Design of Eu³⁺-Doped Fluoride Phosphor with Zero Thermal Quenching Property Based on Density Functional Theory

Although being applied in various fields, white light emitting diodes (WLEDs) still have drawbacks that urgently need to be conquered: the luminescent intensity of commercial phosphors sharply decreases at working temperature. In this study, we calculated the forming energy of defects and confirmed that the V_Na defect state can stably exist in β-NaGdF₄, by density functional theory (DFT) calculation. Furthermore, we predicted that the V_Na vacancies would provide a zero thermal quenching (ZTQ) property for the β-NaGdF₄-based red-light phosphor. Then, a series of β-NaGdF₄:xEu³⁺ and β-NaGdF₄:0.25Eu³⁺,yYb³⁺ red-light phosphors were synthesized by the hydrothermal method. We found that β-NaGdF₄:0.25Eu³⁺ and β-NaGdF₄:0.25Eu³⁺,0.005Yb³⁺ phosphors possess ZTQ properties at a temperature range between 303-483 K and 303-523 K, respectively. The thermoluminescence (TL) spectra were employed to calculate the depth and density of the V_Na vacancies in β-NaGdF₄:0.25Eu³⁺ and β-NaGdF₄:0.25Eu³⁺,0.005Yb³⁺. Combining the DFT calculation with characterization results of TL spectra, it is concluded that electrons stored in V_Na vacancies are excited to the exited state of Eu³⁺ to compensate for the loss of Eu³⁺ luminescent intensity. This will lead to an increase of luminescent intensity at high temperatures and facilitate the samples to improve ZTQ properties. WLEDs were obtained with CRI = 83.0, 81.6 and CCT = 5393, 5149 K, respectively, when phosphors of β-NaGdF₄:0.25Eu³⁺ and β-NaGdF₄:0.25Eu³⁺,0.005Yb³⁺ were utilized as the red-light source. These results indicate that these two phosphors may become reliable red-light sources with high antithermal quenching properties for WLEDs.