DFT plus U calculations on U3O8(100) surface: thermodynamic stability and polarity compensation

Employing the DFT+U framework we investigate the thermodynamic stability of U3O8(100) surface. The most stable structure is predicted to gradually turn from the under-stoichiometric 06- to the over-stoichiometric 56-slab as the oxygen chemical potential increases, showing a continuous surface oxidation process. Oxygen atoms are found to always firstly adsorb on the lower oxidized U ions. Due to the polarity compensation, significant modification of surface charge presents on considered terminations, which is transferred via both U-51 and O-2p states, implying the nature of U3O8 as an intermediate state between the Mott-Hubbard and charge-transfer materials. It is also shown that the displacement of the outmost U6O10 layer and the surface energy are strongly related to the redistribution of surface charge. The static effect between charged layers is the major factor which influences the layer displacement. The increase in surface energy is shown to be roughly linearly dependent on the amount of the surface charge transfer.