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Study on the reduction of Eu3+→Eu2+ in Sr4Al14O25 Eu prepared in air atmosphere

Chemical Physics Letters 371 (2003) 1–6 www.elsevier.com/locate/cplett

Study on the reduction of Eu3? ! Eu2? in Sr4Al14O25: Eu prepared in air atmosphere
Mingying Peng, Zhiwu Pei *, Guangyan Hong, Qiang Su
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People?s Republic of China Received 9 October 2002; in ?nal form 24 December 2002

Abstract Compounds of Sr4 Al14 O25 : Eu were prepared in air atmosphere by high temperature solid state reaction. The reduction of Eu3? ! Eu2? was ?rstly observed in the aluminate phosphor of Sr4 Al14 O25 : Eu synthesized in air condition. This made aluminate a new family and Sr4 Al14 O25 a new member of compounds in which Eu3? ion could be reduced to Eu2? form when ?red in air atmosphere. The reduction of Eu3? ! Eu2? in Sr4 Al14 O25 : 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. ? 2003 Elsevier Science B.V. All rights reserved.

1. Introduction Divalent europium ions (Eu2? ) have been widely used as activators in blue-emitting phosphors [1–3] and long-persistence phosphors [4]. Therefore, many researchers worldwide have studied the luminescent properties of Eu2? ions in various matrix compounds, and the reduction processes of Eu3? ! Eu2? in phosphor preparations for a long period of time [5,6]. The signi?cant di?erences of Eu3? reduction processes in solid state compounds have deeply attracted our attention. On one hand, Eu3? could be easily reduced to its Eu2? form in most of

*

Corresponding author. Fax: +86-431-5698041. E-mail address: zpei@ciac.jl.cn (Z. Pei).

matrix compounds if preparations were carried out in reducing atmospheres. On the other, it was impossible to get Eu2? ion even the preparation was done in a strong reducing atmosphere of pure H2 ?ow [7]. Between the two situations mentioned above, it has been found that in some special compounds the reduction of Eu3? ! Eu2? could occur when samples were prepared in air condition at high temperature [8–13]. The reduction of Eu3? ! Eu2? in air atmosphere had been reported by Tle et al. in Ba3 ?PO4 ?2 : Eu2? a years ago [14]. But no further study had been done since then. Our group has systemically studied this ?abnormal? reduction of Eu3? ! Eu2? in oxide complexes when compounds were prepared in air condition at high temperature [8–12]. Research interests were focussed on ?nding more matrix compounds in which Eu3? ion could be reduced to its

0009-2614/03/$ - see front matter ? 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00044-7

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Eu2? form in air condition at high temperature, and establishing a model with which this ?abnormal? reduction could be explained reasonably. Nowadays, the reduction phenomena of Eu3? ! Eu2? in air condition have been found and studied in sulfates (BaSO4 : Eu) [15], phosphates (Ba3 ?PO4 ?2 : Eu) [14], borophoshpates (MBPO5 : Eu (M ? Ca, Sr, Ba)) [13], silicates (BaMgSiO4 : Eu) [16] and borates ?SrB4 O7 : Eu, SrB6 O10 : Eu and BaB8 O13 : Eu) [8–11]. In this Letter, we reported the study on the reduction of Eu3? ! Eu2? in an aluminate matrix of Sr4 Al14 O25 : Eu, when the compound was prepared in air at high temperature. The mechanism of this reduction was discussed according to the four rules and a charge compensation model. Experiments based on the model were designed and carried out. The results supported our model.

