RESUMO
Herein, high-efficiency Nb-based oxyfluoride K3(NbOF5)(HF2):Mn4+ and fluoride K2NbF7:Mn4+ phosphors were successfully synthesized using different amounts of HF acid solutions by a simple co-precipitation method. XRD, SEM and EDS were used to characterize the crystal structure, morphology and elemental composition of the phosphors. The emission spectra, excitation spectra and luminescence decay curves were used to study the luminescence characteristics of the samples. The thermal stability of the phosphors was tested and the mechanism of temperature quenching was discussed. Meanwhile, the moisture resistance and application of the phosphors were investigated in detail. The results show that the K3(NbOF5)(HF2):Mn4+ phosphor has stronger luminous intensity, lower color temperature, and better moisture resistance compared with the K2NbF7:Mn4+ phosphor. The correlated color temperature (CCT) and color rendering index (CRI) of warm white LEDs can be significantly improved by using the K3(NbOF5)(HF2):Mn4+ (CCT = 3430 K, CRI = 87.3) phosphor as a red light component. So the K3(NbOF5)(HF2):Mn4+ phosphor has broader application prospect in the field of warm white LEDs.
RESUMO
Nowadays, the development of Mn4+-activated fluoride phosphors with efficient water and thermal stabilities continues to pose a huge challenge with regard to prolonging the service life and stabilizing the light output for phosphor-converted white light-emitting diodes (pc-WLEDs). Therefore, the synthesis strategy of simple crystal structure optimization is proposed to realize simultaneously the high hydrophobic and thermal stabilities of fluoride phosphors. Herein, Mn4+-doped Na2Si1-yGeyF6 red phosphors are successfully synthesized by a simple coprecipitation method. Satisfactorily, the optimization of Ge4+ and Mn4+ concentrations successfully enhances the luminescence intensity of the original phosphor (Na2SiF6:Mn4+) and an obvious red shift can be found. Moreover, the CIE coordinates of red light show that the phosphor has low correlated color temperature and excellent color purity. Based on excitation and emission spectra, the crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic ratio (ß1) show that a new Na2Si0.5Ge0.5F6 matrix can meet the high requirements of the crystal field environment when Mn4+ becomes the fluorescence center. Interestingly, the local structure modulation stabilizes the state of existence of Mn4+ in the matrix and enhances the moisture resistance of the phosphor. In addition, the as-prepared Na2Si0.5Ge0.5F6:Mn4+ phosphor possesses admirable thermal quenching behavior and color stability at high temperature. More importantly, low correlated color temperature (3408 K), high color rendering index (89.4) and preeminent luminous efficiency (112.89 Im W-1) are achieved using the YAG:Ce3+-Na2Si0.5Ge0.5F6:0.06Mn4+ system as color converters for warm pc-WLEDs. The work provides a new insight into the construction of red phosphors with favorable water and thermal stabilities for warm pc-WLEDs.