白光LED用钼酸盐红色荧光粉的制备及发光性能研究
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摘要
白光LED由于其具有节能、环保、长寿命、体积小等优异的特点而引起人们的广泛关注,被称为第四代照明光源。荧光粉涂敷光转变法获取白光LED已经成为了发展的主流,同时也给荧光粉的发展带来了新的空间。
传统白光LED若要完全取代荧光灯成为新一代室内照明光源,除需要降低成本外,还需要白光LED具有更高的流明效率、更高的显色指数以及白光发射的色温具有更好的可调节性等。此外,已经商业化的由蓝光LED+黄色荧光粉组合发光的白光LED本身也存在一些缺陷,如：在高的电流下蓝光光谱的电光强度要比长波长的光即黄光增加得快,随着电流的改变就会导致光谱的不匹配从而很容易导致色温的改变和低的显色指数。而紫外和近紫外系统则不存在以上的情况,紫外和近紫外体系的白光LED有成本较低、颜色的控制较蓝光LED容易得多、色彩均匀度极佳、显色性好等优点。因此,紫外和近紫外体系的白光LED取代传统蓝光LED+黄色荧光粉系统也是白光LED发展的趋势。当前,用于近紫外InGaN基LED的三基色商用荧光粉主要是：红色荧光粉Y2O2S:Eu3+,绿色荧光粉ZnS:(Cu+,A13+)和蓝色荧光粉BaMgAl10O17:Eu2+。然而,红色荧光粉Y2O2S:Eu3+在近紫外范围不能有效的吸收,其相对发光亮度值只有商用蓝色荧光粉和绿色荧光粉发光亮度值的八分之一；为了合成理想的白光和形成较好的色纯度,所需要红、绿、蓝荧光粉的质量分别为8：1：1。此外,在近紫外光的激发下红色荧光粉Y2O2S:Eu3+性能不稳定、寿命不长。红色荧光粉Y2O2S:Eu3+这些不足已经成为制约白光LED发展的主要瓶颈。因此,研究新型的能与紫外(近紫外)LED相匹配的红色荧光粉具有重要的理论和现实意义。
本论文系统的研究了现有红色荧光粉CaMoO4:Eu3+体系的制备方法和发光性能,并通过稀土离子共掺以及引进Si原子替代部分基质Mo原子来提高其发光强度；新合成了三种可被紫外(近紫外)或蓝光激发的新型钼酸盐红色荧光粉体系,较系统地研究了它们的制备方法和光谱性能等,并得出以下相关研究结论：
1.红色荧光粉CaMoO4:Eu3+体系发光性能研究
系统的研究了高温固相法制备CaMoO4:Eu3+体系的工艺条件,确定了CaMoO4:Eu3+制备的最佳条件是煅烧温度为900℃保温3小时,并确定了CaMoO4:Eu3+中Eu3+的最佳掺杂浓度为25at.%,把Eu3+的掺杂浓度从20at.%(业界普遍认为在CaMoO4中Eu3+的最大掺杂浓度是20at.%)提高到了25at.%具有重要的积极意义,即在钼酸钙体系中激活剂Eu3+的掺杂浓度提高发光强度随着增强。系统的探讨了电荷补偿剂(Li+,Na+,K+)对CaMoO4:Eu3+发光性能的影响,发现补偿剂对提高CaMoO4:Eu3+的发光强度是按Li+>Na+>K+顺序。虽然Li+作为电荷补偿剂时样品发光强度最大,但是样品硬度较高,所以综合考虑选择Na+作为电荷补偿剂为最佳。
使用Bi3+作为共激活剂,成功合成了红色荧光粉系列Ca0.5MoO4:Eu0.25-x3+,Bix3+,Na0.25+。实验结果显示该系列荧光粉能被UV(396 nm)光和蓝光(467 nm)有效激发,发射出波峰位于615 nm的红光。在Eu3+-Bi3+共掺CaMoO4中,存在从Bi3+到Eu3+的能量传递,适量的Bi3+引进到Ca0.5MoO4:Eu0.253+,Na0.25+中能使得Eu3+(5D0→7F2)的发射强度提高43%。在本实验中同时发现激活剂离子Eu3+-Bi3+之间发生能量迁移的可能与Bi3+的掺杂浓度有很密切的关系。Bi3+的最佳掺杂浓度为1at.%。
利用高温固相法制备了Eu3+和Sm3+共掺的钼酸钙(CaMoO4)荧光粉。研究发现该系列荧光粉在近紫外光范围内有很强的能量吸收,并发射出位于615 nm处的红光,非常合适白光LED制造。此外,通过引入少量的Sm3+可以增加406 nm处的激发光谱强度,同时可以加宽400 nm处的激发光谱。文章分别通过样品发光光谱和发光寿命证明了Sm3+对Eu3+的敏化作用；实验结果显示适量的Sm3+引进不但能提高样品发光强度而且可以提高色纯度,样品的最佳掺杂量是0.5at.%。
在CaMoO4:Eu3+体系中首次引进Si取代部分Mo原子的位置来达到提高发光强度的目的,实验结果显示Si的加入并没有影响激发和发射光谱的形状和发射峰的位置。随着引入适量的Si4+样品发射强度明显提高,主要是因为改变了Eu3+发光中心的次晶格对称而释放了Eu3+的f-f跃迁。此外,实验还发现当Si4+的掺杂含量超过2at.%时,样品的颗粒度明显增大,这与基质含Si原子的荧光粉样品粒径偏大的现象相同。综合考虑当Si4+的掺杂含量为1at.%时,样品的发光强度达到最大值同时平均粒径2.15μm为最佳。样品Ca0.5Mo0.99Si0.01O4:Eu0.253+,Na0.25+色坐标值明显比未掺杂Si4+的样品Ca0.5MoO4:Eu0.253+,Na0.25+的色坐标值更加接近美国国家电视标准委员会(National Television System Committee,缩写为NTSC)标准值。所有实验结果显示该系列荧光粉非常适合于白光LED制造。
2.一种新型白光LED用红色荧光粉Na0.5Gd0.5MoO4:Eu3+体系发光性能研究
采用高温固相法成功合成了一种与CaMoO4同构的新型钼酸盐红色荧光粉Na0.5Gd0.5MoO4:Eu3+体系。实验结果显示在Eu3+激活的Na0.5Gd0.5MoO4体系中,不发生Eu3+浓度猝灭。即Na0.5Gd0.5-xMoO4:Eux3+的发光强度随着掺杂Eu3+浓度的增加而增强。当Gd3+完全被Eu3+取代时样品的发光强度达到最大值,此时样品的相对发光亮度分别是Ca0.80MoO4:Eu0.203+和传统商用红粉Y2O2S：Eu3+的2.26和6倍。
