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The nature of the electronic band gap in lanthanide oxides

Accurate electronic structures of the technologically important lanthanide/rare earth sesquioxides (Ln2O3, with Ln=La,...,Lu) and CeO2 have been calculated using hybrid density functionals HSE03, HSE06 and screened-exchange (sX-LDA). We find that these density functional methods describe the strongly correlated Ln f-electrons as well as the recent G0W0@LDA+U results, generally yielding the correct band gaps and trends across the Ln-period. For HSE, the band gap between O 2p states and lanthanide 5d states is nearly independent of the lanthanide, while the minimum gap varies as filled or empty Ln 4f states come into this gap. sX-LDA predicts the unoccupied 4f levels at higher energies, which leads to a better agreement with experiments for Sm2O3, Eu2O3 and Yb2O3.