  Lanthanides in the trivalent oxidation state are typically described using an
ionic picture that leads to localized magnetic moments. The hierarchical energy
scales associated with trivalent lanthanides produce desirable properties for
e.g., molecular magnetism, quantum materials, and quantum transduction. Here,
we show that this traditional ionic paradigm breaks down for praseodymium in
the 4+ oxidation state. Synthetic, spectroscopic, and theoretical tools
deployed on several solid-state Pr4+ oxides uncover the unusual participation
of 4f orbitals in bonding and the anomalous hybridization of the 4f1
configuration with ligand valence electrons, analogous to transition metals.
The resulting competition between crystal-field and spin-orbit-coupling
interactions fundamentally transforms the spin-orbital magnetism of Pr4+, which
departs from the Jeff =1/2 limit and resembles that of high-valent actinides.
Our results show that Pr4+ ions are in a class on their own, where the
hierarchy of single-ion energy scales can be tailored to explore new correlated
phenomena in quantum materials.
