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Preparation of divalent rare earth ions in air by aliovalent substitution and spectroscopic properties of Ln2+
Journal of Alloys and Compounds 344 (2002) 132–136
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Preparation of divalent rare earth ions in air by aliovalent substitution and spectroscopic properties of Ln 21 Qiang Su
a,b ,
*, Hongbin Liang a,b , Tiandou Hu c , Ye Tao c , Tao Liu c
a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b State Key Laboratory of Ultrafast Laser Spectroscopy, School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, PR China c Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China
Abstract When alkaline earth ions in borates, phosphates or borophosphates [SrB 4 O 7 , SrB 6 O 10 , BaB 8 O 13 , MBPO 5 (M5Ca,Sr)] are substituted partially and aliovalently by trivalent rare earth ions such as Sm 31 , Eu 31 , these rare earth ions can be reduced to divalent state by the produced negative charge vacancy V 99 M . The matrices must have appropriate structure containing a rigid three-dimensional network of tetragonal AO 4 groups (A5B,P). These groups can surround and isolate the produced divalent RE 21 ions from the reaction with oxygen. Therefore, this reduction reaction can be carried out even in air at high temperature. The produced divalent rare earth ions can be detected by luminescence and XANES methods and their spectroscopic properties are discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Divalent rare earth ions; Reduction; Luminescence; Borate; Borophosphate
1. Introduction In 1993, we provided an aliovalent substitution method to reduce trivalent rare earth ions Sm 31 , Eu 31 or Yb 31 into divalent ions Sm 21 , Eu 21 or Yb 21 even in air when these ions were doped in alkaline earth borate SrB 4 O 7 [1]. One of the necessary conditions for this method is that the alkaline earth borate must contains tetrahedral BO 4 group, as we know that structures of alkaline earth borates such as BaB 8 O 13 , SrB 6 O 10 and Sr 2 B 5 O 9 Cl all contain a tetrahedral BO 4 group. According to the principles proposed by us [1], we achieved reduction of Sm 31 [2,4,5], Eu 31 [3,6] and Yb 31 [4] into Ln 21 (Ln 21 5Sm,Eu,Yb) in BaB 8 O 13 ; reduction of Eu 31 [7,11], Sm 31 [8–11] and Tm 31 [11] 21 21 into Ln (Ln 5Sm,Eu,Tm) in SrB 6 O 10 ; and reduction of Eu 31 into Eu 21 in Sr 2 B 5 O 9 Cl [12,13] in air at high temperature. It is interesting to note that not only the rare earth ions mentioned above can be reduced into divalent state in these alkaline earth borates containing tetrahedral 31 BO 4 group, but Bi ion can also be reduced into divalent ion Bi 21 in BaB 8 O 13 even in air at high temperature [14]. Luminescent methods [1,15], ESR [3,7] and XANES *Corresponding author. Tel. / fax: 186-20-8411-1038. E-mail address: [email protected] (Q. Su).
[16] can be used to determine qualitatively the divalent state of rare earth ions. Peterson et al. used our method to reduce Tm 31 into Tm 21 [17] in 1995 and to stabilize the divalent Nd 21 in 1997 [18] in the same matrix SrB 4 O 7 in air. They extended our method to the actinide and to stabilize the divalent actinide Cf 21 in SrB 4 O 7 [19] in 1996. In 1999, Machida et al. also observed that most of the Eu 31 ions in the starting materials are reduced during the formation of crystalline BaB 8 O 13 host lattice by the simple solid state reaction even in air [20] as we did in 1995 [3]. They also found that the Eu 21 ions in the crystalline BaB 8 O 13 host lattice are oxidized into Eu 31 ion in the vitreous form derived from the melting of BaB 8 O 13 as the BO 4 units in the crystalline BaB 8 O 13 transform into the BO 3 units in the vitreous one. The valence change is structural modification-induced. From the results mentioned above, it is obvious that the tetrahedral BO 4 unit in the alkaline earth borate is an important structural factor for stabilization of the divalent rare earth ions formed in the solid state reaction in air at high temperature. Because phosphate anion exists also in the tetrahedral PO 4 unit, it is possible that the divalent rare earth ions such as Eu 21 or Sm 21 can be stabilized in the alkaline earth phosphate containing the tetrahedral PO 4
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00351-1
Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136
units, even when the synthesis is carried out in air at high temperature. It is proved that Eu 31 can be reduced in host (Ba) 3 (PO 4 ) 2 by solid state reaction at high temperature in air [21]. According to these facts, some rare earth ions such as Eu 31 or Sm 31 can also be reduced by solid state reaction in air at high temperature in the crystalline alkaline earth borophosphate hosts, which contain both tetrahedral BO 4 and PO 4 units. In this paper, we report the preparation of divalent Eu 21 ions in air by aliovalent substitution method in the crystalline alkaline earth borophosphate hosts CaBPO 5 and SrBPO 5 . The research on the phase relationship of the system CaO–B 2 O 3 –P2 O 5 indicates only one ternary compound CaBPO 5 could form in the system [22]. It is suggested by Raman spectra that the silica-like network anion units BPO 4 , where B and P are fourcoordinated exist in borophosphates [23,24]. And by crystal structural study, it is confirmed that CaBPO 5 crystallizes in stillwellite type [25] and contains central three single chains of BO 4 tetrahedron and link to terminal PO 4 tetrahedron to form loop-branched chains [26]. In recent years, the spectral properties of rare earth ions in different hosts in vacuum ultraviolet range (VUV, l, 200 nm) have been intensively studied. There are two reasons compelling the developments in research on rare earth ions in this VUV region. The first reason is to study the high-energy states of rare earth ions. Although energy levels up to 50 000 cm 21 for most rare earth ions have been observed, the reports on 4f n energy levels in VUV region are scarce. The second reason is to explore efficient VUV excited luminescent materials, which could be used in mercury-free fluorescent lamps or plasma display panels.
