Structure and luminescence properties of Dy$^{3+}$ doped quaternary tungstate Li$_3$Ba$_2$Gd$_3$(WO$_4$)$_8$ for application in wLEDs

Quaternary tungstates with the composition Li$_3$Ba$_2$Gd$_3$(WO$_4$)$_8$ doped with different concentrations of Dy$^{3+}$ (from 0.5 to 10 at%) were prepared by the solid-state reaction method at 900 °C. Their structural, spectroscopic and optical properties were studied systematically in this work. X-ray diffraction analysis confirmed the crystallization of Li$_3$Ba$_2$Gd$_3$(WO$_4$)$_8$ to have a monoclinic structure (sp. gr. C2/c); the lattice constants for 1 at% doping concentration of Dy$^{3+}$ are a = 5.2126(2) Å, b = 12.7382(1) Å, c = 19.1884(3) Å, V$_{calc}$ = 1273,40(4) Å$^3$ and β = a × c = 91.890(9)°. The first principles calculations for the undoped crystal revealed a direct bandgap of 2.45 eV, which is very close to the experimental one. The identified broad, and strong excitation peak at 450 nm indicates that Li$_3$Ba$_2$Gd$_3$(WO$_4$)$_8$:Dy$^{3+}$ phosphors are suitable to be pumped by a blue laser diode (LD). Under excitation at 445 nm, the phosphor showed a stronger luminescence peak at 575 nm which corresponds to the Dy$^{3+}$:4F$_{9/2}$ → $^6$H$_{13/2}$ transition, and three weaker emissions peaks at 477, 661, and 750 nm. ... mehrMeanwhile, the effect of different Dy$^{3+}$ contents on the luminescence properties was investigated. The optimum concentration to minimize the quenching effect was 4 at% and the critical distance is 31.209 Å. The phosphor emitted strong greenish-yellow light situated at (0.425, 0.472) in CIE coordinates with a color temperature of 3652 K. All the measured luminescence lifetime curves exhibited a single-exponential nature. Excellent thermal stability was found for this tungstate phosphor (the activation energy is 0.352 ± 0.01 eV). The measured absolute photoluminescence quantum yield was around 10.5%. The results presented in this work show that Li$_3$Ba$_2$Gd$_3$(WO$_4$)$_8$:Dy$^{3+}$ phosphors with strong yellow emission are promising candidates for white-light emitting LED (wLED) applications.