Study on the reduction of Eu3+ -> Eu2+ in Sr4Al14O25: Eu prepared in air atmosphere

Compounds of Sr4Al14O15: Eu were prepared in air atmosphere by high temperature solid state reaction. The reduction of Eu3+--> Eu2+ was firstly observed in the aluminate phosphor of Sr4Al14O25: Eu synthesized in air condition. This made aluminate a new family and Sr4Al14O25 a new member of compounds in which Eu3+ ion could be reduced to Eu2+ form when fired in air atmosphere. The reduction of Eu3+ --> Eu2+ in Sr4Al14O25: Eu was explained by means of a charge compensation model. Experiments based on the model were designed and carried out, and the results supported this model.

Role of B2O3 on the phase stability and long phosphorescence of SrAl2O4 : Eu, Dy

The role of B2O3 addition on the long phosphorescence of SrAl2O4:Eu2+, Dy3+ has been investigated. B2O3 is just not an inert high temperature solvent (flux) to accelerate grain growth, according to SEM results. B2O3 has a substitutional effect, even at low concentrations. by way of incorporation of BO4 in the corner-shared AlO4 framework of the distorted 'stuffed' tridymite structure of SrAl2O4. which is discernible from the IR and solid-state MAS NMR spectral data. With increasing concentrations, B2O3 reacts with SrAl2O4 to form Sr4Al4O25 together with Sr-borate (SrB2O4) as the glassy phase, as evidenced by XRD and SEM studies. At high B2O3 contents, Sr4Al14O25 converts to SrAl2B2O7 (cubic and hexagonal), SrAl12O19 and Sr-borate (SrB4O7) glass. Sr4Al14O25:Eu2+, Dy3+ has also been independently synthesized to realize the blue emitting (lambda(em)approximate to490 nm) phosphor. The afterglow decay as well as thermoluminescence studies reveal that Sr4Al14O25:Eu, Dy exhibits equally long phosphorescence as that of SrAl2O4:Eu2+, Dy3+. In both cases, long phosphorescence is noticed only when BO4 is present along with Dy3+ and Eu2+. Here Dy3+ because of its higher charge density than Eu2+ prefers to occupy the Sr sites in the neighbourhood of BO4, as the effective charge on borate is more negative than that of AlO4. Thus. Dy3+ forms a substitutional defect complex with borate and acts as an acceptor-type defect center. These defects Eu2+ ions and the subsequent thermal release of hole at room temperature followed by the trap the hole generated by the excitation of recombination with electron resulting in the long persistent phosphorescence. (C) 2003 Elsevier Science B.V. All rights reserved.

Investigations on valence-change behaviors of europium ions in Eu-doped aluminate and silicate phosphors

Compounds of Sr3Al2O6: Eu, Sr4Al14O25: Eu, and BaZnSiO4: Eu were synthesized by high-temperature solid state reactions. The doping Eu3+ ions were partially reduced to Eu2+ in Sr4Al14O25: Eu and BaZnSiO4: Eu prepared in an oxidizing atmosphere, N-2 + O-2. However, such an abnormal reduction process could not be performed in Sr3Al2O6: Eu, which was also prepared in an atmosphere of N-2 + O-2. Moreover, even though Sr3Al2O6: Eu was synthesized in a reducing condition CO, only part of the Eu3+ ions was reduced to Eu2+. The existence of trivalent and divalent europium ions was confirmed by photoluminescent spectra. The different valence-change behaviors of europium ions in the hosts were attributed to the difference in host crystal structures. The higher the crystal structure stiffness, the easier the reduction process from Eu3+ to Eu2+.

非还原条件下稀土离子的还原现象及相应光谱性质的研究

继在空气气氛下制备的硼酸盐（如srB4O7，含有BO4四面体），硼磷酸盐（如MBPO5（M＝Ca，Sr，Ba），含有PO4和日04四面体）和硫酸盐（如BaSO4，含SO4四面体）体系中发现稀土离子的还原现象（如Eu3+→Eu2+ ）以来，本工作首次在铝酸盐Sr4Al14O25（含有AIO4四面体）和硅酸盐BaMgsiO4（含有SiO4四面体）中发现，在空气气氛下亦能够发生Eu3+→Eu2+的异常还原现象，并尝试用电荷补偿模型对这种现象进行解释。在基子这个模型的主要思想（即带负电荷的缺陷所带的电荷越多，越有利于上述Eu3+→Eu2+的还原现象的发生）的基础上，本工作设计了离子共掺实验，试验结果证明了上述推断是合理的。在荧光粉BaMgSiO4：Eu2+中，两价Eu2+铕离子占据三种截然不同的晶格位置，形成三种类型的发光中心。在Ba（3）格位的两价Eu2+馆离子呈现的位于398nm发射；在Ba（1）和Ba（2）的两价Eu2+铕离子呈现位于500nm的宽带发射；在室温下，荧光粉BaMgSiO4：Eu2+中两价Eu2+馆离子的位于500nm的宽带发射“没有淬灭。这同Blasse等人报道的结果差别很大。在研究铕离子在铝酸盐S以All4025，SrAl2O4和Sr3A12O6中的变价现象时，本工作发现在Sr4Al4O25和SrAl2O4中，Eu3+→Eu2+还原过程比较容易进行，而在Sr3A12O6中，即使是在还原性气氛下，完全实现由三价铕离子变为两价铕离子也是很困难的，同样的现象在硼酸盐基质中也出现了。发生上述现象的主要原因是基质化合物的晶体结构的差别引起的，坚硬的封闭的晶体结构，有利于Eu3＋分Eu2＋还原过程的发生。为了探索新材料，本工作研究了其他的稀土离子在基质化合物掺入基质Sr4Al14O25、BaMgSiO4、Sr3Al2O6中的光谱性质，发现在空气气氛下制备的化合物S以All 4025、BaMgsi04、Sr3Al2O6中同样能够发生Ce4＋→Ce3+的还原过程，这个过程亦可以用电荷补偿模型进行解释。在化合物Sr4Al14O25中共掺入Ce3＋和Tb3+或Eu2+，发现三价Ce3＋离子能够敏化Tb3+和Eu2＋。在化合物Sr4Al14O25、BaMgSiO4、Sr3Al2O6以及CaYBO4中掺入Tb3＋后，都看到了来自5D3拼口5D4两个能级的发射。在254nm激发下，荧光粉CaYBO4：Eu3+的主发射峰位于609nm，这使其有可能用作三基色灯粉的红粉；比较有意思的是虽然在空气下和还原气氛下合成的BaMgsio4：Ce3＋样品的主发射峰均位于371nm，但是在空气下合成的样品的峰宽明显的窄于还原气氛下合成的。

Sr_4Al_(14)O_(25)∶RE~(3+)(RE=Eu,Ce,Tb)中稀土离子的发光性质

采用高温固相反应合成了Sr4Al14O25∶RE3+(RE=Eu,Ce,Tb)样品,研究了其中Eu3+,Ce3+和Tb3+的光谱性质,以及Ce3+与Tb3+共掺时的能量传递现象;发现Eu3+,Ce3+和Tb3+占有两个格位,与Eu2+在此基质中的情况相似;在Tb3+的发射光谱中同时观察到了来自5D3与5D4的发射,表明两能级间无辐射跃迁过程不显著;Ce3+对Tb3+有敏化作用。