Influence of doping concentration on valence states of europium in SrAl₂O₄:Eu phosphors

SrAl₂O₄:Eu phosphors with various doping concentrations were synthesized via a urea assisted solution-combustion method in a reducing atmosphere without any post-annealing process. The phosphors were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (SEM), photoluminescence (PL), X-ray excited luminescence (XEL), and X-ray photoelectron spectroscopy (XPS). The XRD results show that the samples exhibit a monoclinic phase in crystal structure. The PL measurements show that the intensity of luminescence emission from divalent Eu²⁺ ions is increased with increasing europium concentration up to a maximum of 2.5 mol%, then decreases at higher concentration due to the concentration quenching effect. The emission from trivalent Eu³⁺ ions occurs when the doping concentration is higher than 12.5 mol%. The XPS measurements also confirm the trends of Eu²⁺ and Eu³⁺ with changes in concentration. The results indicate that the luminescence color components of the phosphors can be adjusted by the doping concentration; this is a very important result for practical applications.