Sr4 Al14 O25 is composed of octahedral AlO6 anion groups and tetrahedral AlO4 ones. The X-ray di?raction pattern of our sample of Sr4 Al14 O25 : Eu was shown in Fig. 1a. Fig. 1b presented the X-ray di?raction pattern of [18]. The consistence of Figs. 1a and b proved that our sample was the compound of Sr4 Al14 O25 . 3.1. Observation of the reduction of Eu3? ! Eu2? As well known, the emission of Eu2? ion in a solid state compound generally shows a broad band character with a 4f 6 5d ! 4f 7 transition nature (except the f–f line emission at about 360 nm in some special compounds [19]); while that of Eu3? ion always gives a series of typical emission lines in the spectral region of 570–750 nm corresponding to its 5 D0 –7 FJ ?J ?0;1;2;3;4? transitions. This provided us a very convenient way for detecting the presence of Eu2? ion in solid state compounds. The luminescent properties of Sr4 Al14 O25 : Eu2? prepared in reducing atmospheres have been studied by other authors [20–24]. In Sr4 Al14 O25 : Eu2? ; Eu2? ions showed two broad emission bands at about 400 and 490 nm corresponding to two Eu2? centers, which were due to the substitution of Eu2? ions for Sr2? ions in two di?erent lattice sites. The emission spectrum of Sr4 Al14 O25 :1 mol% Eu prepared in air of this work was shown in

2. Experimental The host compounds were synthesized by ?ring the intimate mixtures of SrCO3 (A.R.) and Al2 O3 (A.R.) at high temperature. H3 BO3 was employed as ?ux during preparations. Na2 CO3 (A.R.), Eu2 O3 (4N) and La2 O3 (4N) were used as the sources of doping ions, respectively. The concentrations of the doping ions of Eu3? ; Na? and La3? were ?xed to 1 mol% of the Sr2? ions in 4SrO ? 7Al2 O3 . The mixtures of the corresponding starting materials were thoroughly ground and then heated at 700 °C for 2 h. After reground, they were calcined at 1400 °C for 5 h in air and/or in a thermal-carbon reducing atmosphere. The X-ray powder di?raction pattern was measured with a Rigaku D/max-IIB X-ray Di?rac tometer, using CuKa (k ? 1:5405 A) radiation. Photoluminescence measurements were performed on a Hitachi F-4500 Fluorescence Spectro?uorometer at room temperature.

3. Results and discussion The crystal structure of Sr4 Al14 O25 was originally described by Nadzhina et al. [17], and then by Wang and Wang [18]. It crystallizes in the orthorhombic system with space group of Pmma .

Fig. 1. The XRD patterns of Sr4 Al14 O25 : Eu of this work (a) and Ref. [18] (b).

M. Peng et al. / Chemical Physics Letters 371 (2003) 1–6

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Fig. 2a. There were two broad emission bands peaking at about 400 and 490 nm and several weak line emissions in the region of 550–700 nm (see the ampli?ed spectrum in Fig. 2). Since the blank sample of Sr4 Al14 O25 did not emit any light, we could conclude that the observed spectrum of Fig. 2a should be closely related to the doping europium ions. The emission lines of Fig. 2a in the range from 550 to 700 nm clearly belong to the f–f transitions of Eu3? ion, and the broad emission bands at about 400 and 490 nm should be due to the emissions of Eu2? ions in the compound. For further con?rming the presence of Eu2? ion in Sr4 Al14 O25 : 1 mol% Eu prepared in air, we synthesized the phosphor of Sr4 Al14 O25 : 1 mol% Eu2? in a thermal-carbon reducing atmosphere (TCRA), and its emission spectrum was shown in Fig. 2b. In the TCRA sample, two broad emission bands of Eu2? ions were situated at about 400 and 490 nm respectively, which is exactly consistent with those reported by Smets et al. [20]. By comparing the spectral characters of Figs. 2a and b, it is easily to notice that the shapes and positions of the emission bands of the two samples are almost the same each other. Keeping other authors? results in mind, we could therefore conclude that Eu2? ion was existed and the reduction of Eu3? ! Eu2? occurred in Sr4 Al14 O25 : Eu prepared in air at high temperature.

