Green synthesis strategy to construct strong zero-phonon line red emission phosphor Na2SiF6:Mn4+: Photoluminescence properties, thermal stability and application in warm white LEDs

Nowadays, Mn4+-activated fluoride phosphors because of the excellent luminescence properties have been widely used to improve the luminous efficiency of white LED. However, it is inevitable to use toxic HF as solvent in the preparation of such phosphors, which can cause a series of potential safety hazards. Hence, an effective green synthesis strategy should be adopted to prepare Mn4+-activated fluoride phosphors by simple cation exchange method at room temperature. In this work, the non-toxic reaction system (HNO3/CH3COOH + NH4F) can successfully replace HF as the reaction solution to synthesize a series of Na2SiF6:Mn4+ red phosphors with strong zero-phonon line emission. The crystal structure, morphology and elemental composition of as-prepared Na2SiF6:Mn4+ phosphors are systematically studied. Na2SiF6:Mn4+ phosphor shows bright red-light emission because of the spin-forbidden 2Eg → 4A2g transition of Mn4+ under the excitation of blue and UV light, and the correlated color temperature and the color purity of red fluorescence are calculated in detail. The crystal field strength (Dq) and the Racah parameters (B and C) are calculated to evaluate the crystal field environment of Mn4+ in Na2SiF6. The effect of synthesis conditions on the fluorescence properties of the as-prepared phosphors is also thoroughly discussed, including different sodium sources and acid amounts. The optimal Mn4+ doping concentration is 0.03, and the concentration quenching mechanism is determined to be a dipole-dipole interaction. Impressively, Na2SiF6:Mn4+ phosphor presents exceptional thermal quenching behavior. Within the test temperature range of 298–423 K, the thermal quenching activation energy, chromaticity shift and chromaticity coordinate variation are 0.76 eV, 14.14 × 10−3 and (−0.0055, 0.0055), respectively. Importantly, warm white LED is fabricated using Na2SiF6:Mn4+ and Y3Al5O12:Ce3+ under blue LED excitation, which exhibits excellent optical properties (CCT = 3987 K and Ra = 91.8). Moreover, it also shows good output stability at high drive current. Therefore, this work provides a new strategy to synthesize highly efficient Mn4+-activated fluoride red phosphors through a green synthesis route for the potential application in high-performance warm white LED.