Red-emitting CaWO4:Eu3+,Tm3+ phosphor for solid-state lighting: Luminescent properties and morphology evolution

CaWO₄:xEu³⁺,yTm³⁺ crystals were obtained by facile synthesis at low temperature by the microwave-assisted hydrothermal method (MAH). The phase formation, morphology, luminescent properties and energy transfer were investigated. The X-ray diffraction (XRD) results show the formation of a scheelite-like tetragonal structure without the presence of secondary phases. The growth mechanism of hierarchical microstructures based on self-assembly and Ostwald-ripening processes was evaluated, obtaining different types of morphologies. The luminescence spectra of CaWO₄:Eu³⁺,Tm³⁺ at 325 nm excitation show the predominance of red emission at the ⁵D₀ → ⁷F₂ (Eu³⁺) transition at 624 nm. This feature signals dominant behavior of the electric dipole type. The presence of Tm³⁺ is notably evident in the absorption spectra by the related excitation transitions: ³H₆ → ¹G₄, ³H₆ → ³F₃ and ³H₆ → ³H₄. Color parameters are discussed to characterize CaWO₄:Eu³⁺,Tm³⁺ emission. The study of the emission spectrum as a function of the concentration of Eu³⁺ (x mol%) and Tm³⁺ (y mol%) indicates that the CaWO₄:Eu³⁺,Tm³⁺ phosphors show stronger red emission intensity and exhibit the CIE value of x = 0.63 and y = 0.35. The photoluminescence results show 97% high color purity for CaWO₄:4 mol%Eu³⁺, a high CRI (92%) and a low CCT of 1085 K. These results demonstrate that the CaWO₄:Eu³⁺,Tm³⁺ red phosphors are promising as color converters for application in white light-emitting diodes and display devices.