In our previous work, we have pointed out four conditions that seem to be necessary for the reduction of Eu3? ! Eu2? in solid state compounds, when prepared in air at high temperature. These are: (1) no oxidizing ions present in the host compounds; (2) the dopant trivalent Eu3? ion replaces the divalent cation in the hosts; (3) the substituted cation has similar radius to the divalent Eu2? ion; and (4) the host compound has an appropriate structure, which is composed of the tetrahedral anion groups (BO4 ; SO4 ; PO4 and SiO4 ) [8]. Sr4 Al14 O25 has the enclosed and sti? structure like SrB4 O7 [17,18,25]. The structure is built up of alternating planes, consisting of octahedral AlO6 and tetrahedral AlO4 anion groups. The octahedral and tetrahedral planes are interconnected by additional AlO4 -tetrahedra, which constitute a three-dimensional network structure. In this structure, the strontium ions are located in the cavities formed by AlO4 tetrahedral and AlO6 octahedral anion groups. Now, let us consider Sr4 Al14 O25 based on the four conditions mentioned above. In Sr4 Al14 O25 , there are no oxidizing ions, which agrees with condition (1); the dopant Eu3? ion nonequivalently replaces Sr2? ion, which consists with condition (2); the substituted Sr2? ion has a similar  radius to that of Eu2? ion (e.g. rSr2? ? 1:18 A,  rEu2? ? 1:17 A in the six-coordination sites [26]), which ?ts condition (3); and the structure is a three-dimensional network composed of the tetrahedral anion groups of AlO4 , which favors condition (4). Therefore, we may expect that the reduction of Eu3? ! Eu2? occur in the compound of Sr4 Al14 O25 : Eu prepared in air condition, and in fact, it was the case. This was the ?rst observation of this kind reduction in an aluminate compound. 3.2. Study on the mechanism of Eu3? ! Eu2? reduction In the following, we are trying to explain the reduction phenomenon of Eu3? ! Eu2? in Sr4 Al14 O25 : Eu prepared in air condition with a charge compensation model. When trivalent Eu3? ions were doped into Sr4 Al14 O25 , they would replace the Sr2? ions of

Fig. 2. The emission spectra of Sr3:96 Al14 O25 : 1 mol% Eu prepared in air (a) and in TCRA (b), kex: ? 360 nm. The section marked with dot line was ampli?ed by factor of 40.

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Sr4 Al14 O25 . In order to keep the charge balance, two Eu3? ions should be needed to substitute for three Sr2? ions (the total charge of the two trivalent Eu3? ions is equal to that of the three Sr2? 00 ions). Hence, one vacancy defect represented as VSr with two negative charges and two positive defects of Eu?Sr would be created by each substitution of every two Eu3? ions in the compound of 00 Sr4 Al14 O25 : Eu. The vacancy of VSr then acted as a donor of electrons, while the two Eu?Sr defects became acceptors of the electrons. Consequently, by thermal stimulation, the electrons in the vacancy 00 defects of VSr would be transferred to Eu?Sr sites and reduced Eu3? to Eu2? form. The whole process could be presented in the following equations:
00 3Sr2? ? 2Eu3? VSr ? 2Eu?Sr 00 x VSr ? VSr ? 2e

When Eu3? and Na? were co-doped, one positive defect of EuSr and one negative defect of Na0Sr would be created since Eu3? and Na? occupy two Sr2? sites in Sr4 Al14 O25 : Eu3? ; Na1? . The e?ective charge of electron on Na0Sr is less than 1 for the attraction of positive charge of Na? ion. Therefore, Ra (ratio of the number of e?ective electrons on negative defects and the number of Eu3? ions to be reduced in the compound) is also less than 1. In the case of co-doping with Eu3? and Sr2? , two EuSr 00 defects and one VSr vacancy appear. Because there is no any positive charge on the vacancy, the two 00 electrons of VSr vacancy remain their total e?ect negative charge as 2. Then Ra equals 1. With the similar consideration, we could calculate the ratio of Ra as 2 in the cases of co-doping with Eu3? and 00 La3? (one EuSr and one VSr ), as shown in column 4, Table 1. According to this model, the increment of

2Eu?sr ? 2e ? 2Eux sr If this model worked, we might infer that the more electrons carried by negative defects were created, the more Eu3? ions would be reduced to Eu2? ions in Sr4 Al14 O25 : Eu. Based on these ideas, we designed and conducted the following co-doping experiments. Together with Eu3? ion, we doped a second cation of Mn? ?Mn? ? Na? ; Sr2? ; La3? ) into the matrix of Sr4 Al14 O25 at the same time. By this way, we made di?erent defects with various charges in the mentioned compounds. The emission spectra of the co-doping samples of Sr4 Al14 O25 : 1 mol%Eu, 1 mol% Mn? ?Mn? ? Na? ; Sr2? ; La3? ) were measured under the excitation of 360 nm with the same emission and excitation slits of 2.5 and 5.0 at room temperature, as shown in Fig. 3. Table 1 summarized the experiment results.

