4[functionof] electron delocalization and volume collapse in praseodymium metal

We study the pressure evolution of the 4[functionof] 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 4[functionof] occupation numbers, the key quantum observable related to the delocalization of the strongly correlated 4[functionof] 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.