Mn4+ nonequivalent-doped Al3+-based cryolite high-performance warm WLED red phosphors

Abstract

Herein, a series of red-emitting Mn⁴⁺-activated fluoride cryolite phosphors were prepared via the coprecipitation and hydrothermal methods, and the nonequivalent substitution of Mn⁴⁺ for Al³⁺ in the Na₃AlF₆ host was investigated; moreover, systematic characterizations by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were conducted to investigate the phase purity, morphology, elemental composition and distribution of the samples, respectively. The diffuse reflectance, excitation, and emission spectra and fluorescence decay curves indicated that the Na₃AlF₆:Mn⁴⁺ red phosphors possessed distinguished optical properties. The as-prepared Na₃AlF₆:Mn⁴⁺ phosphors have two broad and intense absorption bands in the UV (360 nm) and blue light (468 nm) regions; this indicates that they can be applied in commercial WLEDs based on UV and blue light chips. All samples exhibited red emissions around 630 nm with outstanding color purity owing to the ²E_g → ⁴A_2g transition of Mn⁴⁺. The theoretical calculated results demonstrated that the Na₃AlF₆ host could provide strong crystal field strength for Mn⁴⁺, and the nephelauxetic effect of Mn⁴⁺ was evaluated by the Racah parameters (B and C), which were compared with those of other previously reported fluoride and oxide hosts. The doping amount of K₂MnF₆, reaction time, raw materials, HF concentration and reaction solvent were optimized to improve the morphology and optical properties of the Na₃AlF₆:Mn⁴⁺ red phosphors. Concentration quenching occurred when the doping content was 7%, and the energy transfer mechanism was theoretically identified to be a dipole–dipole interaction. Importantly, red (Na₃AlF₆:Mn⁴⁺) and yellow (YAG:Ce³⁺) phosphors at different proportions were introduced into WLEDs to optimize the color rendering index from 73.1 to 89 and the color temperature from 5675 K to 3472 K. The emission light of the WLEDs has been successfully changed from cold white light to warm white light; this makes the Na₃AlF₆:Mn⁴⁺ red phosphor a promising candidate for application in WLEDs.