Synthesis and Optical Characterization of Red-Emitting BaTa2O6:Eu3+ Phosphors

Boron doping effect on the structural, spectral properties and charge transfer mechanism of orthorhombic tungsten bronze β-SrTa2O6:Eu3+ phosphor

In the study, the effect of boron doping on spectral properties and CTB mechanism was investigated by using Eu³⁺ doped orthorhombic β-SrTa₂O₆. A phosphor series of Eu³⁺ doped SrTa₂O₆, and Eu³⁺ and B³⁺ co-doped SrTa₂O₆ polycrystals were fabricated by solid-state reaction at 1400 °C for 20 h in an air atmosphere. The X-ray diffractions of the main phase structure for all the ceramics maintained up to 10 mol% Eu³⁺ concentration, while the increase of XRD intensity for Eu³⁺ and B³⁺ co-doped samples was attributed to somewhat improvement of crystallization. SEM morphologies of grains showed that the presence of boron promotes agglomeration and grain growth. The doping of boron up to 20 mol% led to an increase in PL intensity, CTB energy slightly shifted to low energy, and also an increase occurred in the asymmetry ratio of the phosphor. Therefore, the low crystal field symmetry of the Eu³⁺ sites and some improvement in crystal structure properties for Eu³⁺, B³⁺ co-doped samples supported the PL increase. The trend of Judd-Ofelt parameters (Ω ₂, Ω ₄) is SrTa₂O₆:xEu³⁺, 0.1B³⁺ > SrTa₂O₆:xEu³⁺. The high Ω ₂ parameter for boron co-doped samples showed a covalent Eu-O bond character with low symmetry of Eu³⁺ environment, while the high Ω ₄ value indicated the reduction in electron density of the ligands. Some increase in the short decays of Eu³⁺, B³⁺ co-doped samples is probably due to the surface effect and low crystal field symmetry. The quantum efficiency of 0.05Eu³⁺, 0.1B³⁺ co-doped phosphor with the highest PL intensity increased by about 21% compared to that without boron.

Influence of europium doping on the crystallization, morphology, and the cathodoluminescent properties of PbNb2O6:Eu3+ phosphors

Undoped PbNb2O6 and Eu3+ ion doped PbNb2O6 samples were synthesized by high temperature mixed oxide method, applying a heat treatment temperature of 1250°C and an annealing time of 6 hours. In order to elucidate the structural and optical behavior of PbNb2O6:Eu3+ phosphors, XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy dispersive spectroscopy), CL (cathodoluminescence) and absoption analyses were performed. The X-ray diffraction results showed that the undoped PbNb2O6 sample crystallized in a rhombohedral symmetry while Eu3+ doped samples formed in orthorhombic symmetry. The morphologies of the rhombohedral and orthorhombic grains were examined by SEM-EDS. The CL spectra showed spectral profiles between 580 and 780 nm in relation to the 4f–4f transitions of Eu3+. A strong emission was observed at about 620 nm, corresponding to the red color and associated with the 5D0 → 7F2 transition of Eu3+, while the undoped sample did not exhibit CL emission of the host which is probably due to the presence of lead in the host structure. In addition, the CL analysis results showed that the emission intensity increased with the increase of Eu3+ ion concentration. The increase in magnetic dipole transition caused by the electron beam radiation effect of the CL with increasing doping concentration is associated with the change of dipole moments of the Eu3+ doped tungsten bronze host and thus differentiating the emission spectrum. UV lamp excited photograph of undoped sample showed blue-violet color while Eu3+ doped phosphors with red color became more significant with increasing Eu3+ concentration.

Novel Eu3+-activated Ba2Y5B5O17 red-emitting phosphors for white LEDs: high color purity, high quantum efficiency and excellent thermal stability

Eu³⁺-activated Ba₂Y₅B₅O₁₇ (Ba₂Y_5−xEu_xB₅O₁₇; x = 0.1–1) red-emitting phosphors were synthesized by the conventional high temperature solid-state reaction method in an air atmosphere. Powder X-ray diffraction (XRD) analysis confirmed the pure phase formation of the as-synthesized phosphors. Morphological studies were performed using field emission-scanning electron microscopy (FE-SEM). The photoluminescence spectra, lifetimes, color coordinates and internal quantum efficiency (IQE) as well as the temperature-dependent emission spectra were investigated systematically. Upon 396 nm excitation, Ba₂Y_5−xEu_xB₅O₁₇ showed red emission peaking at 616 nm which was attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ ions. Meanwhile, the influences of different concentrations of Eu³⁺ ions on the PL intensity were also discussed. The optimum concentration of Eu³⁺ ions in the Ba₂Y_5−xEu_xB₅O₁₇ phosphors was found to be x = 0.8. The concentration quenching mechanism was attributed to the dipole–dipole interaction and the critical distance (R_c) for energy transfer among Eu³⁺ ions was determined to be 5.64 Å. The asymmetry ratio [(⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁)] of Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors was calculated to be 3.82. The fluorescence decay lifetimes were also determined for Ba₂Y_5−xEu_xB₅O₁₇ phosphors. In addition, the CIE color coordinates of the Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors (x = 0.653, y = 0.345) were found to be very close to the National Television System Committee (NTSC) standard values (x = 0.670, y = 0.330) of red emission and also showed high color purity (∼94.3%). The corresponding internal quantum efficiency of the Ba₂Y_4.2Eu_0.8B₅O₁₇ sample was measured to be 47.2%. Furthermore, the as-synthesized phosphors exhibited good thermal stability with an activation energy of 0.282 eV. The above results revealed that the red emitting Ba₂Y_4.2Eu_0.8B₅O₁₇ phosphors could be potential candidates for application in near-UV excited white light emitting diodes.

A novel Ba₂Y₅B₅O₁₇:Eu³⁺ red-emitting phosphor with high color purity, high quantum efficiency and excellent thermal stability was developed.