A New Allotropic form of Cerium Due to its Transition Under Pressure to the Tetravalent State

An x-ray structural analysis on cerium under pressure was made in order to investigate its valence behavior in the metallic state. A new form, ${\alpha }^{\prime }-\mathrm{Ce}$, always fcc but strongly collapsed, was found to exist at pressures higher than 50 kbar. It was deduced that Ce is tetravalent in the ${\alpha }^{\prime }$ form. The atomic radius of ${\mathrm{Ce}}^{\mathrm{IV}}$ was calculated, and the probable values of the valency in $\gamma -\mathrm{Ce}$ and $\alpha -\mathrm{Ce}$ are also estimated.