引入适量的Li+取代部分的Na+在荧光粉Na0.5Eu0.5MoO4中能明显提高粉体发光强度。Li+的最佳掺杂浓度是25at.%,样品Na0.25Li0.25Eu0.5MoO4的发光强度比未掺杂Li+的样品的相对亮度提高了37%,其亮度值更是传统商用红粉Y2O2S:Eu3+发光亮度的8.2倍；同时其色坐标值(x=0.654,y=0.346)比商用红粉Y2O2S:Eu3+的色坐标值(x=0.631,y=0.350)更加接近NTSC标准值。主要原因是因为引入适量的Li+到Na0.5Eu0.5MoO4中是能够改变Eu3+发光中心的次晶格结构使得Eu3+远离反演对称中心而释放更多的电偶极跃迁来提高发光强度。此外,样品Na0.5Eu0.5MoO4和Na0.25Li0.25Eu0.5MoO4的荧光寿命τ值分别为0.375和0.416 ms,同时发现添加适量的Li+,样品的荧光寿命τ值也随着增加。所有实验结果表明Na0.25Li0.25Eu0.5MoO4是一种高效的红色荧光粉适用于三基色荧光粉系统白光LED制造或可以作为“蓝光LED+黄色荧光粉系统”的红光补偿粉。
3.钼钨酸盐红色荧光粉的制备及光谱性能研究
合成了钾铕双钼酸盐荧光粉KEu(MoO4)2并通过引进钨酸来提高发光强度,确定了钾铕双钼酸盐荧光粉KEu(MoO4)2制备的最佳工艺条件是煅烧温度为750℃保温5小时。在样品KEu(MoO4)2中,引入WO42-的最佳掺杂量是50at.%即摩尔量的比MoO42-/WO42-=3/1结构式表达为KEu(WO4)0.5(MoO4)1.5。系统的分析比较了荧光粉MEu(WO4)0.5(MoO4)1.5(M=Li, Na, K)的发光性能,发现样品的发射强度随着碱金属离子半径的增大而减小(碱金属离子半径排序：K+>Na+>Li+),综合各方面因素考虑碱金属和铕双钼钨酸盐的最佳结构表达式选择为：LiEu(WO4)0.5(MoO4)1.5。
合成了一种新型白光LED用红色荧光粉LiEu1-xYx(WO4)0.5(MoO4)1.5(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.8)系列,确定了Y3+掺杂浓度最佳值x为0.5,同时研究表明该系列荧光粉可以被近紫外光(396 nm)和蓝光(466 nm)有效激发,发射峰值位于615 nm(Eu3+离子的5D0→7F2跃迁)的红光,并且证明了Eu3+离子在晶体结构中占据了非反演对称中心的位置,激发波长与目前广泛使用的蓝光和紫外光LED芯片相符合,适用于白光LED的制造。此外,研究了助熔剂对该体系荧光粉发光特性的影响。XRD的测量结果表明加入适量的助熔剂有利于LiEu1-xYx(WO4)0.5(MoO4)1.5荧光粉的结晶化,并且不引入杂相。适量的助熔剂的加入可增大该系列荧光粉的相对发光强度,并能有效降低荧光粉的平均粒径,确定了在制备LiEu0.5Y0.5(WO4)0.5(MoO4)1.5荧光粉的焙烧过程中按1：1添加占总质量1%的AlF3和H3B03作为助熔剂时荧光粉的相对发光强度最大,平均粒径较小综合效果最佳。
合成了一系列新型红色荧光粉LiEu1-xBix(WO4)0.5(MoO4)1.5(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.8)并详细研究了它们的发光性能。样品的最强发射峰位于615 nm处对应于Eu3+的5D0→7F2跃迁；实验结果表明在LiEu(WO4)0.5(MoO4)1.5中加入适量的Bi3+,不但可以提高荧光粉的发光强度,而且有利于改善样品的色纯度。Bi3+的最佳掺杂浓度为10at.%,当Bi3+浓度超过最佳值时,Bi3+-Bi3+之间形成团聚且吸收的能量也通过非辐射跃迁损失,导致Bi3+对Eu3+的敏化作用明显减小。计算出不同Bi3+浓度x=0.05,0.1,0.15,0.2,0.3,0.4,0.5时Eu3+-Bi3+之间的能量传递平均距离RBi→Eu分别为22.3,23.7,24.9,26.1,28.1,29.9,31.4A。并确定了能量传递的临界距离(Rc)约为27.1 A。
4.一种新型白光LED用三钼酸盐红色荧光粉LiKGd2(MoO4)4:Eu3+体系发光性能研究
采用高温固相法成功合成了一系列新型三铝酸盐红色荧光粉LiKGd2-xEux(MoO4)4。确定了三铝酸盐红色荧光粉LiKGd2(MoO4)4:Eu3+制备的最佳工艺条件是煅烧温度为850℃保温5小时。采用“绝热法”对目标样品做了半定量分析,计算出物相质量分数证明了所制备样品为纯相。为半定量分析样品相纯度提供了一种新的实用的方法。
荧光粉LiKGd2(MoO4)4:Eu3+在紫外(396 nm)和蓝光(466 nm)区域有两个很强的吸引峰分别能与近紫外光LED芯片和蓝光LED芯片发射很好的匹配,并发射出峰值位于615 nm的红光。实验结果显示：Eu3+掺杂LiKGd2(MoO4)4基质的最佳浓度值x为1.1,经比较发现样品LiKGd0.9(MoO4)4:Eu1.13+的发光强度分别是Ca0.80MoO4:Eu0.203+和商用红粉Y2O2S:Eu3+发光强度的1.40和3.68倍。而且荧光粉LiKGd0.9(MoO4)4:Eu1.13+的色坐标值比传统商用红粉Y2O2S:Eu3+的色坐标值更加接近NTSC标准值。基于Dexter的多极相互作用能量传递公式和Reisfeld提供的近似值原理,详细分析了荧光粉LiKGd2-xEux(MoO4)4系列中Eu3+离子之间的能量传递是电偶极-偶极相互作用,并按临界无辐射能量传递距离公式计算得出Eu3+离子之间能量传递临界距离Rc=8.24A。所有实验结果表明三钼酸盐LiKGd0.9(MoO4)4:Eu1.13+是一种高效的红色荧光粉,适用于三基色荧光粉系统白光LED制造或可以作为“蓝光LED+黄色荧光粉系统”的红光补偿粉。
White light-emitting diodes (LEDs), known as the fourth generation solid-state lighting, have many advantages over the existing incandescent and halogen lamps in reliability, energy saving, long life, small size, maintenance and safety, and therefore are gaining lots of attentions. Today, the white LEDs market is dominated by phosphor-converted LEDs (pcLEDs), which brings new prospect to the development of phosphors.
