Formation of oxygen vacancies in Li2FeSiO4: first-principles calculations.

The formation of oxygen vacancies could affect various properties of oxides. Herein we have investigated the formation energies of an oxygen vacancy (VO) with the relevant charge states in bulk Pnma-Li2FeSiO4 using first-principles calculations. The formation energies of the VO are essentially dependent on the atomic chemical potentials that represent the experimental conditions. The calculated formation energies of an oxygen vacancy in different charge states indicate that it would be energetically favorable to fully ionize the oxygen vacancy in Li2FeSiO4. The presence of VO is accompanied by a distinct redistribution of the electronic charge densities only around the Fe and Si ions next to the O-vacancy site, which shows a very local influence on the host material arising from VO. This local characteristic is also confirmed by the calculated partial densities of states (PDOS). We also studied the influence of substitutional (MnFe and CoFe) and cation vacancy defects (i.e., VFe and VLi) in the vicinity of an O-vacancy on the formation of an O-vacancy, respectively. We find that the calculated interaction energies between these defects and the oxygen vacancy are all negative, which implies that the formation of an oxygen vacancy becomes easier when the above defects are introduced. Compared to the substitutional defects, the interaction energies between the vacancy defects and the oxygen vacancy are significantly larger. Among them, the interaction energy between VFe and VO is the largest.