Design of Eu-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 defect state can stably exist in β-NaGdF, by density functional theory (DFT) calculation. Furthermore, we predicted that the vacancies would provide a zero thermal quenching (ZTQ) property for the β-NaGdF-based red-light phosphor. Then, a series of β-NaGdF:Eu and β-NaGdF:0.25Eu,Yb red-light phosphors were synthesized by the hydrothermal method. We found that β-NaGdF:0.25Eu and β-NaGdF:0.25Eu,0.005Yb 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 vacancies in β-NaGdF:0.25Eu and β-NaGdF:0.25Eu,0.005Yb. Combining the DFT calculation with characterization results of TL spectra, it is concluded that electrons stored in vacancies are excited to the exited state of Eu to compensate for the loss of Eu 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-NaGdF:0.25Eu and β-NaGdF:0.25Eu,0.005Yb 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.