Electron polarons and donor point defects in americium dioxideAmO2

Abstract

Intrinsic donor point defects and electron polarons are investigated in bulk ${\mathrm{AmO}}_{2}$ using density functional theory $+U$ calculations. Oxygen vacancies are deep double-donor defects, with transition energy levels closer to the valence band maximum than to the conduction band minimum. Americium interstitials are unlikely, due to prohibitive formation energies. Self-trapped electron polarons (which locally correspond to reducing one ${\mathrm{Am}}^{4+}$ in ${\mathrm{Am}}^{3+}$) are found extremely stable (self-trapping energy $=\ensuremath{-}1.01\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$). The electron is even more stable in the self-trapped state (far from an oxygen vacancy) rather than in association with an oxygen vacancy, indicating that oxygen vacancies have the tendency to spontaneously ionize, and thus automatically liberate electron polarons in the lattice. This large stability of the electron polarons confines the accessible range of Fermi levels to a very narrow interval between the valence band maximum ${E}_{\text{VBM}}$ and $\ensuremath{\sim}{E}_{\text{VBM}}+0.09\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. In oxygen-poor conditions, oxygen vacancies may be formed in rather large concentration in ${\mathrm{AmO}}_{2}$ and have a strong probability to be doubly or singly ionized, with charge compensation being mostly ensured by a large number of electron polarons. The electron polaron hopping from an Am atom onto the nearest one involves a rather large activation energy of $\ensuremath{\sim}0.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and probably takes place by a nonadiabatic mechanism.