Paramagnetic Resonance in Uranium Salts

Abstract

Paramagnetic resonance has been observed in ${\mathrm{U}}^{+3\mathrm{}}$ and ${\mathrm{U}}^{+4\mathrm{}}$ ions in powdered samples of U${\mathrm{F}}_3$ and U${\mathrm{F}}_4$, respectively. The resonance curve in U${\mathrm{F}}_3$ was found to be asymmetric, with two unresolved peaks yielding $g_{\perp }\approx 2.1$ and $g_{\mathrm{II}}\approx 2.8$. The resonance is broad and weak; the over-all half-width is about 5 kilogauss at room temperature. The curve obtained for U${\mathrm{F}}_4$ is symmetric and yields $g={2.1}_5$. Its half-width is about 3.5 kilo-gauss at room temperature. Although the resonance in U${\mathrm{F}}_3$ appeared somewhat stronger at liquid air than at room temperature, that in U${\mathrm{F}}_4$ could be detected only at room and at elevated temperatures. The results favor the interpretation that the two unpaired electrons in ${\mathrm{U}}^{+4\mathrm{}}$ are $6d$ electrons, that the crystalline field in U${\mathrm{F}}_4$ is essentially cubic, and that either the lowest component of the Stark multiplet is a nonmagnetic singlet or that the electrons have a high degree of antiferromagnetic alignment. The results on U${\mathrm{F}}_3$ show that the crystalline field has an axially symmetric component.