Preparation and Photoluminescence of a Novel White-Emitting Phosphor Sr1．5Ca0．5SiO4∶Eu3+，Tb3+，Eu2+

CHEN Xi，ZHAO Jun-feng，YU Li-ping，RONG Chun-ying，LI Cheng-zhi，LIAN Shi-xun

（Key Laboratory of Chemical Biology＆Traditional Chinese Medicine Research（Ministry of Education）; Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province; College of Chemistry and Chemical Engineering，Hunan Normal University，Changsha 410081，China）

Preparation and Photoluminescence of a Novel White-Emitting Phosphor Sr1．5Ca0．5SiO4∶Eu3+，Tb3+，Eu2+

CHEN Xi，ZHAO Jun-feng，YU Li-ping，RONG Chun-ying，LI Cheng-zhi，LIAN Shi-xun*

（Key Laboratory of Chemical Biology＆Traditional Chinese Medicine Research（Ministry of Education）; Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province; College of Chemistry and Chemical Engineering，Hunan Normal University，Changsha 410081，China）

A series of the nominal composition Sr1.5Ca0.5SiO4∶0.01 Eu3+，nTb3+（n=3.0×10－4，7.0×10－4，1.5×10－3mol）phosphors with single phase was synthesized by high temperature solid－state reaction.Upon the excitation by near－UV light（394 nm），the phosphors give blue，green and orange－red emissions which are originated from Eu2+5d→4f，Tb3+5D4→7FJand Eu3+5D0→7FJtransitions，respectively，with comparable intensities，resulting in white light emission.The results of CIE calculation show that the Sr1.5Ca0.5SiO4∶0.01 Eu3+，7.0×10－4Tb3+phosphor can provide white emission（x=0.321，y= 0.322）very close to the standard white（x=0.33，y=0.33），indicating it is a potential single－phased white phosphor for LED－based UV－chip.

Sr1.5Ca0.5SiO4;white－emitting phosphor;LED

The white light－emitting diodes（LEDs）composed of InGaN blue LEDs and a yellow YAG:Ce phosphor havebeen investigated extensively due to their applications［1］.But this kind of white light has a low color rendering index（Ra＜80）.As the near－ultraviolet（UV）or ultraviolet chip has been applied in consumer，white light generation has been proposed to combine near－UV or ultraviolet LEDs with a single－phased white－light（SPWL）phosphor.Therefore，SPWL phosphors［2－9］have been an active research area in the study of luminescent materials.In this letter，we demonstrate that it is possible to obtain SPWL phosphor via chemical approaches with appropriate tuning of activator contents of Eu3+，Tb3+and Eu2+ions in Sr1.5Ca0.5SiO4host lattices.

A series of phosphors whose design the nominal composition is Sr1.5Ca0.5SiO4∶0.01 Eu3+，nTb3+（n=3.0× 10－4，7.0×10－4，1.5×10－3mol）was synthesized by a two－stage solid－state reaction.The well－mixed starting materials were placed in the tube furnace and calcined at 1 200℃in the air for 4 h，then，the obtained powders were fired at the same temperature in reducing atmosphere（5%H2and 95%N2）for 0.5～1.0 h in order for Eu3+ions in the host lattice to be partially reduced to Eu2+to obtain a final product of Sr1.5Ca0.5SiO4∶Eu3+，Tb3+，Eu2+（abbreviated as SCS∶ETE）.The crystalline phase of the prepared samples was examined by the XRD，which indicated only a single phase belonging to Sr1.5Ca0.5SiO4.

As shown in Fig.1，under the excitation of 394 nm，the emission spectra of these SCS∶ETE phosphors exhibited three emission bands:one broad band in the blue area，a second band with sharp lines peaked in green（about 548 nm），and the third band in the orange－red region（588～720 nm）.These bands are originated from Eu2+5d→4f，Tb3+5D4→7FJand Eu3+5D0→7FJtransitions，respectively，with comparable intensities，which resulted in white light emission.With the increasing of Tb3+content，both of broad Eu2+emission and sharp Eu3+emission increase.

The white emission can be further confirmed by the CIE（Commission International de I′Eclairage 1931）chromaticity coordinates for the emission spectra of SCS∶ETE.Fig.2 showed the ca.CIE chromaticity coordinates of phosphors vary from cold white to warm white（black dots a→c）with changing of Tb3+concentration（n）from 3.0× 10－4～1.5×10－3mol;when n=7.0×10－4mol，phosphor gives the pure white emission（black dot b:x=0.321，y= 0.322），which is very close to the standard white（x=0.33，y=0.33）.The nominal composition Sr1.5Ca0.5SiO4∶0.01 Eu3+，7.0×10－4Tb3+is found to be the optimal composition for single－phased white－light phosphor.

