Effect of Zn substitution in Cr³⁺ doped MgAl₂O₄ mixed spinel nanoparticles on red/NIR emission properties

Red/Near-infrared (NIR) emitting Mg_1-xZn_xAl_1.98Cr_0.02O₄ nanoparticles were synthesized by co-precipitation followed by firing(1200 °C). Gradual shift of XRD peaks with Zn-content confirmed complete solid-solution formation. HRTEM revealed hexagonal sheet-like structures and lattice fringes revealed their single-crystallinity. Owing to refractory nature of MgAl₂O₄, particles were smaller(∼38 nm) in Mg-rich samples and gradually increased(∼58 nm) with Zn-substitution. Due to trigonal distortion, the fine splitting of the ⁴T_1g level of Cr³⁺ manifests as asymmetric band(400 nm) in the excitation spectrum. Cr³⁺ emission was clearly observed, comprising of R-line and multi-phonon side-bands on either side, with maximum R-line intensity for x = 0.5. With increasing x, side-bands were more pronounced probably due to the band-gap difference of end members. High Dq/B (3.5–3.9), suggested Cr³⁺ in a strong crystal-field. Racah-parameter(B) variations suggest increased electron delocalization over molecular orbitals for higher Mg²⁺ substitutions. Longer lifetimes (23–34 ms) are well-suited for bioimaging and optoelectronic applications. Such a simple synthesis can be easily extended to other important phosphors.