DFT+U study of the structures and properties of the actinide dioxides

The actinide oxides play a vital role in the nuclear fuel cycle. For systems where current experimental measurements are difficult, computational techniques provide a means of predicting their behaviour. However, to date no systematic methodology exists in the literature to calculate the properties of the series, due to the lack of experimental data and the computational complexity of the systems. Here, we present a systematic study where, within the DFT+U formulism, we have parametrized the most suitable Coulombic (U) and exchange (J) parameters for different functionals (LDA, PBE, PBE-Sol and AM05) to reproduce the experimental band-gap and lattice parameters for ThO2, UO2, NpO2, PuO2, AmO2 and CmO2. After successfully identifying the most suitable parameters for these actinide dioxides, we have used our model to describe the electronic structures of the different systems and determine the band structures, optical band-gaps and the Bulk moduli. In general, PBE-Sol provides the most accurate reproduction of the experimental properties, where available.

We have employed diamagnetic order for ThO2, PuO2 and CmO2, transverse 3k antiferromagnetic order for UO2 and AmO2, and longitudinal 3k antiferromagnetic order for NpO2. The Fm m cubic symmetry is preserved for diamagnetic ThO2, PuO2 and CmO2 and longitudinal 3k NpO2. For UO2 and AmO2, the transverse 3k antiferromagnetic state results in Pa symmetry, in agreement with recent experimental findings. Although the electronic structure of ThO2 cannot be reproduced by DFT or DFT+U, for UO2, PuO2, NpO2, AmO2 and CmO2, the experimental properties are very well represented when U = 3.35 eV, 6.35 eV, 5.00 eV, 7.00 eV and 6.00 eV, respectively, with J = 0.00 eV, 0.00 eV, 0.75 eV, 0.50 eV and 0.00 eV, respectively.