Over the past ten years, white light emitting diodes (WLEDs) that combining the InGaN blue chip with YAG:Ce 3+ yellow-emitting phosphors have proverbially been applied in the fields of lighting, indicators and displays. However, there are also some problems with such “blue+yellow” WLEDs. For example, the color temperature is high and the color rendering index is low. Coating blue, green and red phosphors onto a near ultraviolate LED chips can potentially overcome the above problems. So far, many groups have focused on the development of tricolor emitting phosphors for the near ultraviolate chip excitation, especially the red emitting phosphors. Rare earth ions are widely used as emitting centers due to rich energy levels and metastable states with long lifetime. Among them, Eu 3+ is particularly suitable for the red emission. The 4f-4f transitions of Eu 3+ , however, are strongly dependent on its own local surrounding, and thus the selection of host materials is crucial. CaSnO 3 with low phonon energy and favorable thermal stability is a good choice. Unfortunately, the mismatch between Eu 3+ and Ca 2+ in charge and ions radii limits the doping level of Eu 3+ . On the other hand, although some studies on thermal properties of 5 D 0 → 7 F J transitions have been reported, there still exists a need to comprehensively understand the thermal quenching mechanism. In this paper, CaSn 1 − x Li x O 3 :Eu 3+ ( x =0, 2, 5, 10) phosphors are synthesized by a simple solid state method. Firstly, the starting materials including CaCO 3 (AR), SnO 2 (CP), EuNO 3 ·6H 2 O (4N) and Li 2 CO 3 (AR) are mixed in stoichiometric ratio. After throughout grinding in agate mortar, the mixture are put into corundum crucible and then calcined in a silicon-molybdenum stove at 1300°C for 3 h. Finally, the white products are obtained. XRD analysis indicates that the solubility limit of Eu 3+ in CaSnO 3 lattice is low, but the doping level of Eu 3+ can be enhanced by introducing Li + ions. At 10% Li + doping, 8% Eu 3+ ions are completely incorporated into CaSnO 3 lattice. On the basis of the shift of diffraction peaks, it is confirmed that the enhanced solubility limit of Eu 3+ in CaSnO 3 is related to the charge compensation of Li + ions. More importantly, the enhanced 5 D 0 → 7 F J transitions of Eu 3+ are observed after introducing the Li + ions. When the Li + doping concentration is 10%, the strongest one is obtained. The related mechanism is discussed by using the corrected emission spectra. Take the optimal sample CaSn 0.9 Li 0.1 O 3 :8%Eu 3+ as an example, the thermal properties are also investigated. With increasing the temperature from 300 K to 500 K, the quenching of fluorescence is observed. Because the ratio of intensity for 5 D 1 → 7 F J to 5 D 0 → 7 F J increases with the increase of temperature, the weaken emissions are related to the thermal population from 5 D 0 to 5 D 1 . But due to the different activation energy for thermal quenching under 394 nm and 464 nm excitation, there must be some other thermal quenching processes. On the basis of the excitation spectra at various temperature, it is found that the weaken excitation efficiency is another key factor. Furthermore, it is clarified that the thermal population from 7 F 0 to 7 F J ( J >0) is responsible for the decrease of excitation efficiency by using a steady-state luminescent rate equation.
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Open Access
Feb 22, 2024
Harsha P Uskaikar 
