Study of luminescent behavior and crystal defects of different MNa[PO₄]-Dy³⁺ phosphors (M = Mg, Ca, Sr and Ba)

Dysprosium (Dy³⁺) doped alkaline earth based sodium-ortho phosphate (MNa[PO₄], where M = Mg, Ca, Sr and Ba) phosphors were prepared by solution combustion method. The phosphors were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), fluorescent spectrophotometry and UV–vis spectroscopy. The XRD and FE-SEM results confirm that the samples contain mixture of phases of crystals. The band gap of the phosphors was calculated from diffuse reflectance spectra data using the Kubelka–Munk function. The excitation spectra of the phosphors were characterized by broadband extending from 250 to 500 nm. Upon near-UV excitation, the phosphor emits intense blue and yellow light with a weak red bands, which originated from ⁴F_9/2 → ⁶H_15/2, ⁶H_13/2, ⁶H_11/2 transitions of Dy³⁺ ion. Luminescence features of MNa[PO₄]-Dy³⁺ are discussed on the basis of crystal structures of the host lattices. The effect of alkaline metal ions on the luminescence properties of MNa[PO4]-Dy³⁺ phosphors was studied. The Dy³⁺ ions have structure-dependent transitions due to the lattice sites they occupy in each particular host. The calculated CIE (Commission Internationale de l'Eclairage) coordinates were found to be lying in the white and blue regions of the horse shoe plot of color gamut. The PL excitation and emission data suggest that the studied MNa[PO₄]-Dy³⁺ phosphors could be used as near UV excited white LEDs.