Paramagnetic Resonance in Uranium Salts

Abstract

Paramagnetic resonance has been observed in ${\mathrm{U}}^{+3}$ and ${\mathrm{U}}^{+4}$ 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 ${g}_{\ensuremath{\perp}}\ensuremath{\approx}2.1$ and ${g}_{\mathrm{II}}\ensuremath{\approx}2.8$. 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 ${\mathrm{U}}^{+4}$ 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.