Synthesis and luminescent properties of rare earths doped Gd2Te4O11 tellurite phosphors

A series of rare earth Dy<sup>3+</sup>, Tb<sup>3+</sup>, Eu<sup>3+</sup> singly doped Gd<sub>2</sub>Te<sub>4</sub>O<sub>11</sub> (GTO) tellurite phosphors with intrinsic polarity were prepared by hydrothermal method. The phase structure, morphology and thermal stability of the phosphors were characterized. Their luminescence properties were tested in detail. The results show all those phosphors were crystalized into single phase of digadolinium tellurite with short rod-like shape. The maximum size achieved microns in axial direction. The phosphors have good thermal stability. For the GTO:Dy<sup>3+</sup>, the fluorescence emission under UV excitation is mainly located in the yellow-green region. The optimal doping concentration corresponding to the strongest excitation and emission is 2.5%, and the CIE color coordinates are (0.39, 0.43). The fluorescence decay curves show that the lifetime of the GTO:Dy<sup>3+</sup> on <sup>4</sup>F<sub>9/2</sub> energy level decreases gradually with increasing doping concentration of Dy<sup>3+</sup>, which may be related to the cross relaxation (CR) between Dy<sup>3+</sup> ions. For the GTO:Eu<sup>3+</sup>, the fluorescence emission under UV excitation is mainly located in the red and orange-red regions. The emission intensity was enhanced with increasing doping concentration of Eu<sup>3+</sup>. When the doping concentration is 10%, the CIE color coordinates are (0.62, 0.38), which located in the orange-red region with high color purity. The fluorescence lifetime of Eu<sup>3+</sup> on <sup>5</sup>D<sub>0</sub> energy level is hardly affected by the change of Eu<sup>3+</sup> doping concentration. For the GTO:Tb<sup>3+</sup>, with increasing the Tb<sup>3+</sup> concentration, the fluorescence emission under UV excitation changes from blue-violet region to yellow-green region, which can be ascribed to the influence of CR between Tb<sup>3+</sup> ions. The fluorescence decay behavior revealed that the Tb<sup>3+</sup> ions on <sup>5</sup>D<sub>4</sub> excited state may undergo energy transfer and reabsorption, which deviated fluorescence decay from the single exponential model. When the concentration of Tb<sup>3+</sup> is 0.5%, the sample exhibits white light emission, having the CIE color coordinates of (0.33, 0.35) and color rendering index of 86. The measurements of temperature-dependent emission spectra show that the above-mentioned phosphors have good luminescent thermal stability. The internal quantum efficiencies (IQE) of those three types of phosphors were tested, and the IQE of GTO:Eu<sup>3+</sup> are better than those of GTO:Dy<sup>3+</sup> and GTO:Tb<sup>3+</sup>. All those phosphors still have much room for improvement in the luminescent performance. These phosphors have potential for the use of UV-excited white LED.