What is the valence of a correlated solid? The double life of delta-plutonium

Plutonium displays phase transitions with enormous volume differences among its phases and both its Pauli like magnetic susceptibility and resistivity are an order of magnitude larger than those of simple metals. Curium is also highly resistive but its susceptibility is Curie-like at high temperatures and orders antiferromagnetically at low temperatures. The anomalous properties of the late actinides stem from the competition between the itinerancy and localization of its f electrons, which makes the late actinides elemental strongly correlated materials. A central problem in this field is to understand the mechanism by which these materials resolve these conflicting tendencies. In this letter we identify the electronic mechanisms responsible for the anomalous behaviour of late actinides. We revisit the concept of valence using theoretical approach that treats magnetism, Kondo screening, atomic multiplet effects, spin orbit coupling and crystal field splitting on the same footing. Plutonium is found to be in a rare mixed valent state, namely its ground state is a superposition of two distinct valencies. Curium settles in a single valence magnetically ordered state at low temperatures. The f7 atomic configuration of Curium is contrasted with the multiple configuration manifolds present in Plutonium ground state which we characterize by a valence histogram. The balance between the Kondo screening and magnetism is determined by the competition between spin orbit coupling and the strength of atomic multiplets which is in turn regulated by the degree of itinerancy. The approach presented here, highlights the electronic origin of the bonding anomalies in plutonium and can be applied to predict generalized valences and the presence or absence of magnetism in other compounds starting from first principles.