Proton-induced unusual valence-state variations and electronic structures in double perovskite La 2 NiMnO 6 films via ionic liquid gating

Ionic liquid gating (ILG) technique has been demonstrated as a powerful manner to manipulate the valence-state and electronic structures of complex oxides through topotactic phase transformations. However, the dominant type of transfer ions of a particular system and the underlying mechanisms for the ion incorporation dynamics remain controversies in different systems. In this article, ILG treatments were applied to double-perovskite La₂NiMnO₆ (LNMO) system. In response to positively gated electric bias, protons (H⁺) incorporate into LNMO films gradually. Based on the combined results from synchrotron-based x-ray absorption and photoelectron spectroscopies, the doped electrons not only induce the valence-state reductions of both Mn and Ni sites, but also the downshift of the valence-band maximum away from the Fermi level, which is distinct from the valence-state and electronic-state variations induced by electron doping from oxygen vacancies (Vₒ). This peculiar valence-state reduction has been verified by first-principles calculations, which not only enlightens the roles of energy competitions between H⁺ insertion and Vₒ formation, but also the electronic structures near the Fermi level in determining the electrochemical gating mechanism of complex oxides.