Structure and luminescence properties of Dy³⁺ doped quaternary tungstate Li₃Ba₂Gd₃(WO₄)₈ for application in wLEDs

Abstract

Quaternary tungstates with the composition Li₃Ba₂Gd₃(WO₄)₈ doped with different concentrations of Dy³⁺ (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 Li₃Ba₂Gd₃(WO₄)₈ to have a monoclinic structure (sp. gr. <i>C</i>2/<i>c</i>); the lattice constants for 1 at% doping concentration of Dy³⁺ are <i>a</i> = 5.2126(2) Å, <i>b</i> = 12.7382(1) Å, <i>c</i> = 19.1884(3) Å, <i>V</i>_calc = 1273,40(4) ų and <i>β</i> = <i>a</i> × <i>c</i> = 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 Li₃Ba₂Gd₃(WO₄)₈:Dy³⁺ 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 Dy³⁺:⁴F_9/2 → ⁶H_13/2 transition, and three weaker emissions peaks at 477, 661, and 750 nm. Meanwhile, the effect of different Dy³⁺ 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 Li₃Ba₂Gd₃(WO₄)₈:Dy³⁺ phosphors with strong yellow emission are promising candidates for white-light emitting LED (wLED) applications.