Nanocrystalline MgCr_xAl_2-xO₄: Facile synthesis and thermal dependency of photoluminescence

MgCr_xAl_2-xO₄ nanoparticles were synthesized by facile co-precipitation method. The x = 0.02 sample showed intense emission and considered for studying the effect of annealing temperature (500–1200 °C) on the structural and optical properties. On annealing, crystallinity improved and the material gradually turned pink at 1200 °C. Hexagonal sheet-like nanoparticles (∼52 nm) were observed by HRTEM and the lattice fringes suggested their single crystalline nature. Excitation spectra showed two peaks at 546 and 419 nm where the latter was asymmetric, attributable to the trigonal distortion. The photoluminescence consisted of intense zero-phonon line and associated multi-phonon side bands, whose intensities increased with annealing. Higher Dq/B (3.5), confirmed strong crystal field for Cr³⁺, suggesting that the photoluminescence emission could be associated with spin-parity forbidden transitions. The emission lifetimes (λ_em = 688 nm) ranged between ∼20 and 24 ms. Other oxide nanoparticles suitable for phosphors, biomarkers, etc. can be conveniently synthesized similarly.