DFT plus 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

A DFT plus U study of the structural, electronic, magnetic, and mechanical properties of cubic and orthorhombic SmCoO3

SmCoO3 is a perovskite material that has gained attention as a potential substitute for La1−xSrxMnO3−d as a solid oxide fuel cell cathode. However, a number of properties have remained unknown due to the complexity of the material. For example, we know from experimental evidence that this perovskite exists in two different crystal structures, cubic and orthorhombic, and that the cobalt ion changes its spin state at high temperatures, leading to a semiconductor-to-metal transition. However, little is known about the precise magnetic structure that causes the metallic behavior or the spin state of the Co centers at high temperature. Here, we therefore present a systematic DFT+U study of the magnetic properties of SmCoO3 in order to determine what magnetic ordering is the one exhibited by the metallic phase at different temperatures. Similarly, mechanical properties are difficult to measure experimentally, which is why there is a lack of data for the two different phases of SmCoO3. Taking advantage of our DFT calculations, we have determined the mechanical properties from our calculated elastic constants, finding that both polymorphs exhibit similar ductility and brittleness, but that the cubic structure is harder than the orthorhombic phase