Multiple intermediate valence in plutonium

On the basis of the concepts of an intermediate-valence (IV) regime, an empirical model is proposed that quantitatively describes the magnetic susceptibility, specific heat, and effective atomic volume of the low-temperature α phase and the gallium or aluminum-stabilized face-centered cubic (fcc) δ phase of plutonium metal. The results of the paper allow one to estimate the entropy change associated with the α → δ structural phase transition, the value of this change being very close to the experimental value. According to the model, both phases of plutonium are systems with multiple intermediate valence, whose ground state is a many-particle Kondo singlet, and fluctuations occur between 5f electron configurations with integer valences of 3+, 4+, and 2+.