Dependence of manganese phosphorescence on crystal lattice sites of spinel aluminate hosts

This work investigates the structural and optical properties of SrAl2O4: 2%Mn2+ and BaAl2O4: 2%Mn2+ nano-phosphors. These nano-phosphors were synthesized by a conventional combustion method at 600 °C, using urea as a fuel. The effects of crystal lattice sites, hence the crystals field on the electronic transitions of Mn2+ have been investigated. The crystal structures, morphology, and photoluminescence properties of SrAl2O4: 2%Mn2+ and BaAl2O4: 2%Mn2+ nano-phosphors have been studied. The crystal structures, morphology, and photoluminescence studies were carried out using X-ray diffraction (XRD) spectrometer, field emission scanning electron microscope (HRSEM), and photoluminescence (PL) spectrofluorometer respectively. XRD patterns confirmed the hexagonal and monoclinic structures of BaAl2O4 and SrAl2O4 respectively. The crystallites sizes were also averaged from XRD peaks in all samples using scherrer equation, and they were found to be at nano scale. SEM micrographs revealed irregular shaped particles both in SrAl2O4: 2%Mn2+ and BaAl2O4: 2%Mn2+ nano-phosphors. Energy dispersive x-ray spectroscopy (EDS) analysis confirmed the presence of Mn in BaAl2O4: 2%Mn2+ and SrAl2O4: 2%Mn2+ samples. Under 426 nm excitation, PL emission spectrum of SrAl2O4: 2%Mn2+ revealed a green emission peaking at 512 nm, together with a red emission at 650 nm, which collectively form a bright yellow emission. The 512 and 650 nm emissions are ascribed to the T4→A6(S6) spin-forbidden transition of the Mn2+ ions residing in tetrahedral and octahedral sites of the SrAl2O4 matrix respectively. Only a green emission peaking at 512 nm due to the T4→A6(S6) transition of Mn2+ ions in the tetrahedral sites was observed on the PL emission spectrum of BaAl2O4: 2%Mn2+ phosphor after a 426 nm excitation.