Rietveld refinement and optical properties of SrWO4:Eu3+ powders prepared by the non-hydrolytic sol-gel method

Abstract: We investigated the effect of annealing time on the structure and optical properties of SrWO₄:Eu³⁺ powders prepared by the non-hydrolytic sol-gel method and heat treated at 800 °C for 2, 4, 8 and 16 h. Thermogravimetric and differential thermal analyses revealed that SrWO₄:Eu³⁺ powders were obtained at about 800 °C. X-ray diffraction patterns and Rietveld refinement data confirmed that all powders had a scheelite-type tetragonal structure. Micro-Raman and Fourier transform infrared spectra indicated structural order at short range and anti-symmetric stretching vibrations of O–W–O bonds associated with tetrahedral [WO₄] clusters. Optical properties were investigated by ultraviolet-visible (UV-vis) diffuse reflectance, and photoluminescence (PL) data which provided the evolution of quantum efficiency (η) and lifetime (τ). UV-vis spectroscopy evidenced intermediate energy levels within the band gap of SrWO₄:Eu³⁺ powders. PL properties validated that the Eu³⁺ electric-dipole (⁵D₀→⁷F₂) transition in PL emission spectra was dominant which proved that Eu³⁺ ions were positioned in a site without an inversion center. [(⁵D₀→⁷F₂)/(⁵D₀→⁷F₁)] band ratios showed that Eu³⁺ ions were located in a low symmetry environment. The PL emission, η and τ proved the dependence on the annealing time in the behavior of SrWO₄:Eu³⁺powders with a higher relative emission PL intensity as well as higher η and τ values related to other samples when heat treated at 800 °C for 8 h.