To completely replace fluorescent lamp to be a new generation indoor illumination source, it is necessary that the traditional white LED lighting possesses lower costs, higher luminous efficiency and color rendering index and better adjustability in color temperature of white light emitting and so on. There exist some defect in the commercialized white LEDs by using pcLEDs comprising a blue emitting InGaN semiconductor (420-480 nm) coated with yellow phosphor (YAG:Ce), such as electro-optical intensity of blue spectra increases faster than that of yellow light at high current, the current changes can engender spectrum mismatch, consequently result in the changes of color temperature and low color rendering index. The above circumstances are absent in the ultraviolet (UV) and near-UV LEDs systems. Since the conversion of UV and near-UV LEDs has the virtue of relative lower cost, more easily controlling color than the blue LED, excellent color uniformity, good color rendering etc, it is also a development tendency of white LEDs that UV and near-UV LEDs systems supplant the traditional white pcLEDs operating on the basis of blue InGaN dies. The current commercially applicable red phosphor for UV InGaN-based LEDs is Y2O2S:Eu3+ However, the Y2O2S:Eu3+ red phosphor cannot efficiently absorb in near-UV region and its brightness is about eight times less than that of the blue (BaMgAl10O17:Eu2+) and green (ZnS:(Cu+, Al3+)) phosphors. In addition, the lifetime of the Y2O2S:Eu3+is inadequate under near-UV irradiation for its instability. The drawback of red emitting phosphors has been the main obstacle for the development of white LEDs. Therefore, the study on the red phosphors matching to UV (near-UV) LED is of great significance in theory and practice.
In this work, the preparation methods and luminescent properties of the existing red emitting phosphors CaMoO4:Eu3+ systems were systematically investigated, and their light-emitting properties were improved by co-doping rare earth ions. Three types of novel molybdate red emitting phosphors excited by near-UV or blue light were synthesized, whose synthesis method and spectral characteristics were researched in detail. The major research contents and appropriate conclusions are as following:
1. Synthesis and photoluminescence properties of red emitting phosphors CaMoO4:Eu3+ for
white LEDs
The technological conditions for preparation of red emitting phosphors CaMoO4:Eu3+ by traditional solid state reaction were investigated. The optimum calcination temperature and the holding time were determined to be 900℃and 3 hours, respectively. And the superlative Eu3+-doped concentration in red phosphors CaMoO4:Eu3+ was concluded to be 25at.%. It was important and positive to improve doping concentration of Eu3+ from 20at.% to 25at.%. Namely, the luminescence intensity increased with the increase of doping concentration of Eu3+ in the molybdenum calcium system. The effect of charge compensator (Li+, Na+, K+) on the luminescence behavior of Eu3+ activated CaMoO4 phosphor was discussed in detail. The relative intensity decreases with the increases of the ionic radii of alkali metal ions, which are as charge compensator (the ionic radii in the order of Li+ Through the use of Bi as a co-activator, red emitting phosphors Cao.5Mo04:Eu3+0.25-x, Bi3+x, Na+0.25 were synthesized by conventional solid state reaction method. The photo-luminescent results show that all samples can be excited efficiently by UV (396 nm) and blue (467 nm) light and emit the red light at 615nm with line spectra, In the Eu3+-Bi3+ co-doped system, there is an efficient energy transfer from Bi3+ to Eu3+ ions, leading to that the emission intensity (5D0→7F2) is enhanced by 1.43 times and even more when Bi3+ ions are introduced into Cao.5Mo04:Eu3+ 0.25, Na+0.25.The energy transfer probability is strongly dependent upon the Bi3+ doping concentrations. The optimum doping concentration of Bi3+ is found to be 1 at.%
The Eu3+ and Sm3+ co-doped calcium molybdenum (CaMoO4) phosphors were prepared by solid state reaction. The results show that the series phosphors can strongly absorb energy in the range of near UV and emit red light at 615 nm, nicely fitting in with the widely applied output wavelengths of ultraviolet or blue LED chips. In addition, the introduction of a small amount of Sm3+ can enhance the absorption intensity of excitation spectra at 406 nm and the excitation spectra locating near 400 nm become broader. The sensitization effect of Sm3+ on Eu3+ was proved through emission spectroscopy and luminescence lifetime in our paper, respectively. The experimental results indicate that the introduction of appropriate amount of Sm3+ can not only enhance emission intensity but also improve the color purity. The optimal doping volume was 0.5at.%.
For the first time, introducing Si atoms replaced part of the location of Mo to achieve the purpose of improving luminous intensity in CaMoO4:Eu3+. The results show that the addition of Si does not affect the shapes of excitation and emission spectra and the position of emission peak. With the introduction of appropriate amount of Si4+, the emission intensity of samples obviously increases. It was the reason that the sub-lattice symmetry of Eu3+luminescence center was changed and the f-f transitions of Eu3+ were further released. Moreover, it can be found that when the doping content of Si4+ was beyond 2at.%, the particle size of the samples increased significantly, which was the same to the large size phenomenon of silicate phosphors. Given consideration to every aspect, the optimum content of Si4+ is 1at.%, when the luminous intensity of the sample reached the maximum and the average diameter was 2.15μm. The color coordinate values of Ca0.5Mo0.99Si0.01O4:Eu3+0.25, Na+0.25 were closer to the standard value of the U.S. National Television Standards Committee (National Television System Committee, abbreviated as NTSC) than that of Ca0.5MoO4:Eu3+0.25, Na+0.25.All the experimental results showed that this series phosphors were very suitable for white LED manufacturing.
