Extended optical and dielectric properties of non-stoichiometric Mg0.5Ca0.5Fe2O4 nanoparticles

This investigation reports UV–Vis–NIR absorption, impedance spectroscopy, and terahertz time-domain spectroscopic studies on Mg 0.5 Ca 0.5 Fe 2 O 4 nanoparticles. A direct allowed band gap of 1.67 ± 0.02 eV and Urbach energy of 0.566 ± 0.01 eV were obtained. THz-TDS revealed a nearly dispersion-free refractive index n ≈ 2.5 and a very low extinction coefficient (k = 0.02–0.07) while the part of permittivity remained remarkably stable (ε′ ≈ 5.9). Low-frequency dielectric dispersion (up to 1 MHz) followed Maxwell–Wagner interfacial polarization and Koop’s model. Combination of direct narrow band gap and record-low THz losses establishes non-stoichiometric Mg 0.5 Ca 0.5 Fe 2 O 4 NPs as a promising material for passive terahertz photonics and optoelectronic devices.