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The presented work gives an overview of recent advances in research and development of novel phosphors based on Eu2+-doped KxNa1-xLuS2 ternary sulfides which may be used as phosphors in white LED sources with tunable color correlated temperature. Tunability is achieved by using two different kinds of excitation diodes (nearUV and blue) in combination with the proper K/Na ratio in KxNa1-xLuS2:Eu2+ structure. Intense photoluminescence due to the Eu2+ 5d – 4f allowed transition covers a broad emission region in visible range under both nearUV (~390 nm) and blue (~450 nm) excitations. The possibility to change the intensity ratio of the excitation diodes yields the changes of the output color correlated temperature. These changes are clearly demonstrated by shifts in the CIE color space; light quality is also quantified and discussed using CRI (color rendering index) and CQS (color quality scale) metrics [1]. However, to fully understand the Eu2+ emission mechanisms in these sulfidic hosts and to be able to optimize phosphor preparation and its performance, additional experimental techniques are applied to characterize these KxNa1-xLuS2:Eu2+ phosphors. When a focused electron beam impinges on a cathodoluminescent (CL) material in a scanning electron microscope (SEM), secondary electrons, characteristic x-rays and visible photons are emitted. These signals can be used to construct a two-dimensional topographical, chemical and luminescence map, respectively [2]. Furthermore, thermal quenching and quantum efficiency measurements of KxNa1-xLuS2:Eu2+ phosphors will be discussed for the first time. V. Jarý, L. Havlák, J. Bárta, M. Rejman, A. Bystřický, Ch. Dujardin, G. Ledoux, M. Nikl. Circadian Light Source Based on KxNa1-xLuS2:Eu2+ Phosphor. ECS Journal of Solid State Science and Technology, 7 (1), R3182-R3188, (2018). doi: 10.1149/2.0231801jss. L.I.D.J. Martin, D. Poelman, P.F. Smet, J.J. Joos, Microscopic Study of Dopant Distribution in Europium Doped SrGa2S4: Impact on Thermal Quenching and Phosphor Performance. ECS Journal of Solid State Science and Technology, 7 (1) R3052-R3056 (2018). doi: 10.1149/2.0341709jss.

Set of novel Eu2+-doped KxNa1-xLuS2 phosphors (where x = 0 – 1) was successfully synthesized in the form of transparent crystalline hexagonal platelets by chemical reaction under the flow of hydrogen sulfide. Their physical properties were investigated by means of X-ray diffraction, X-ray excited fluorescence, time-resolved luminescence spectroscopy and electron paramagnetic resonance. Special attention is given to photoluminescence emission spectra under the 395 nm and 455 nm excitation aiming to obtain white emission with tunable color temperature, which is desirable for circadian white LEDs construction. EPR method is employed to understand the Eu2+ incorporation and distribution in the KxNa1-xLuS2 hosts. CIE xy-coordinates are used to compare effects of different K/Na concentration in KxNa1-xLuS2 on resulting spectrum (Figure 1). The application potential of mentioned compounds in the field of white LED solid state lighting phosphors with tunable color temperature is thoroughly discussed. The realistic circadian light bulb is presented as well and its properties are addressed. Figure 1

We have recently started to investigate structural, scintillation, luminescence and paramagnetic (in case of Eu2+ doping) properties of the unexplored family of rare earth (RE)-doped ternary sulfides of general formula ALnS2 (A = Rb, K, Na; Ln = La, Gd, Lu, Y), which show prospective application potential in the field of X-ray and solid state white LED phosphors. The X-ray diffraction (XRD) detected only a single crystalline phase of rhombohedral lattice system, except NaLaS2 which has cubic structure. Time-resolved luminescence spectroscopy, XRD and electron paramagnetic resonance were employed to understand the nature of Eu2+ emission centre in these sulfidic hosts and find correlation between structural and optical properties of these materials. Eu2+ and Ce3+ emission features are investigated in a broad temperature and concentration ranges and phenomenological modeling and delayed recombination decay measurements are used to understand better the Eu2+ excited state dynamics. Optical properties of ALnS2 doped by another RE ions are presented as well. Motivating figure 1 shows clear energy shift of the Eu2+ emission spectra maxima with different chemical compositions, peaking from 498 nm (RbLuS2) to 779 nm (NaGdS2). Figure 1. RT Radioluminescence spectra of Eu2+-doped ALnS2.