Modulation of radiative defects in MgAl2O4 nanocrystals probed using NMR, ESR, and PL spectroscopies

Abstract

We have investigated the influence of combustion fuels on radiative defects of MgAl2O4 spinel. Monoethanolamine (MEA), glycine, and urea fuels were employed to synthesize three samples of MgAl2O4 nanocrystals, choice consequence of which has been observed on cation inversion and various intrinsic defects. Synchrotron powder x-ray diffraction patterns were refined by the Rietveld method. All samples exhibited a spinel structure, with a minor secondary phase of MgO in the sample synthesized by urea fuel. 27Al nuclear magnetic resonance (NMR) inferred the distribution of Al cations among tetrahedral and octahedral sites with structural distortion of interstices. Rietveld, NMR, and diffuse reflectance spectra analyses revealed the presence of antisite defects, cation, and anion vacancies, which may be anticipated to induce F(Mg) defect centers through charge compensation. An electron spin resonance (ESR) study was carried out to investigate the paramagnetic defect centers. Two prominent broad ESR signals, attributed to F+ and V− defect centers, originated from the delocalization of paramagnetic spins and their interaction with nearby cations. Photoluminescence studies further confirmed the existence of F(Mg) defect centers, VO-related defects, and MgAl′ and VMg″ defects in all samples. It was found that the nature of the defects did not change, but the concentration of defects varied significantly with the type of fuel employed and excitation wavelength. The MgAl2O4 samples synthesized using glycine, urea, and MEA fuels were found to be rich in F(Mg) defect centers, VO-related defects, and MgAl′, VMg″, VO-related defects, respectively. The tailoring of defect-assisted photoluminescence by varying fuel types in MgAl2O4 spinel can be exploited in various luminescence applications.