Paramagnetic Resonance and Charge Compensation of Tetravalent Uranium (${\mathrm{U}}^{4+}$) in Calcium, Strontium, and Barium Fluorides

A new electron spin paramagnetic resonance spectrum in Ca${\mathrm{F}}_2$ doped with uranium is attributed to ${\mathrm{U}}^{4+}$. The spectrum possesses axial symmetry about the $〈111〉$ directions and is describable by a spin Hamiltonian with $g_{\perp }=0\mathrm{}\pm 0.15$ and $g_{\mathrm{II}}=3.238\mathrm{}\pm 0.005$ at 4.2°K. The trigonal symmetry is due to the presence of charge compensating ${\mathrm{O}}^{2-}$ ions along the $〈111〉$ directions. The spectrum is absent in crystals grown under reducing conditions. Similar results were also observed in Sr${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$.