2. Luminescent properties of a novel red phosphor Na0.5Gd0.5Mo04:Eu3+ for white LEDs
The novel red phosphors Na0.5Gd0.5MoO4:Eu3+ with the similar crystal structure to CaMoO4 was successfully synthesized by the high temperature solid phase reaction. The results show that concentration quenching does not occur in the Eu3+-activated Na0.5Gd0.5MoO4 system, namely the luminous intensity of Na0.5Gd0.5-xMoO4:Eu3+ increases with the increase of the doping concentration of Eu3+. When the Gd3+ was completely replaced by Eu3+, luminescence intensity of the samples reached the maximum, at this time, the relative brightness of samples was 2.26 and 6 time higher than that of Ca0.80MoO4:Eu3+0.20 and traditional commercial Y2O2S:Eu3+ respectively.
The introduction of appropriate amount of Li+ replacing part of the Na+ in the phosphor Na0.5Eu0.5MoO4 can markedly improve the luminous intensity. The optimal doping concentration of Li+ is 25at.%, when the relative luminous intensity of Na0.25Li0.25Eu0.5MoO4 increased by 37% of Na0.5Eu0.5MoO4, the brightness value is 8.2 times bigger than that of traditional commercial red phosphors Y2O2S:Eu3+. In the meantime, the color coordinates (x=0.654, y= 0.346) were closer to the NTSC standard values than those of commercial red phosphors Y2O2S: Eu3+(x=0.631, y=0.350). The chief reason is that the introduction of appropriate amount of Li+ into the Na0.5Eu0.5MoO4 is able to alter the sub-lattice structure of Eu3+ luminescence centers and make the Eu3+ far away from the inversion center of symmetry, so that more electric dipole transitions are released to enhance the luminous intensity. Also, the fluorescence lifetimeτvalues of Na0.5Eu0.5MoO4 and Na0.25Li0.25Eu0.5MoO4 were 0.375 and 0.416 ms respectively. With the introducing appropriate Li+ content, the samples lifetimeτvalues increase. All experimental results showed that Na0.25Li0.25Eu0.5MoO4 was a high efficient red phosphor and suited to the 'UV chip+ tri basic color' system or the complement light phosphors in the'blue chip+ yellow phosphor' system.
3. Preparation and spectral properties of molybdenum-tungstate red emitting phosphors
The red phosphors KEu(MoO4)2 were synthesized and its luminous intensity was enhanced through the introduction of tungstate radical. The optimum technological condition of preparation of KEu(MoO4)2 was calcination at 750℃for 5 hours. The best doping amount of WO42- was 50at.%, namely molar ratio of MoO42-/WO42- was 3/1, and the structural formula is KEu(WO4)0.5(MoO4)1.5. Comparison and analysis of luminescence properties of MEu(WO4)0.5(MoO4)1.5(M=Li, Na, K) had been done, the result indicated that the emission intensity of phosphor MEu(WO4)0.5(MoO4)1.5 (M=Li, Na, K) decreased with increasing radius of alkali metal ions (alkali metal ions radius sort:K+> Na+> Li+). Taking all factors into account, the best expression was LiEu(WO4)0.5(MoO4)1.5.
A series novel red phosphors LiEu1-xYx (WO4)0.5 (MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were prepared, and the best doping concentration of Y3+ was determined to be 0.5mol. The series phosphor can be excited by near-UV(396 nm) and blue (466 nm) and emit red light at 615 nm (5D0→7F2 transition of Eu3+), nicely fitting in with the widely applied output wavelengths of ultraviolet or blue LED chips. Our reserach also proves that the Eu3+ occupies the lattice site of noncentrosymmetric environment in the scheelite phases. All the results indicate that the red phosphor is a suitable candidate of red emitting phosphor for the fabrication of white LEDs.
In addition, the effects of the flux on the luminescent properties were studied. The results of the XRD patterns indicated that the proper flux was beneficial to the crystallization of the phosphor, and no other phases were formed. Proper addition amount of flux could improve the relative luminosity and decrease the particle size. Experiments confirmed that the phosphor had the optimal comprehensive properties when the addition amount of flux (WAlF3/WH3BO3=1/1) was 1%.
A series of novel red phosphor LiEu1-xBix(WO4)0.5(MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were synthesized and their luminescence properties were studied in detail. The emission spectra show the most intense peak is located at 615 nm, which corresponds to the 5D0→7F2 transition of Eu3+. The experimental results show that adding appropriate amount of Bi3+ in the LiEu(WO4)0.5(MoO4)1.5 can not only increase the luminescence intensity of phosphor but also improve the color purity. The optimum doping concentration of Bi3+ was found to be 10at.%. For higher concentration of Bi3+, the absorbed energy is nonradiatively dissipated due to the formation of Bi3+ aggregates, which results in an evident reduction of the sensitized effectiveness of Bi3+ ions on Eu3+ ions. The average separation RBi→Eu (in A) of energy transfer is determined to be 22.3,23.7,24.9,26.1,28.1,29.9, and 31.4 for Bi3+ concentration x=0.05,0.1,0.15,0.2, 0.3,0.4 and 0.5, respectively, in LiEu1-xBix(WO4)0.5(MoO4)1.5. The critical concentration xc, at which the luminescence intensity of Eu3+ is half that in the sample in the absence of Bi3+, is 0.45. Therefore, the critical distance (Rc) of energy transfer was calculated to be about 27.1 A.
4. A potential red emitting phosphors scheelite-like triple molybdates LiKGd2(MoO4)4:Eu3+ for white LEDs applications
A series of novel triple molybdates red emitting phosphors LiKGd2-xEux:(MoO4)4 were successfully synthesized through solid state reaction. The optimum condition was calcination at 850℃for 5 hours. A semi-quantitative estimation of the sample was investigated by adiabatic method. The mass fraction of phase was calculated, which proved that the prepared samples were pure phase. A new practical approach was provided to semi-quantitative analysis of phase purity of the sample.
Phosphor LiKGd2(MoO4)4:Eu3+ has two strong peaks in the UV (396 nm) and blue (466nm) region, which match to the near-UV LED chip and blue LED chip well, and emit red light at 615 nm. The results show the optimum doped concentration of Eu3+ in LiKGd2(MoO4)4 is 1.1. Comparing with Ca0.80MoO4:Eu3+0.20 and commercial red phosphors Y2O2S:Eu3+, the luminous intensity of LiKGdo.9(Mo04)4:Eu3+1.1 are 1.40 and 3.68 time higher, respectively. The color coordinates values of the phosphor LiKGdo.9(Mo04)4:Eu3+1.1 are closer to the NTSC standard values than those of the traditional commercial red phosphors Y2O2S:Eu3+. Based on Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the energy transfer among Eu3+ ions in the phosphor LiKGd2-xEux(MoO4)4 is governed by electric dipole-dipole interaction. The critical distance of energy transfer among Eu3+ ions was calculated to be Rc=8.24 A according to the distance formula of critical non-radiative energy transfer. All results show that the triple molybdate phosphors LiKGd0.9(MoO4)4:Eu3+1.1 is highly efficient red phosphors for the'UV chip+tri basic color' system or the complement light phosphors in the 'blue chip+yellow phosphor' system.
