Semi-empirical models describing thermodynamic properties of f-metals

The unusual thermodynamic properties (abnormal elastic and thermophysical features and complex phase diagrams) of a number of metals with unfilled f-shells (4f and 5f) are caused by the proximity of close energies of different electron configurations. According to the modern concepts, electron states differ either in the degree of screening of the localized f-electron spins (Kondo volume-collapse model) or in localization and delocalization of f-electrons (Mott transition). At finite temperatures, a thermodynamically stable state of a mixture of atoms with different electron configuration becomes feasible due to the configuration entropy contribution, with concentrations of atoms of different sort being determined from thermodynamic potential minimum. A successful semi-empirical model capable of describing behavior of material in this state is the Aptekar-Ponyatovsky (AP) model, which treats a system of atoms of different sort as a substitution solid solution with component-to-component ratio varying as a function of temperature and pressure. The terminal component concentrations correspond to the states of material in adjacent polymorphous modifications. This paper discusses the capabilities of the AP model to describe thermodynamic properties of unalloyed cerium (4f-metal) as well as unalloyed δ-plutonium and δ-Pu-based alloys (5f-metals). The paper shows that the results obtained within a single model provide an adequate description of abnormal behavior of these metals at varying external conditions, this proves the common nature of these anomalies associated with the evolution of the f-electron subsystem.