Ab-initio study of point defects in Th and U alloy

Point defect formation energies are calculated along with several structural and elastic properties for the fcc ( α) and bcc ( β) Th crystal structures. Th-self defects and single uranium impurity defects are modeled in the fcc phase, whereas only Th self-defects are modeled in the high temperature bcc phase. Additionally, an analysis of several well known exchange-correlation functionals is given in regard to defect modeling performance. Overall, defect formation energies in the bcc phase were lower in energy compared to their defect equivalents in the fcc phase. In both phases, vacancies were found be the most stable, with the bcc vacancy being ∼0.8 eV lower in energy than the fcc vacancy, regardless of form of the exchange-functional. The most stable interstitial defects were in the octahedral and <110> dumbbell positions for the fcc and bcc phases, respectively. It was also found that the <110> and <111> dumbbell defects were unstable in the fcc phase, but not in the bcc phase. Overall, the choice of exchange-correlation functional was found to have almost a negligible effect on the calculated formation energies. The most notable difference between functionals was in the prediction of the vacancy defect formation energies, which may be attributed to differences in the way each functional handles exchange and correlation in regions of low electron density relative to the bulk of a material, such as in the internal surface of a vacancy or vacancy cluster.