传统白光LED若要完全取代荧光灯成为新一代室内照明光源,除需要降低成本外,还需要白光LED具有更高的流明效率、更高的显色指数以及白光发射的色温具有更好的可调节性等。此外,已经商业化的由蓝光LED+黄色荧光粉组合发光的白光LED本身也存在一些缺陷,如：在高的电流下蓝光光谱的电光强度要比长波长的光即黄光增加得快,随着电流的改变就会导致光谱的不匹配从而很容易导致色温的改变和低的显色指数。而紫外和近紫外系统则不存在以上的情况,紫外和近紫外体系的白光LED有成本较低、颜色的控制较蓝光LED容易得多、色彩均匀度极佳、显色性好等优点。因此,紫外和近紫外体系的白光LED取代传统蓝光LED+黄色荧光粉系统也是白光LED发展的趋势。当前,用于近紫外InGaN基LED的三基色商用荧光粉主要是：红色荧光粉Y2O2S:Eu3+,绿色荧光粉ZnS:(Cu+,A13+)和蓝色荧光粉BaMgAl10O17:Eu2+。然而,红色荧光粉Y2O2S:Eu3+在近紫外范围不能有效的吸收,其相对发光亮度值只有商用蓝色荧光粉和绿色荧光粉发光亮度值的八分之一；为了合成理想的白光和形成较好的色纯度,所需要红、绿、蓝荧光粉的质量分别为8：1：1。此外,在近紫外光的激发下红色荧光粉Y2O2S:Eu3+性能不稳定、寿命不长。红色荧光粉Y2O2S:Eu3+这些不足已经成为制约白光LED发展的主要瓶颈。因此,研究新型的能与紫外(近紫外)LED相匹配的红色荧光粉具有重要的理论和现实意义。
本论文系统的研究了现有红色荧光粉CaMoO4:Eu3+体系的制备方法和发光性能,并通过稀土离子共掺以及引进Si原子替代部分基质Mo原子来提高其发光强度；新合成了三种可被紫外(近紫外)或蓝光激发的新型钼酸盐红色荧光粉体系,较系统地研究了它们的制备方法和光谱性能等,并得出以下相关研究结论：
1.红色荧光粉CaMoO4:Eu3+体系发光性能研究
系统的研究了高温固相法制备CaMoO4:Eu3+体系的工艺条件,确定了CaMoO4:Eu3+制备的最佳条件是煅烧温度为900℃保温3小时,并确定了CaMoO4:Eu3+中Eu3+的最佳掺杂浓度为25at.%,把Eu3+的掺杂浓度从20at.%(业界普遍认为在CaMoO4中Eu3+的最大掺杂浓度是20at.%)提高到了25at.%具有重要的积极意义,即在钼酸钙体系中激活剂Eu3+的掺杂浓度提高发光强度随着增强。系统的探讨了电荷补偿剂(Li+,Na+,K+)对CaMoO4:Eu3+发光性能的影响,发现补偿剂对提高CaMoO4:Eu3+的发光强度是按Li+>Na+>K+顺序。虽然Li+作为电荷补偿剂时样品发光强度最大,但是样品硬度较高,所以综合考虑选择Na+作为电荷补偿剂为最佳。
使用Bi3+作为共激活剂,成功合成了红色荧光粉系列Ca0.5MoO4:Eu0.25-x3+,Bix3+,Na0.25+。实验结果显示该系列荧光粉能被UV(396 nm)光和蓝光(467 nm)有效激发,发射出波峰位于615 nm的红光。在Eu3+-Bi3+共掺CaMoO4中,存在从Bi3+到Eu3+的能量传递,适量的Bi3+引进到Ca0.5MoO4:Eu0.253+,Na0.25+中能使得Eu3+(5D0→7F2)的发射强度提高43%。在本实验中同时发现激活剂离子Eu3+-Bi3+之间发生能量迁移的可能与Bi3+的掺杂浓度有很密切的关系。Bi3+的最佳掺杂浓度为1at.%。
利用高温固相法制备了Eu3+和Sm3+共掺的钼酸钙(CaMoO4)荧光粉。研究发现该系列荧光粉在近紫外光范围内有很强的能量吸收,并发射出位于615 nm处的红光,非常合适白光LED制造。此外,通过引入少量的Sm3+可以增加406 nm处的激发光谱强度,同时可以加宽400 nm处的激发光谱。文章分别通过样品发光光谱和发光寿命证明了Sm3+对Eu3+的敏化作用；实验结果显示适量的Sm3+引进不但能提高样品发光强度而且可以提高色纯度,样品的最佳掺杂量是0.5at.%。
在CaMoO4:Eu3+体系中首次引进Si取代部分Mo原子的位置来达到提高发光强度的目的,实验结果显示Si的加入并没有影响激发和发射光谱的形状和发射峰的位置。随着引入适量的Si4+样品发射强度明显提高,主要是因为改变了Eu3+发光中心的次晶格对称而释放了Eu3+的f-f跃迁。此外,实验还发现当Si4+的掺杂含量超过2at.%时,样品的颗粒度明显增大,这与基质含Si原子的荧光粉样品粒径偏大的现象相同。综合考虑当Si4+的掺杂含量为1at.%时,样品的发光强度达到最大值同时平均粒径2.15μm为最佳。样品Ca0.5Mo0.99Si0.01O4:Eu0.253+,Na0.25+色坐标值明显比未掺杂Si4+的样品Ca0.5MoO4:Eu0.253+,Na0.25+的色坐标值更加接近美国国家电视标准委员会(National Television System Committee,缩写为NTSC)标准值。所有实验结果显示该系列荧光粉非常适合于白光LED制造。
2.一种新型白光LED用红色荧光粉Na0.5Gd0.5MoO4:Eu3+体系发光性能研究
采用高温固相法成功合成了一种与CaMoO4同构的新型钼酸盐红色荧光粉Na0.5Gd0.5MoO4:Eu3+体系。实验结果显示在Eu3+激活的Na0.5Gd0.5MoO4体系中,不发生Eu3+浓度猝灭。即Na0.5Gd0.5-xMoO4:Eux3+的发光强度随着掺杂Eu3+浓度的增加而增强。当Gd3+完全被Eu3+取代时样品的发光强度达到最大值,此时样品的相对发光亮度分别是Ca0.80MoO4:Eu0.203+和传统商用红粉Y2O2S：Eu3+的2.26和6倍。
引入适量的Li+取代部分的Na+在荧光粉Na0.5Eu0.5MoO4中能明显提高粉体发光强度。Li+的最佳掺杂浓度是25at.%,样品Na0.25Li0.25Eu0.5MoO4的发光强度比未掺杂Li+的样品的相对亮度提高了37%,其亮度值更是传统商用红粉Y2O2S:Eu3+发光亮度的8.2倍；同时其色坐标值(x=0.654,y=0.346)比商用红粉Y2O2S:Eu3+的色坐标值(x=0.631,y=0.350)更加接近NTSC标准值。主要原因是因为引入适量的Li+到Na0.5Eu0.5MoO4中是能够改变Eu3+发光中心的次晶格结构使得Eu3+远离反演对称中心而释放更多的电偶极跃迁来提高发光强度。此外,样品Na0.5Eu0.5MoO4和Na0.25Li0.25Eu0.5MoO4的荧光寿命τ值分别为0.375和0.416 ms,同时发现添加适量的Li+,样品的荧光寿命τ值也随着增加。所有实验结果表明Na0.25Li0.25Eu0.5MoO4是一种高效的红色荧光粉适用于三基色荧光粉系统白光LED制造或可以作为“蓝光LED+黄色荧光粉系统”的红光补偿粉。
