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Hidden Structural Evolution and Bond Valence Control in Near-infrared Phosphors for Light-emitting Diodes
M.-H. Fang, K.-C. Chen, N. Majewska, T. Leśniewski, S. Mahlik, G. Leniec, S. M. Kaczmarek, C.-W. Yang, K.-M. Lu, H.-S. Sheu, and R.-S. Liu* 
We aim to conduct a complete study on the unexpected structure evolution behavior in Cr3+-doped phosphors. A series of Ga2–xScxO3:Cr3+ phosphors are successfully synthesized and confirmed through structural studies, while the lattice parameters change unexpectedly. The unique partial substitution (∼87%) of Sc3+ in the octahedral site is demonstrated via Rietveld refinement. Therefore, the bond valence sum calculation explains the reason for this particular Sc3+ concentration. The photoluminescent bandwidth and electron–lattice coupling energy initially increase and then decrease, implying an inhomogeneous broadening effect. Time-resolved spectra and electron paramagnetic resonance are utilized to further examine the subtle change in the microstructures and the second coordination sphere effect of Cr3+. Ga1.594Sc0.4O3:0.006Cr3+ exhibits high internal quantum efficiency (99%) and high phosphor-converted light-emitting diode output power (66.09 mW), demonstrating its capability as an outstanding infrared phosphor. This work will motivate further research on unexpected partial substitution during the solid solution process.

近年來,以螢光粉轉換之近紅外線發光二極體(LED)被大量應用於各種日常生活中,例如電動車。主要原因是這類螢光粉具有高輸出功率、體積小、高效能等優點。本研究以鉻三價(Cr3+)離子摻雜入不同比例之氧化鎵(Ga)與氧化鈧(Sc)之固溶體系,Ga2−xScxO3:Cr3+。Ga在晶體裡有兩個不同格位,從同步輻射X光粉末繞射搭配Rietveld結構精算結果,發現Sc進入氧化鎵結構中,且具有明顯之擇優取代現象,只有很少量Ga1被Sc取代,當Sc量增加時只會進入Ga2位置,並且當Sc取代量達到約87%時即無法再增加。我們以鍵價總和法(bond valence sum; BVS)計算配位環境之變化,發現當Sc之量達到87%時,Ga與Sc之價數皆到達最佳理論值(3+)。此外,隨著Sc摻入的比例增加,光譜半高寬與聲電作用能量皆呈現先升後降,顯示結構由有序轉為無序後再轉為有序,此一過程亦以電子自旋光譜驗證。最後,進行實際之發光二極體封裝實驗,發現最高放光效率於350 mA之驅動電流下,輸出功率為66.09 mW且半高寬僅為160 nm,具有很高市場價值。