Influences of cation distribution on proton insertion dynamics in complex spinel NiCo2O4 thin films via ionic liquid gating

Ionic liquid gating (ILG) induced proton doping has appeared as an efficient tuning knob to manipulate novel ground states and realize rich applications in complex spinel oxides. We report insights into the influences of cation distribution on local atomic and electronic structure of NiCo2O4 (NCO) epitaxial films as well as its correlation with proton insertion dynamics. For NCO films deposited at lower oxygen partial pressure (PO2), we verify the dominant structural variation as cation distribution rather than oxygen vacancies through electric and magnetic transport analyses. The x-ray absorption spectra at the O K-edge further suggests two couples of cation inversion at octahedral (Oh)-Ni2+ and tetrahedral (Td)-Co2+ sites, resulting in Ni3+ at Td and more Co3+ at Oh sites. Through systematic investigations of the structural and valence states of NCO films with different ILG treatment conditions, our results display the robust ionic reduction procedure, i.e., the Ni ions near the Fermi level exhibit a prior Ni3+ to Ni2+ reduction whereas the Co ions at different coordinated sites show the gradual Co3+ & RARR; Co2+ reduction procedure. However, more stringent conditions are necessary for fully protonating NCO films deposited at low PO2, which can be ascribed to the short relaxation time and poor diffusion coefficient accompanying with the increased cation disorders.