3.钼钨酸盐红色荧光粉的制备及光谱性能研究
合成了钾铕双钼酸盐荧光粉KEu(MoO4)2并通过引进钨酸来提高发光强度,确定了钾铕双钼酸盐荧光粉KEu(MoO4)2制备的最佳工艺条件是煅烧温度为750℃保温5小时。在样品KEu(MoO4)2中,引入WO42-的最佳掺杂量是50at.%即摩尔量的比MoO42-/WO42-=3/1结构式表达为KEu(WO4)0.5(MoO4)1.5。系统的分析比较了荧光粉MEu(WO4)0.5(MoO4)1.5(M=Li, Na, K)的发光性能,发现样品的发射强度随着碱金属离子半径的增大而减小(碱金属离子半径排序：K+>Na+>Li+),综合各方面因素考虑碱金属和铕双钼钨酸盐的最佳结构表达式选择为：LiEu(WO4)0.5(MoO4)1.5。
合成了一种新型白光LED用红色荧光粉LiEu1-xYx(WO4)0.5(MoO4)1.5(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.8)系列,确定了Y3+掺杂浓度最佳值x为0.5,同时研究表明该系列荧光粉可以被近紫外光(396 nm)和蓝光(466 nm)有效激发,发射峰值位于615 nm(Eu3+离子的5D0→7F2跃迁)的红光,并且证明了Eu3+离子在晶体结构中占据了非反演对称中心的位置,激发波长与目前广泛使用的蓝光和紫外光LED芯片相符合,适用于白光LED的制造。此外,研究了助熔剂对该体系荧光粉发光特性的影响。XRD的测量结果表明加入适量的助熔剂有利于LiEu1-xYx(WO4)0.5(MoO4)1.5荧光粉的结晶化,并且不引入杂相。适量的助熔剂的加入可增大该系列荧光粉的相对发光强度,并能有效降低荧光粉的平均粒径,确定了在制备LiEu0.5Y0.5(WO4)0.5(MoO4)1.5荧光粉的焙烧过程中按1：1添加占总质量1%的AlF3和H3B03作为助熔剂时荧光粉的相对发光强度最大,平均粒径较小综合效果最佳。
合成了一系列新型红色荧光粉LiEu1-xBix(WO4)0.5(MoO4)1.5(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.8)并详细研究了它们的发光性能。样品的最强发射峰位于615 nm处对应于Eu3+的5D0→7F2跃迁；实验结果表明在LiEu(WO4)0.5(MoO4)1.5中加入适量的Bi3+,不但可以提高荧光粉的发光强度,而且有利于改善样品的色纯度。Bi3+的最佳掺杂浓度为10at.%,当Bi3+浓度超过最佳值时,Bi3+-Bi3+之间形成团聚且吸收的能量也通过非辐射跃迁损失,导致Bi3+对Eu3+的敏化作用明显减小。计算出不同Bi3+浓度x=0.05,0.1,0.15,0.2,0.3,0.4,0.5时Eu3+-Bi3+之间的能量传递平均距离RBi→Eu分别为22.3,23.7,24.9,26.1,28.1,29.9,31.4A。并确定了能量传递的临界距离(Rc)约为27.1 A。
4.一种新型白光LED用三钼酸盐红色荧光粉LiKGd2(MoO4)4:Eu3+体系发光性能研究
采用高温固相法成功合成了一系列新型三铝酸盐红色荧光粉LiKGd2-xEux(MoO4)4。确定了三铝酸盐红色荧光粉LiKGd2(MoO4)4:Eu3+制备的最佳工艺条件是煅烧温度为850℃保温5小时。采用“绝热法”对目标样品做了半定量分析,计算出物相质量分数证明了所制备样品为纯相。为半定量分析样品相纯度提供了一种新的实用的方法。
荧光粉LiKGd2(MoO4)4:Eu3+在紫外(396 nm)和蓝光(466 nm)区域有两个很强的吸引峰分别能与近紫外光LED芯片和蓝光LED芯片发射很好的匹配,并发射出峰值位于615 nm的红光。实验结果显示：Eu3+掺杂LiKGd2(MoO4)4基质的最佳浓度值x为1.1,经比较发现样品LiKGd0.9(MoO4)4:Eu1.13+的发光强度分别是Ca0.80MoO4:Eu0.203+和商用红粉Y2O2S:Eu3+发光强度的1.40和3.68倍。而且荧光粉LiKGd0.9(MoO4)4:Eu1.13+的色坐标值比传统商用红粉Y2O2S:Eu3+的色坐标值更加接近NTSC标准值。基于Dexter的多极相互作用能量传递公式和Reisfeld提供的近似值原理,详细分析了荧光粉LiKGd2-xEux(MoO4)4系列中Eu3+离子之间的能量传递是电偶极-偶极相互作用,并按临界无辐射能量传递距离公式计算得出Eu3+离子之间能量传递临界距离Rc=8.24A。所有实验结果表明三钼酸盐LiKGd0.9(MoO4)4:Eu1.13+是一种高效的红色荧光粉,适用于三基色荧光粉系统白光LED制造或可以作为“蓝光LED+黄色荧光粉系统”的红光补偿粉。
White light-emitting diodes (LEDs), known as the fourth generation solid-state lighting, have many advantages over the existing incandescent and halogen lamps in reliability, energy saving, long life, small size, maintenance and safety, and therefore are gaining lots of attentions. Today, the white LEDs market is dominated by phosphor-converted LEDs (pcLEDs), which brings new prospect to the development of phosphors.
