High-brightness red-emitting double-perovskite phosphor Sr₂LaTaO₆:Eu³⁺ with high color purity and thermal stability

Emission spectrum and CIE chromaticity coordinates of the as-prepared Sr₂LaTaO₆:0.2Eu³⁺ red phosphors.

At present, the environmental problems such as resource depletion, water pollution and air pollution caused by the development of nature resources have are raised great social concerns. Among the environmental issues, lighting electricity accounts for about 20% of global electricity consumption. Compared with traditional lighting sources such as incandescent lamps and fluorescent lamps, white light emitting diodes (LEDs) have many outstanding advantages like high energy efficiency, long working life, environmental friendliness, and etc. Therefore, the development of LEDs lighting is an effective solution to save power and protect the global environment.

Nowadays, the widely used white LEDs are commonly manufactured by combining InGaN blue LEDs chips with Y₃Al₅O₁₂:Ce³⁺ yellow-emitting phosphors. However, due to lack of red light, there are some limitations in these white LEDs, including high correlated color temperature and low color rendering index. Instead, the near-ultraviolet (near-UV) pumped white LEDs based on the combination of near-UV LED chips and tri-color (blue, green, and red) phosphors can exhibit warm white light with high color rendering index. Yet, there is still a lack of red phosphors excited by near-UV light with good performance for white LEDs. Thus, research on near-UV-light-excitable red phosphor is of great importance to both practical application and scientific research.  It is well-known that Eu³⁺ ion is an excellent activator for inorganic phosphors because of its bright red emissions due to the ⁵D₀→⁷F_J (J = 1, 2, 3, 4) transitions. The optical properties of Eu³⁺ ions are critically influenced by the used host materials. In recent years, the double-perovskite compounds with molecular formula of A₂BMO₆ have been widely selected as outstanding host materials for Eu³⁺ ions.

A research group led by Prof. Xiaoyong Huang from Taiyuan University of Technology discovered a novel bright red-emitting Eu³⁺ ions doped Sr₂LaTaO₆ double-perovskite phosphor. The related results are published in Chinese Optics Letters, Volume 19, Issue 3, 2021 (Z. Zhang, et al., High-brightness red-emitting double-perovskite phosphor Sr₂LaTaO₆:Eu³⁺ with high color purity and thermal stability). A series of Sr₂LaTaO₆:Eu³⁺ phosphors doped with different Eu³⁺ concentrations were prepared by using a high-temperature solid-state reaction method. Impressively, the obtained Sr₂LaTaO₆:Eu³⁺ phosphor exhibited intense absorption in the near-UV wavelength range peaking at 394 nm, thus this phosphor could be used in near-UV-pumped white LEDs. Moreover, under 394 nm excitation, the Sr₂LaTaO₆:Eu³⁺ phosphor can emit bright red light around 613 nm. The optimal doping concentration of Eu³⁺ ions was found to be 20 mol%, and the integrated emission intensity of Sr₂LaTaO₆:0.2Eu³⁺ phosphor was about three times higher than that of commercial Y₂O₂S:Eu³⁺ red phosphor. Furthermore, the CIE chromaticity coordinates of Sr₂LaTaO₆:0.2Eu³⁺ phosphors were measured as (0.650, 0.349), which were close to that of ideal red light (0.67, 0.33). The color purity can reach about 92%. In addition, these Sr₂LaTaO₆:0.2Eu³⁺ red phosphors show good thermal stability.

The experimental results not only showed that these novel red-emitting phosphors have great potential as a red phosphor for near-UV-pumped white LEDs but also verified that the double-perovskite compound can be a good host for Eu³⁺ luminescent materials. The research team will further investigate on Eu³⁺ doped double-perovskite phosphors to obtain luminescent materials with better performances in the near future.

稀土离子助力高显色性暖白光LED：高效双钙钛矿型红色荧光粉

Sr₂LaTaO₆:0.2Eu³⁺红色荧光粉的发射光谱和CIE色坐标

当前，能源开发利用引发的水污染、空气污染和资源枯竭等环境问题日益严峻，引发了人们的广泛关注。在电能的广泛用途中，仅照明一项就占全球用电量的约20%。提升照明用电的效率，将对资源保护带来不可估量的贡献。

与白炽灯和荧光灯等传统光源相比，白光发光二极管（LED）具有能源效率高、使用寿命长和环境友好等优点。因此，大力发展LED照明是节省电能和保护全球环境的一个重要解决方案。当今，广泛使用的白光LED通常由InGaN蓝光芯片与Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺)黄色荧光粉组合而成。然而，由于缺少红光组分，这些白光LED存在一定的局限性，如高色温和低显色指数。而基于近紫外光LED芯片与三基色（蓝色、绿色和红色）荧光粉组合的近紫外光激发白光LED可以发射出具有高显色指数的暖白光。

遗憾的是，目前无论工业或研究领域仍然缺少性能良好的可被近紫外光激发的LED用红色荧光粉，相关研究具有巨大的实际应用和科研价值。众所周知，Eu³⁺离子是无机荧光粉常用的发光离子，这是由于其⁵D₀→⁷F_J（J = 1, 2, 3, 4）跃迁可以发出明亮的红光。Eu³⁺离子的光学性能很大程度上受其所用基质材料的影响。近年来，分子式为 A₂BMO₆的双钙钛矿化合物已被广泛选作Eu³⁺离子的良好基质材料。

太原理工大学黄小勇教授领导的研究小组首次发现了一种新型高效的Eu³⁺离子掺杂Sr₂LaTaO₆双钙钛矿红色荧光粉。相关结果发表在Chinese Optics Letters第19卷第3期（Z. Zhang et al., High-brightness red-emitting double-perovskite phosphor Sr₂LaTaO₆:Eu³⁺ with high color purity and thermal stability [Invited]）。该研究小组采用高温固相法成功制备了一系列掺杂不同Eu³⁺浓度的Sr₂LaTaO₆:Eu³⁺红色荧光粉。

研究发现，Sr₂LaTaO₆:Eu³⁺荧光粉在峰值为394 nm的近紫外波长范围内表现出强烈的吸收，因此该荧光粉可用于近紫外激发的白色LED器件。并且，在394 nm激发下，Sr₂LaTaO₆:Eu³⁺荧光粉可发出波长为613 nm左右的明亮红光。研究还发现Eu³⁺离子的最佳掺杂浓度为20 mol％，同时Sr₂LaTaO₆:0.2Eu³⁺红色荧光粉的发光积分强度约为商业Y₂O₂S:Eu³⁺红色荧光粉的3倍。此外，Sr₂LaTaO₆:0.2Eu³⁺荧光粉的CIE色坐标为（0.650，0.349），接近理想红光的CIE色坐标（0.67，0.33），其色纯度可以达到约92％。同时，这些Sr₂LaTaO₆:0.2Eu³⁺红色荧光粉表现出良好的热稳定性。

实验结果表明，Sr₂LaTaO₆:Eu³⁺红色荧光粉可用于近紫外光激发的白光LED器件中，并进一步证明了双钙钛矿化合物可以作为Eu³⁺掺杂红色荧光粉的良好基质材料。该研究小组将进一步对Eu³⁺掺杂双钙钛矿型发光材料进行深入研究，以期获得性能更优异的发光材料。