Fig．1Photoluminescence spectra of Sr1.5Ca0.5SiO4∶0.01 Eu3+，n Tb3+phosphors at different Tb3+concentration excited by 394 nm（a→c:n=3.0×10－4，7.0×10－4，1.5×10－3mol）.

Fig．2The corresponding CIE chromaticity diagram of Sr1.5Ca0.5SiO4∶0.01 Eu3+，n Tb3+phosphors（dots a→c are agreement with n=3.0×10－4，7.0×10－4，1.5×10－3mol）.

The luminescent mechanism of SCS∶ETE can be illustrated by the energy levels.The excited electrons from the7F0→5L6transition of Eu3+at 394 nm yields the emission spectrum（Fig.1），which consists of all the emission sharp lines from the5D0excited state to the7FJground state of Eu3+，i．e．，5D0→7F0（578 nm），5D0→7F1（587 nm），5D0→7F2（612 nm），5D0→7F3（650 nm），5D0→7F4（700 nm）;from the5D4excited state to the7FJground state of Tb3+，i．e．，5D4→7F5（544 nm），and a broad band range of 400～540 nm centered at 462 nm from the 5d excited state to the8S7/2ground state（4f→5d）of Eu2+.The above emissions have convincingly shown that white light emission of SCS∶ETE is originated from the combination of Eu3+，Tb3+，Eu2+multicolor emitting.

On the other hand，the no－resonance energy transfer process in the system of SCS∶ETE will occur from 5d level of excited Eu2+ions to5D4level of Tb3+ion，enhancing5D4→7F5emitting.The energy transfer efficiency（ηT） increases with increasing Tb3+concentration，which is the same as Tb3+and Eu2+co－doping with（Sr，Ba）2SiO4phosphor［9］.In addition，the charge transfer（CT）process from Tb3+to Eu3+has been discussed by many studies［10－13］and can be explained as the following equation:Eu3++Tb3+=Eu2++Tb4+.The CT process holds the equilibrium of the concentration of Tb3+，Eu3+and Eu2+ions in the Ca0.5Sr1.5SiO4host lattice.Therefore，we are sure that tunable white－light emission was resulted from the system of SCS∶ETE as a result of the competition and equilibrium between these two processes when the Tb3+concentration varies.

In conclusion，a series of phosphors Sr1.5Ca0.5SiO4∶Eu3+，Tb3+，Eu2+has been prepared by a two－step hightemperature solid state process.Under the excitation of 394 nm，these emission spectra consist of a broad blue band，a sharp green line and some sharp orange－red lines，which correspond to Eu2+5d→4f transitions，Tb3+5D4→7FJand Eu3+5D0→7FJtransitions，respectively.The white emission（x=0.321，y=0.322）provided by a phosphor with nominal composition Sr1.5Ca0.5SiO4∶0.01 Eu3+，7.0×10－4Tb3+is very close to the standard white，which shows it is a potential single－phased white－light phosphor for UV－light emitting diodes.

This work is partially supported by NSFC（Grant Nos.20971042，50772035），and the Science and Technology Office of Education Department of Hunan Province（No.10A070）.

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一種新型白光熒光粉Sr1．5Ca0．5SiO4∶Eu3+，Tb3+，Eu2+的合成與光致發光

陳茜，趙君風，余麗萍，榮春英，李承志，廉世勛*

（化學生物學和中藥研究教育部重點實驗室;資源精細化與先進材料湖南省高校重點實驗室;湖南師范大學化學化工學院，中國長沙 410081）

采用高溫固相法合成了名義組成為Sr1.5Ca0.5SiO4∶0.01 Eu3+，nTb3+（n=3.0×10－4，7.0×10－4，1.5× 10－3mol）的熒光粉.X射線衍射測試表明熒光粉樣品為單一物相.在紫外光（394 nm）激發下，樣品同時產生藍光、綠光和紅橙光發射，分別對應于Eu2+離子的5d→4f，Tb3+離子的5D4→7FJ和Eu3+離子的5D0→7FJ躍遷，表明部分Eu3+離子在還原氣氛下被還原成Eu2+.紅光、綠光和藍光發射強度相當，復合得到白光.色坐標（CIE）計算結果顯示，熒光粉Sr1.5Ca0.5SiO4∶0.01 Eu3+，7.0×10－4Tb3+的白色發光（CIE:x=0.321，y=0.322）接近純白色（CIE: x=0.33，y=0.33），表明它是一種很有應用前景的基于紫外光芯片的單基白光熒光粉.

Sr1.5Ca0.5SiO4;白光發射熒光粉;發光二極管

2011－01－18

國家自然科學基金資助項目（20971042;50772035）;湖南省教育廳基金資助項目（10A070）

*通訊作者，E－mail:lianshixun@yahoo.com.cn

（編輯楊春明）