Actinide materials are systems rich in interesting physics, while playing an important role in environmental preservation, and a good comprehension of their electronic structure is therefore of particular importance.
A first-principles computational methodology is proposed in this work, affording accurate density functional theory studies in such strongly-correlated crystalline systems. The main ingredient behind the success of this approach is the use of hybrid density functionals, which provide good agreement with known experimental data for the optimum structural and electronic properties of the actinide oxides considered in this study.
The predictive capabilities of the method help understand certain unusual properties and phenomena associated with these compounds, from puzzling experimental findings to the relative stability of heavy actinide oxides.
Plutonium dioxide is taken as a case study in this work and detailed structural investigations are considered for this compound, such as the effects of non-stoichiometry and of various lattice distortions. The interstitial oxygen in PuO2.25 for example is found to be singly charged, consistent with experimental observations and contrary to the O2- previously proposed theoretically. A tetragonal distortion of the PuO2 lattice, with a very small orthorhombic component, is energetically favorable in our description at zero temperature. Such lattice deformation anticipates the experimentally observed orthorhombic phase, to which many actinide dioxides transform at high pressures. We also confirm that non-hydrostatic effects could be responsible for the intriguing value of the only measurement to date of the bulk modulus of PuO2.
Unexpected f orbital populations are predicted in heavy actinide dioxides, and they could elucidate certain perplexing structural measurements made on these compounds. These occupancies point to an early-occurring half-filled shell effect, and can also explain the lack of experimental evidence for the heavier actinide dioxides. These findings suggest that accepted models of electronic structure for certain open-shell compounds are not always warranted, and that their theoretical descriptions should be revised accordingly.