Local structure modulation of Mn4+-doped Na2Si1−yGeyF6 red phosphors for enhancement of emission intensity, moisture resistance, thermal stability and application in warm pc-WLEDs

Abstract

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 Na₂Si_1−yGe_yF₆ 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 (D_q), Racah parameters (B and C) and nephelauxetic ratio (β₁) show that a new Na₂Si_0.5Ge_0.5F₆ 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 Na₂Si_0.5Ge_0.5F₆: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⁺–Na₂Si_0.5Ge_0.5F₆:0.06Mn⁴⁺ 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.