Correlating dopant-induced lattice modifications with optical and magnetic properties in Ni²⁺-doped TiO₂ nanostructures

This study investigates the structural, optical, and magnetic properties of Ni²⁺-doped TiO₂ nanostructures (1.0–10.0 wt%) synthesized via an acidic sol–gel route. Dopant-induced lattice modifications are primarily expressed through microstructural strain, lattice parameter distortion, and phonon softening rather than macroscopic lattice defects visible by TEM. X-ray diffraction and Rietveld refinement reveal systematic shifts of the anatase (101) peak, lattice contraction, and increasing compressive strain with Ni incorporation, as confirmed by Williamson–Hall analysis. Raman and electron paramagnetic resonance (EPR) spectra indicate oxygen vacancy formation and localized electronic states. Optical absorption and Tauc analysis show progressive bandgap narrowing, while crystal field theory (CFT) confirms Ni²⁺ ions occupying both octahedral and tetrahedral coordination environments. EPR signals further evidence Ni²⁺ magnetic centers and Ti³ ⁺/oxygen vacancy-related species. These results demonstrate that defect-mediated lattice distortion governs the optoelectronic and magnetic tuning of Ni-doped TiO₂, offering insights for advanced photocatalytic and spintronic material design.