Influence of alkaline earth metal cations; Ca2+, Sr2+ and Ba2+ on the structural and optical properties of MAl2O4: Eu2+, Nd3+ phosphors.

Abstract

Eu2+ doped and Nd3+ co-doped MAl2O4:Eu2+, Nd3+ (M = Ca, Sr, and Ba) phosphors were prepared by a solution-combustion method. The obtained powders were investigated in terms of their phase composition, particle morphology and photoluminescence (PL) by X-Ray diffraction (XRD), Scanning Electron Microscope (SEM) and Ultraviolet-Visible Spectroscopy (UV-VIS) techniques respectively. XRD analysis depicts a monoclinic phase for CaAl2O4:Eu2+, Nd3+ and SrAl2O4:Eu2+, Nd3+ and a hexagonal structure for BaAl2O4:Eu2+, Nd3+ phosphor. SEM results showed generally agglomerated particles with non-uniform shapes and sizes with irregular network structures having lots of voids and pores. PL excitation revealed broadband spectra with peaks corresponding to the crystal field splitting of the Eu2+ d-orbital. The emission spectra were also broadband with peaks at 447 nm for CaAl2O4:Eu2+, Nd3+, at 507 nm for SrAl2O4:Eu2+, Nd3+ and at 497 nm for BaAl2O4:Eu2+, Nd3+ due to the 4f65d1-4f7 emission of Eu2+. Sharp emission lines were observed arising from the f-f transitions of the Eu3+ ions. The differences in emissions from the three phosphors arise from the crystal field splitting of the 5d electron shell due to the changes in the crystalline environment of the Eu2+ ions caused by the substitution of the divalent alkaline earth metal cations with different ionic sizes in the MAl2O4 host lattice. UV-VIS spectra showed absorption edges at 330, 342 and 340 nm in agreement with the observed PL excitation peaks. The luminescence decay characteristics showed that these materials possess persistent luminescence whereby BaAl2O4: Eu2+; Nd3+ gave a longer afterglow as compared to the other two phosphors.