Unravelling The Effect of Nitrogen Doping on The Structure of LaFeO₃ Epitaxial Thin Films

Heteroatom doping has proven to be an effective approach for fine-tuning the electronic and magnetic properties, as well as enhancing the electrocatalytic performance in ABO₃-type perovskite oxides. Beyond the conventional cation doping at A or B-sites, partial replacement of oxygen anions in perovskite oxides by other elements, such as nitrogen (N), sulfur (S), and fluorine (F), has also been explored to modify their structure and properties. In this study, we aim to advance our understanding of how N doping in LaFeO₃ films, influences their composition and structure, subsequently affecting their electronic and electrochemical properties. High-quality N-doped LaFeO₃ (LFON) epitaxial thin films with different doping levels were grown on (001)-oriented Nb:SrTiO₃ substrates using nitrogen-plasma-assisted pulsed laser deposition. Our X-ray diffraction and scanning transmission electron microscopy (STEM) measurements revealed that while the films are coherently strained in plane, with no observed dislocations at the interface, there is a notable, up to 4% lattice expansion in the out-of-plane direction. By employing an integrated differential phase contrast (iDPC) imaging technique in STEM, we examined orthorhombic relaxation in both undoped and doped samples. We found no significant differences in octahedral tilting between them, while the doped LFON sample exhibited a higher prevalence of two orthogonal in-plane rotated structural domains. Moreover, atomically resolved electron energy loss spectroscopy (EELS) indicated that the Fe valence remains as Fe³⁺ after N doping, suggesting that the generation of oxygen vacancies serves as a compensatory mechanism for the charge difference between nitride and oxide ions. However, DFT calculations indicated that oxygen vacancies and substitutional N had little impact on the out-of-plane lattice parameter. Further experimental and theoretical investigations are necessary to uncover the underlying cause of the 4% lattice expansion in the out-of-plane direction. In summary, these findings provide a crucial guide for understanding how N doping affects the structure and properties of complex oxides, essential for designing novel electrocatalysts for water splitting.