Structural distortion, photoluminescence, and magnetism in Fe -doped ZnAl₂O₄ spinel nanostructures synthesized by Pechini method

Undoped and Fe³⁺-doped ZnAl₂O₄ spinel nanostructures were synthesized via the Pechini method to investigate the correlation between structural distortion, optical transitions and magnetic behaviour. To confirm the phase, X-ray diffraction (XRD) revealed the formation of a cubic spinel phase (Fd3̅m), with a secondary phase (α-Fe₂O₃) emerging at higher Fe concentrations. The crystal structure was further refined by MAUD Rietveld, which provided reliable lattice parameters, cell volumes, and atomic coordinates. Refinement also showed that Fe³⁺ ions are partially substituted at tetrahedral (8a) sites at higher concentrations, resulting in a slight lattice expansion and local structural distortions. Scanning electron microscopy (SEM) revealed agglomerated polyhedral particles with homogeneous surface morphology, while transmission electron microscopy (TEM) revealed semi-spherical nanoparticles with a diameter of 22–44 nm. Thermogravimetric and differential thermal analyses (TGA/DGA) showed high thermal stability with negligible weight loss above 600 °C, which is consistent with the formation of a stable spinel. Diffuse reflectance spectroscopy (DRS) showed a systematic bandgap narrowing with increasing Fe content, while photoluminescence (PL) showed dual emission bands at 499 nm and 750 nm, attributed to Fe³⁺ ions in the tetrahedral and octahedral coordination sites, respectively. Room-temperature vibrating sample magnetometry (VSM) measurements revealed weak ferromagnetism, with magnetic behaviour strongly influenced by Fe³⁺ dopant concentration and domain structure. Overall, these findings provide a framework of structure and properties that link Fe-induced site occupancy, crystal field effects and domain magnetism, thus highlighting the multifunctional potential of ZnAl₂O₄-doped nanostructures for optoelectronics, spintronics, and biomedical applications.