Red-emitting phosphors Na2XF6:Mn4+ (X = Si, Ge, Ti) with high colour-purity for warm white-light-emitting diodes

Red-emitting phosphors Na2XF6:Mn4+ (X = Si, Ge, Ti) were synthesized by a simple co-precipitation method. The crystal structure, morphology and composition of as-prepared phosphors were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDS) and atomic absorption spectroscopy (AAS). Their optical properties were investigated by photo-luminescent spectra (PL), diffuse reflectance spectra (DRS), and the luminescence decay curves. These phosphors exhibit broad excitation band in blue regions and intense emission in red regions with high color-purity. The sample Na2SiF6:Mn4+ shares bright red emission under blue light excitation, and its CIE coordinates are x = 0.684, y = 0.315, which are close to the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). The white-light-emitting diodes (WLEDs) fabricated with this phosphor emits intense warm white-light with lower color temperature (3930 K) and higher color rendering index (92.3). The above results indicated that red phosphor Na2SiF6:Mn4+ maybe serve as a red-emitting phosphor for application in warm WLEDs.