To completely replace fluorescent lamp to be a new generation indoor illumination source, it is necessary that the traditional white LED lighting possesses lower costs, higher luminous efficiency and color rendering index and better adjustability in color temperature of white light emitting and so on. There exist some defect in the commercialized white LEDs by using pcLEDs comprising a blue emitting InGaN semiconductor (420-480 nm) coated with yellow phosphor (YAG:Ce), such as electro-optical intensity of blue spectra increases faster than that of yellow light at high current, the current changes can engender spectrum mismatch, consequently result in the changes of color temperature and low color rendering index. The above circumstances are absent in the ultraviolet (UV) and near-UV LEDs systems. Since the conversion of UV and near-UV LEDs has the virtue of relative lower cost, more easily controlling color than the blue LED, excellent color uniformity, good color rendering etc, it is also a development tendency of white LEDs that UV and near-UV LEDs systems supplant the traditional white pcLEDs operating on the basis of blue InGaN dies. The current commercially applicable red phosphor for UV InGaN-based LEDs is Y2O2S:Eu3+ However, the Y2O2S:Eu3+ red phosphor cannot efficiently absorb in near-UV region and its brightness is about eight times less than that of the blue (BaMgAl10O17:Eu2+) and green (ZnS:(Cu+, Al3+)) phosphors. In addition, the lifetime of the Y2O2S:Eu3+is inadequate under near-UV irradiation for its instability. The drawback of red emitting phosphors has been the main obstacle for the development of white LEDs. Therefore, the study on the red phosphors matching to UV (near-UV) LED is of great significance in theory and practice.
In this work, the preparation methods and luminescent properties of the existing red emitting phosphors CaMoO4:Eu3+ systems were systematically investigated, and their light-emitting properties were improved by co-doping rare earth ions. Three types of novel molybdate red emitting phosphors excited by near-UV or blue light were synthesized, whose synthesis method and spectral characteristics were researched in detail. The major research contents and appropriate conclusions are as following:
1. Synthesis and photoluminescence properties of red emitting phosphors CaMoO4:Eu3+ for
white LEDs
The technological conditions for preparation of red emitting phosphors CaMoO4:Eu3+ by traditional solid state reaction were investigated. The optimum calcination temperature and the holding time were determined to be 900℃and 3 hours, respectively. And the superlative Eu3+-doped concentration in red phosphors CaMoO4:Eu3+ was concluded to be 25at.%. It was important and positive to improve doping concentration of Eu3+ from 20at.% to 25at.%. Namely, the luminescence intensity increased with the increase of doping concentration of Eu3+ in the molybdenum calcium system. The effect of charge compensator (Li+, Na+, K+) on the luminescence behavior of Eu3+ activated CaMoO4 phosphor was discussed in detail. The relative intensity decreases with the increases of the ionic radii of alkali metal ions, which are as charge compensator (the ionic radii in the order of Li+
The Eu3+ and Sm3+ co-doped calcium molybdenum (CaMoO4) phosphors were prepared by solid state reaction. The results show that the series phosphors can strongly absorb energy in the range of near UV and emit red light at 615 nm, nicely fitting in with the widely applied output wavelengths of ultraviolet or blue LED chips. In addition, the introduction of a small amount of Sm3+ can enhance the absorption intensity of excitation spectra at 406 nm and the excitation spectra locating near 400 nm become broader. The sensitization effect of Sm3+ on Eu3+ was proved through emission spectroscopy and luminescence lifetime in our paper, respectively. The experimental results indicate that the introduction of appropriate amount of Sm3+ can not only enhance emission intensity but also improve the color purity. The optimal doping volume was 0.5at.%.
For the first time, introducing Si atoms replaced part of the location of Mo to achieve the purpose of improving luminous intensity in CaMoO4:Eu3+. The results show that the addition of Si does not affect the shapes of excitation and emission spectra and the position of emission peak. With the introduction of appropriate amount of Si4+, the emission intensity of samples obviously increases. It was the reason that the sub-lattice symmetry of Eu3+luminescence center was changed and the f-f transitions of Eu3+ were further released. Moreover, it can be found that when the doping content of Si4+ was beyond 2at.%, the particle size of the samples increased significantly, which was the same to the large size phenomenon of silicate phosphors. Given consideration to every aspect, the optimum content of Si4+ is 1at.%, when the luminous intensity of the sample reached the maximum and the average diameter was 2.15μm. The color coordinate values of Ca0.5Mo0.99Si0.01O4:Eu3+0.25, Na+0.25 were closer to the standard value of the U.S. National Television Standards Committee (National Television System Committee, abbreviated as NTSC) than that of Ca0.5MoO4:Eu3+0.25, Na+0.25.All the experimental results showed that this series phosphors were very suitable for white LED manufacturing.
2. Luminescent properties of a novel red phosphor Na0.5Gd0.5Mo04:Eu3+ for white LEDs
The novel red phosphors Na0.5Gd0.5MoO4:Eu3+ with the similar crystal structure to CaMoO4 was successfully synthesized by the high temperature solid phase reaction. The results show that concentration quenching does not occur in the Eu3+-activated Na0.5Gd0.5MoO4 system, namely the luminous intensity of Na0.5Gd0.5-xMoO4:Eu3+ increases with the increase of the doping concentration of Eu3+. When the Gd3+ was completely replaced by Eu3+, luminescence intensity of the samples reached the maximum, at this time, the relative brightness of samples was 2.26 and 6 time higher than that of Ca0.80MoO4:Eu3+0.20 and traditional commercial Y2O2S:Eu3+ respectively.
