Preparation of ZnFe1La1O4 spinel ferrites followed by dielectric, electrical and optical investigations for energy conservation applications

We report synthesis of La³⁺-doped zinc ferrites via sol-gel auto-combustion, followed by stepwise annealing up to 900 °C for 2 h. Growth in average crystallite size from 12.2 nm to 13.5 nm has been observed as a result of Ostwald ripening process. Lattice parameter for the most prominent peak (311) increases from 8.1855 to 8.2179 Å consistent with Vegard's law. The material exhibited high specific surface areas and real part of dielectric constant coupled with low values of imaginary part of dielectric constant and dielectric tangent loss lead towards the potential utilization in energy conservation particularly in high frequency super-capacitors. Nyquist and Cole-Cole plots confirmed non-Debye relaxation mechanism with distinct contributions from grain and grain boundaries. The semiconducting nature was verified by resistivity values in the range of 16.8 × 10⁻⁵ to 20.5 × 10⁻⁵ Ω m. The optical band gaps decreased from 2.53 eV to 1.31 eV, broadening the applicability of the material to photocatalysis and sensor technologies. Furthermore, the higher refractive index values and metallization criterion (0.25–0.36) highlights the suitability of La³⁺-doped zinc ferrites for advanced non-linear optical applications and hetero-laser systems. The Urbach energy variation from 796 to 6102 meV reflects a delicate balance between crystallization and defect generation, crucial for tuning the optical behavior of nanostructured ferrites. The magnetic permeability (μ_m) response across energy ranges highlights the influence of thermal treatment on the magnetic, optical, and dielectric interplay, reinforcing the material's potential for energy storage and optoelectronic applications.