An orthorhombic spectrum of ${\mathrm{U}}^{3+}$ in Ca${\mathrm{F}}_2$ was measured. The orthorhombic $X$, $Y$ axes are found to lie in the (110) plane and the $Z$ axis perpendicular to these axes. The $X$ axes are at ±(19±1)° with the [$1\overline{1}0$] direction. There are, in general, twelve inequivalent magnetic sites. The $g$ factors corresponding to one set of axes are $g_x=1.38\mathrm{}\pm 0.01$, $g_y=2.85\mathrm{}\pm 0.02$, and $g_z=2.94\mathrm{}\pm 0.01$. Various models explaining this spectrum are discussed. The most likely model is that of an interstitial ${\mathrm{F}}^{-}$ in the second-nearest neighbor position.

• Received 18 March 1963

DOI:https://doi.org/10.1103/PhysRev.131.920