The introduction of appropriate amount of Li+ replacing part of the Na+ in the phosphor Na0.5Eu0.5MoO4 can markedly improve the luminous intensity. The optimal doping concentration of Li+ is 25at.%, when the relative luminous intensity of Na0.25Li0.25Eu0.5MoO4 increased by 37% of Na0.5Eu0.5MoO4, the brightness value is 8.2 times bigger than that of traditional commercial red phosphors Y2O2S:Eu3+. In the meantime, the color coordinates (x=0.654, y= 0.346) were closer to the NTSC standard values than those of commercial red phosphors Y2O2S: Eu3+(x=0.631, y=0.350). The chief reason is that the introduction of appropriate amount of Li+ into the Na0.5Eu0.5MoO4 is able to alter the sub-lattice structure of Eu3+ luminescence centers and make the Eu3+ far away from the inversion center of symmetry, so that more electric dipole transitions are released to enhance the luminous intensity. Also, the fluorescence lifetimeτvalues of Na0.5Eu0.5MoO4 and Na0.25Li0.25Eu0.5MoO4 were 0.375 and 0.416 ms respectively. With the introducing appropriate Li+ content, the samples lifetimeτvalues increase. All experimental results showed that Na0.25Li0.25Eu0.5MoO4 was a high efficient red phosphor and suited to the 'UV chip+ tri basic color' system or the complement light phosphors in the'blue chip+ yellow phosphor' system.
3. Preparation and spectral properties of molybdenum-tungstate red emitting phosphors
The red phosphors KEu(MoO4)2 were synthesized and its luminous intensity was enhanced through the introduction of tungstate radical. The optimum technological condition of preparation of KEu(MoO4)2 was calcination at 750℃for 5 hours. The best doping amount of WO42- was 50at.%, namely molar ratio of MoO42-/WO42- was 3/1, and the structural formula is KEu(WO4)0.5(MoO4)1.5. Comparison and analysis of luminescence properties of MEu(WO4)0.5(MoO4)1.5(M=Li, Na, K) had been done, the result indicated that the emission intensity of phosphor MEu(WO4)0.5(MoO4)1.5 (M=Li, Na, K) decreased with increasing radius of alkali metal ions (alkali metal ions radius sort:K+> Na+> Li+). Taking all factors into account, the best expression was LiEu(WO4)0.5(MoO4)1.5.
A series novel red phosphors LiEu1-xYx (WO4)0.5 (MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were prepared, and the best doping concentration of Y3+ was determined to be 0.5mol. The series phosphor can be excited by near-UV(396 nm) and blue (466 nm) and emit red light at 615 nm (5D0→7F2 transition of Eu3+), nicely fitting in with the widely applied output wavelengths of ultraviolet or blue LED chips. Our reserach also proves that the Eu3+ occupies the lattice site of noncentrosymmetric environment in the scheelite phases. All the results indicate that the red phosphor is a suitable candidate of red emitting phosphor for the fabrication of white LEDs.
In addition, the effects of the flux on the luminescent properties were studied. The results of the XRD patterns indicated that the proper flux was beneficial to the crystallization of the phosphor, and no other phases were formed. Proper addition amount of flux could improve the relative luminosity and decrease the particle size. Experiments confirmed that the phosphor had the optimal comprehensive properties when the addition amount of flux (WAlF3/WH3BO3=1/1) was 1%.
A series of novel red phosphor LiEu1-xBix(WO4)0.5(MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were synthesized and their luminescence properties were studied in detail. The emission spectra show the most intense peak is located at 615 nm, which corresponds to the 5D0→7F2 transition of Eu3+. The experimental results show that adding appropriate amount of Bi3+ in the LiEu(WO4)0.5(MoO4)1.5 can not only increase the luminescence intensity of phosphor but also improve the color purity. The optimum doping concentration of Bi3+ was found to be 10at.%. For higher concentration of Bi3+, the absorbed energy is nonradiatively dissipated due to the formation of Bi3+ aggregates, which results in an evident reduction of the sensitized effectiveness of Bi3+ ions on Eu3+ ions. The average separation RBi→Eu (in A) of energy transfer is determined to be 22.3,23.7,24.9,26.1,28.1,29.9, and 31.4 for Bi3+ concentration x=0.05,0.1,0.15,0.2, 0.3,0.4 and 0.5, respectively, in LiEu1-xBix(WO4)0.5(MoO4)1.5. The critical concentration xc, at which the luminescence intensity of Eu3+ is half that in the sample in the absence of Bi3+, is 0.45. Therefore, the critical distance (Rc) of energy transfer was calculated to be about 27.1 A.
4. A potential red emitting phosphors scheelite-like triple molybdates LiKGd2(MoO4)4:Eu3+ for white LEDs applications
A series of novel triple molybdates red emitting phosphors LiKGd2-xEux:(MoO4)4 were successfully synthesized through solid state reaction. The optimum condition was calcination at 850℃for 5 hours. A semi-quantitative estimation of the sample was investigated by adiabatic method. The mass fraction of phase was calculated, which proved that the prepared samples were pure phase. A new practical approach was provided to semi-quantitative analysis of phase purity of the sample.
Phosphor LiKGd2(MoO4)4:Eu3+ has two strong peaks in the UV (396 nm) and blue (466nm) region, which match to the near-UV LED chip and blue LED chip well, and emit red light at 615 nm. The results show the optimum doped concentration of Eu3+ in LiKGd2(MoO4)4 is 1.1. Comparing with Ca0.80MoO4:Eu3+0.20 and commercial red phosphors Y2O2S:Eu3+, the luminous intensity of LiKGdo.9(Mo04)4:Eu3+1.1 are 1.40 and 3.68 time higher, respectively. The color coordinates values of the phosphor LiKGdo.9(Mo04)4:Eu3+1.1 are closer to the NTSC standard values than those of the traditional commercial red phosphors Y2O2S:Eu3+. Based on Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the energy transfer among Eu3+ ions in the phosphor LiKGd2-xEux(MoO4)4 is governed by electric dipole-dipole interaction. The critical distance of energy transfer among Eu3+ ions was calculated to be Rc=8.24 A according to the distance formula of critical non-radiative energy transfer. All results show that the triple molybdate phosphors LiKGd0.9(MoO4)4:Eu3+1.1 is highly efficient red phosphors for the'UV chip+tri basic color' system or the complement light phosphors in the 'blue chip+yellow phosphor' system.
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