Correlation between Optical Band Gap and Microstructural Defects in Nickel Chromite Nanoparticles

This work reports dopant-driven tuning of the optical band gap in nickel chromite nanoparticles and correlates it with the evolution of microstructural defects. Nickel chromite samples doped with rare-earth ions (Eu, Dy, Sm and Tb) in the 1-9 mol% range were systematically examined to extract crystallite size and defect-related parameters and relate them to optical band-gap variations. The crystallite size and defect characteristics show clear dopant- and concentration-dependent changes. In the Eu-doped series, the crystallite size decreases with increasing dopant content, accompanied by a rise in dislocation density, indicating progressive lattice distortion at higher Eu levels. In contrast, Dy-, Sm- and Tb-doped samples exhibit distinct dopant-specific trends in crystallite size and dislocation density, reflecting different defect-generation pathways within the host lattice. Optical analysis reveals that the band gap is significantly modulated by the choice of dopant: Eu incorporation increases the band gap, whereas Dy, Sm and Tb incorporation decreases the band gap with increasing concentration. Overall, the results establish a strong qualitative correlation between band-gap evolution and defect characteristics, highlighting rare-earth doping as an effective route to engineer defect states and tailor the optical response of nickel chromite nanoparticles for potential optoelectronic and functional oxide applications.