{Am}_{}^{4+}{}_{}{}^{}$ and $A=(45.\mathrm{}3\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${}_{}{}^{243}{}_{}{}^{}\mathrm{Am}_{}^{4+}{}_{}{}^{}$ in Th${\mathrm{O}}_2$. Spectra observed for both ${\mathrm{Pu}}^{3+}$ and