Fig. 3. The emission spectra of Sr3:92 Al14 O25 : Eu: 1 mol%Eu, 1 mol%Mn? (Mn? ? Na? (c), Sr2? (b), La3? (a)), kex: ? 360 nm. Excitation and emission slits: 5.0, 2.5.

Table 1 Experimental results of the co-doping samples of Sr3:92 Al14 O25 : 1 mol% Eu, 1 mol% Mn? (Mn? ? Na? ; Sr2? ; La3? ) Substitution Eu3? ? Na? ! 2Sr2? 2Eu3? ! 3Sr2? ?2Eu3? ? 2Sr2? 5Sr2? ? Eu3? ? La3? ! 3Sr2? Positive defect Eu?Sr 2Eu?Sr Eu?Sr LaSr Negative defect Na0Sr 00 VSr
00 VSr

Ra <1=1 )<1 2=2 ) 1 2=1 ) 2

Rint: 16 800 23 000 33 000

Ra: ratio of the number of e?ective electrons on negative defects and the number of Eu3? ions to be reduced. Rint: : integrated value of Eu2? emission intensity of the co-doping samples.

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Ra should have an advantageous e?ect on the reduction of Eu3? ! Eu2? ; in other words, the amount of Eu2? ions reduced in Sr4 Al14 O25 : Eu; Mn? ?Mn? ? Na? ; Sr2? ; La3? ) would increase with the increasing of the charge of second doping ion from Na? to La3? . Below quenching concentration of Eu2? luminescence, the emission intensity of Eu2? d ! f transition is proportional to the content of Eu2? ions existing in compounds. In our samples, the doping concentration of europium ion for each sample was ?xed to 1 mol% of the Sr2? ion in Sr4 Al14 O25 ; which is below the quenching concentration of Eu2? emission reported by Song et al. in [22] (around 2–3 mol%). Fig. 3 clearly showed that the intensities of Eu2? emissions in Sr4 Al14 O25 : Eu, Mn? ?Mn? ? Na? ; Sr2? ; La3? ) increase with the order of Na? ! Sr2? ! La3? . The integrated values of the Eu2? emission intensities were listed in column 5, Table 1. With the co-doping of Eu3? and Na? in 00 Sr4 Al14 O25 , one EuSr defect and one VSr defect with <1 negative charge were created. This made Ra < 1 and a few Eu3? ions were reduced as shown in Table 1 and Fig. 3. In the case of the co-doping 00 with Eu3? and Sr2? , two EuSr defects and one VSr defect with 2 negative charges were crated. Since the ratio of the e?ective electrons and the number of EuSr defects was 1, which is larger than that in the Eu3? and Na? co-doping, more Eu3? ions were reduced to Eu2? form. Similarly, with the increase of Ra to 2 when co-doped with Eu3? and La3? , the strongest Eu2? emission was observed, which revealed a largest amount of Eu2? ions reduced in Sr4 Al14 O25 : Eu, La3? . The above results indicated that the more electrons on negative defects were created, the more Eu3? ions were reduced to their lower valence state of Eu2? in Sr4 Al14 O25 : Eu. This agrees well with our model proposed above.

Eu3? ! Eu2? reduction occurs in air condition. The reduction process of Eu3? ! Eu2? in air condition was explained with a charge compensation model. Experiments based on the model were designed and performed. Experiment results have proved that the more electrons on negative defects were created in compounds, the more Eu3? ions were reduced to their lower valence state of Eu2? in air condition. Acknowledgements The authors acknowledge the ?nancial aid of National Natural Science Foundation of China (No. 29831010). References
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4. Conclusions The reduction of Eu3? ! Eu2? in air condition was ?rstly observed in an aluminate compound of Sr4 Al14 O25 : Eu prepared at high-temperature. This observation made aluminate a new family in which

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