Tadeusz Lesniewski

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes

Red phosphors AMF6:Mn(4+) (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn(4+) as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF6(2-) octahedron with strong ZPL intensity. The larger distorted SiF6(2-) octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressure-dependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce(3+) (YAG) and KNaSiF6:Mn(4+) phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn(4+) presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

Thermal stabilization and energy transfer in narrow-band red-emitting Sr[(Mg2Al2)1−y(Li2Si2)yN4]:Eu2+ phosphors

Cuboid-coordinated nitridomagnesoaluminate Sr[Mg2Al2N4]:Eu2+ and a solid solution of Sr1−x[(Mg2Al2)1−y(Al2Si2)yN4]:Eux2+ were prepared using all-nitride precursors by gas pressure sintering. X-ray diffraction (XRD) data of the phosphors were validated by Rietveld refinement of synchrotron X-ray powder XRD data with the space group I4/m indexed to a tetragonal crystal system. The 7Li solid-state magic-angle spinning nuclear magnetic resonance (ss-MAS-NMR) data prove the successful incorporation of the Li+–Si4+ couple. The blue-light excitable property with the excitation band peaking at 460 nm gave rise to emission at 620–630 nm at x = 0.004, with a full-width-at-half-maximum of ∼77 nm. The inhomogeneous broadening of the Eu2+ luminescence in the system due to the effective energy transfer from one activator to another was observed which concurrently resulted in spectral peak shifts from ∼615 nm to ∼680 nm as a function of the amount of Eu2+. The Li+–Si4+-tuned solid solution increased the emission intensity without significantly shifting the emission wavelength. Such a phenomenon was accompanied by an improvement in thermal stability from Δ = 965 cm−1 for y = 0, to Δ = 1365 cm−1 for y = 0.1 wherein Δ corresponds to the activation energy from the 5d states to the conduction band. The simple gas pressure sintering strategy using all-nitride starting materials resulted in the desired blue light-excitable narrowband red emitting thermally stabilized phosphor.