Mechanistic insights into defect generation and tuning of optical properties in Zn_1−xFe_xAl₂O₄ (0.01 ≤ x ≤ 0.40) nanocrystals

The correlation of several defects and optical and magnetic properties with Fe content in Zn_1−xFe_xAl₂O₄ (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT–IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while , and Fe_Al^× antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe³⁺ ions at distorted octahedral and non-symmetric tetrahedral sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange–brown is observed in Zn_1−xFe_xAl₂O₄ nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries.