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

Abstract

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.