Fe Substitution Suppresses Oxygen Vacancy Formation and Stabilizes High-Valence Ni in Epitaxial La_0.5Sr_0.5Ni_1-xFe_xO_3-δ Thin Films

Recent discovery of superconductivity in Nd_0.8Sr_0.2NiO₂ has inspired further exploration of nickelates to gain insights into the origins of high-temperature superconductivity. However, the synthesis of Sr- or Ca-doped nickelate thin films is challenging due to the instability of high-valence Ni and the competition between perovskite and Ruddlesden–Popper (RP) phases. Our recent study revealed that Sr doping in LaNiO₃thin films significantly reduces the Ni valence from Ni³⁺ to Ni²⁺ and results in the formation of numerous RP phases as the Sr doping level increases from 0% to 100%. Furthermore, DFT simulations on the La_1-ySr_yNiO_3-δperovskite structures indicated a substantial decrease in the oxygen vacancy formation energy from 2.96 to 1.85 eV as y varies from 0 to 0.5. In our current work, we synthesized a series of epitaxial La_0.5Sr_0.5Ni_1-xFe_xO_3-δ(LSNFO) thin films on LSAT(001) substrates using oxide MBE. We found that partial Fe substitution for Ni in La_0.5Sr_0.5NiO_3-δsignificantly enhances the structure quality and stabilizes the perovskite structure. Ni <i>L</i>-edge XAS results reveal an increase in the Ni valence of LSNFO films, approaching Ni³⁺ as x increases from 0 to 0.5. At x = 0.75, the Ni valence is even higher than Ni³⁺. This is in contrast to the LaNi_1-xFe_xO₃ (LNFO) system (without Sr), where the average Ni valence decreases with increasing Fe content due to charge transfer from Fe to Ni. STEM measurements confirm that the x = 0.75 sample remains in the perovskite phase but exhibits lots of RP faults and defects. On the other hand, Fe <i>L</i>-edge XAS results indicate that films of all Fe content show a nominally higher Fe oxidation state than a LaFeO₃ (Fe³⁺) reference. In-plane transport measurements demonstrate that the resistivity increases with Fe content in the LNFO system, but decreases initially in the LSNFO system, reaching the lowest value at x = 0.375, before rising again with x greater than 0.375. The metal-insulator transition in the LSNFO system strongly depends on Fe content, increasing with x, similar to the LNFO system. Further theoretical investigations are necessary to elucidate the influence of Fe doping on the electronic structure of the LSNFO system. In summary, these findings provide valuable insights into the design of perovskite structures featuring high-valence Ni and Fe, crucial for developing effective electrocatalysts for water splitting, as well as the creation of advanced electronic devices for solid oxide fuel cells.