4f electron delocalization and volume collapse in praseodymium metal

We study the pressure evolution of the 4f electrons in elemental praseodymium metal compressed through several crystallographic phases, including the large volume-collapse transition at 20 GPa. Using resonant x-ray emission, we directly and quantitatively measure the development of multiple electronic configurations with differing 4f occupation numbers, the key quantum observable related to the delocalization of the strongly correlated 4f electrons. These results provide a high-fidelity test of prior predictions by dynamical mean-field theory, and support the hypothesis of a strong connection between electronic and structural degrees of freedom at the volume-collapse transition.