Uranium at the conditions of the Earth’s inner core: Fe-U forms and implications

Abstract

We have carried out density functional theory (DFT) calculations to study the form on which U exists inside the Earth's inner core. The question of whether U exists in the inner core at all has been controversial and attracted a lot of attention because of its potential role in the Earth's heat budget as a radioactive element. We have carried an extensive structural search and then calculated the stability, at the pressure of the inner core, of Fe-U systems in various forms: pure crystalline U, FeU_n (n=1…4) compounds, and U impurities in bulk crystalline Fe, both in its bcc (α-Fe) and hcp (ε-Fe) phases. We have found that Fe-U compounds will not form in the inner core, and if they form, they are thermodynamically unstable, which agrees with the literature. However, we have found that U as point defects in crystalline Fe may exist under certain conditions. These U impurities appear to stabilize the α-Fe phase significantly, which lowers the barrier of the phase transitions between ε-Fe and α-Fe phase transition. This is a potentially quite significant contribution to the Earth's energy budget.