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 ThO 2 , UO 2 , NpO 2 , PuO 2 , AmO 2 and CmO 2 . 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 ThO 2 , PuO 2 and CmO 2 , transverse 3k antiferromagnetic order for UO 2 and AmO 2 , and longitudinal 3k antiferromagnetic order for NpO 2 . The Fm 3 ¯ m cubic symmetry is preserved for diamagnetic ThO 2 , PuO 2 and CmO 2 and longitudinal 3k NpO 2 . For UO 2 and AmO 2 , the transverse 3k antiferromagnetic state results in Pa 3 ¯ symmetry, in agreement with recent experimental findings. Although the electronic structure of ThO 2 cannot be reproduced by DFT or DFT+U, for UO 2 , PuO 2 , NpO 2 , AmO 2 and CmO 2 , 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.