Effect of the reducing atmospheres on the photoluminescent and phosphorescent properties of Sr4Al14O25:Eu2+, Dy3+, Cr3+ phosphors

This work reports the effects of two reducing atmospheres (N2 + H2 and carbon) on the luminescent, morphological, and structural properties of Sr4Al14O25:Eu2+ (1 mol%), Dy3+ (2 mol%), Cr3+ (1–4 mol%) [SALO:Eu, Dy, Cr] phosphors. They were synthesized by a combustion method followed by an annealing treatment in a reducing atmosphere. According to scanning electron microscopy (SEM) images, the samples annealed in both types of reducing atmospheres present microparticles with irregular shape. An increase of Cr concentration from 1 to 4 mol% produced microparticles with higher porosity. X-ray diffraction patterns demonstrated that the orthorhombic phase was maintained in all the samples synthesized with different Cr concentrations or reducing atmospheres. Two main emission bands centered at 490 and 693 nm were observed in the SALO:Eu, Dy, Cr phosphors under excitation at 375 nm. In general, the annealing under carbon atmosphere produced the strongest luminescent emission and the longest phosphorescence times in SALO:Eu, Dy, Cr phosphors. When the Cr concentration is increased from 1 to 4 mol%, the phosphorescence times for the NIR and VIS bands were drastically reduced from ≈1830 to 40–60 s. The phosphorescence intensity was favored by the presence of defects such as oxygen vacancies (Vo) and Dy3+–Vo traps, which were formed after the introduction of Dy3+ and Cr3+ ions in the SALO host. The presence of these defects was confirmed by absorbance measurements. The annealing in carbon atmosphere is simpler than that in H2 + N2 atmosphere and leads to enhanced luminescent properties of SALO:Eu, Dy, Cr phosphors.