Optical properties of ZnMn1.5Al0.5O4 spinel nanoparticles: direct band gap, low Urbach energy, and estimated nonlinear optical response.

In this work, the single-phase spinel oxide ZnMn1.5Al0.5O4(ZMAO) was successfully synthesized via the sol-gel auto-combustion method. X-ray diffraction analysis confirmed that the sample crystallizes in a tetragonal spinel structure with the I41/amd space group. The optical properties, based on UV-visible-NIR reflectance measurements, reveal the semiconducting behavior of the ZMAO sample, with a direct optical band gap of approximately E g = 2.160 ± 0.017 eV and a remarkably low Urbach energy (E U = 0.376 ± 0.006 eV). The analysis of the optical constants of ZMAO reveals a refractive index of n 0 ≈ 2.05 and a very low extinction coefficient of k = 1.8 × 10-5, indicating low optical absorption and promising potential for optical and optoelectronic applications. Using the Wemple-DiDomenico single effective oscillator model, the dispersion parameters E 0 = 5.150 eV and E d = 11.360 eV were determined, confirming the dispersive behavior of the material. In addition, the low dielectric losses (tan δ = 2.1 × 10-5) at high frequencies indicate a stable electrical response, further supporting the potential of ZMAO for electronic and functional devices. The third-order nonlinear optical susceptibility χ(3) = 2.264 × 10-21 m2 V-2 and the nonlinear refractive index n 2 = 2.662 × 10-20 m2·V-2 confirm the strong nonlinear optical response of ZMAO, suggesting its potential for applications in various nonlinear optical devices such as optical switches, modulators, and fiber-optic communication systems. The photoluminescence spectrum was deconvoluted into five emission bands. The 571 nm emission is attributed to near-band-edge recombination, whereas the 415-520 nm emissions are associated with localized electronic states, Mn-related states, and defects. The green emission around 520 nm is particularly linked to defect-related luminescence. Finally, CIE chromaticity analysis confirms that ZMAO is a promising material for optical applications, particularly as a phosphor for LEDs and solid-state lighting.