Improved SMTB-Q model applied to oxygen migration and pressure phase transitions in UO₂

The second-moment tight-binding variable-charge (SMTB-Q) interatomic potentials have been implemented in the molecular dynamics (MD) code LAMMPS in order to study the static and dynamical properties of uranium dioxide UO₂. With respect to a previous work on UO₂ the SMTB-Q model has been slightly modified in introducing a splitting energy of the U 5f  orbitals. This improvement results in a better description of the electronic structure of UO₂ namely the gap estimation which is now close to the experimental value (~2 eV). The structural and mechanical properties along with the cohesive energy of bulk UO₂ are in good agreement with the experimental data. The ionic charges on uranium and oxygen are respectively equal to 2.86 and  -1.43, very close to the Bader charges derived from ab initio calculations. The migration energies and the diffusion coefficient calculated respectively for oxygen vacancy (VO) and oxygen interstitials (IO) in under and over stoichiometry compare well with ab initio calculations and experimental data. The oxygen diffusivity is consistent at high temperature when additional Frenkel thermally formed swamps the effect of single IO and VO defects with recent prediction from EAM semi-empirical potentials. Additionally, a study on phase transitions between high pressure polymorphs of UO₂ has been performed and has shown the good transferability of the SMBT-Q potential over different coordination. It is found that the UO₂ phases stability order under tensile and compressive stresses, compared with stable fluorite phase at 0 GPa, are respected.