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mined at room temperature and performed on a Hitachi F-4500 fluorescence spectrophotometer (resolution 0.2 nm) and a xenon lamp as excitation source. VUV spectra and XANES at Eu–L 3 edge of the samples were performed at Beijing Synchrotron Radiation Facilities (BSRF) on beam 3B1B at VUV spectral experimental station or on beam 4W1B at XAFS experimental station under normal operating conditions (2.2 GeV, |80 mA), respectively. For the determination of VUV spectra, an ARC-502 monochromator was used for the excitation spectrum, an ARC-308 monochromator was used for the emission spectrum and the signal was detected by a H7421-50 photomultiplier. The relative VUV excitation intensities of the sample were corrected by comparing the measured excitation intensities of the samples with the excitation intensities of sodium salicylate at the same excitation condition. For XANES measurements, X-ray photon was defined by a slit of size 1.0(H)310(V) mm 2 and monochromatized by a Si(111) double crystal monochromator whose energy resolution was 1.2310 24 DE /E. All VUV and XANES experimental data were collected at 293 K. In this work, the reduced behavior of europium in CaBPO 5 :Eu and SrBPO 5 :Eu prepared in air was checked by XANES at Eu–L 3 edge and luminescence of europium. In addition, the VUV excitation and VUV excited luminescent spectra of the material were reported.
3. Results and discussion
3.1. The reduced behavior of europium in CaBPO5 : Eu prepared in air
2. Experimental
3.1.1. Excitation spectra The excitation curves for Eu 21 and Eu 31 in the sample Ca 0.97 Eu 0.03 BPO 5 are presented in Fig. 1. They were obtained by f–d transition emission band of divalent
Powder samples, Sr 0.99 Eu 0.01 BPO 5 and Ca 0.97 Eu 0.03 BPO 5 , were synthesized by solid state reaction at high temperature in air. The powder sample Ca 0.97 Eu 0.03 BPO 5 was prepared by the method of solid phase reaction in air atmosphere. The mixed reactants, analytical-grade calcium carbonate, boric acid (excess 3 mol% to compensate the evaporation), ammonium dihydrophosphate and europium oxide (99.99%) were heated at 400 8C for 4 h, then firing them at 1000 8C for another 4 h in air atmosphere. The powder X-ray diffraction patterns of SrBPO 5 showed the prepared samples were single phases, corresponding to JCPDS 18-1270 (SrBPO 5 ) or Ref. [27]. The XRD data of CaBPO 5 indicated all of the synthesized samples were single hexagonal phase, and which were coincident with JCPDS 18-283. The constants of matrix crystal cell which calculated from d values were ˚ and c56.6234 A. ˚ a56.6772 A The emission and UV excitation spectra were deter-
Fig. 1. The UV excitation spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at room temperature: (a) excitation spectrum of Eu 21 , lem 5402 nm; (b) excitation spectrum of Eu 31 , lem 5592 nm.
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europium at 402 nm and f–f transition emission line of trivalent europium at 592 nm, respectively. The excitation spectrum exhibited a broad band with a maximum at 312 nm and which was caused by the permitted 4f 6 5d– 4f 7 ( 8 S 7 / 2 ) transition of Eu 21 . The f–f transition of Eu 31 led to the sharp lines, and the peaking of the prominent one was at 396 nm.
3.1.2. Emission spectra As shown in Fig. 2, when the sample was excited with wavelength 312 nm, a broad emission band appeared, it was caused by 4f 7 ( 8 S 7 / 2 )–4f 6 5d transition of divalent europium and the maximum centered at 402 nm with half width about 25 nm. When the sample was excited with wavelength 396 nm, the emission narrow lines of trivalent europium could be observed, which were due to the 5 D 0 – 7 FJ (J50, 1, 2) transitions of Eu 31 ions and were situated at 586, 592, 610 and 619 nm, respectively. The luminescent spectrum of Eu 21 in CaBPO 5 :Eu prepared in air is coincident with that prepared in H 2 / N 2 reducing atmosphere [28]. From excitation and emission spectra, it could be observed that parts of trivalent europium ions had been reduced to the divalent in the sample Ca 12x Eu x BPO 5 (x50.03) prepared in air at high temperature.
Fig. 3. The VUV spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at 293 K: solid line, emission spectrum, lex 5147 nm; dash–dot line, excitation spectrum of Eu 21 , lem 5410 nm; dash line, excitation spectrum of Eu 31 , lem 5594 nm.
Eu 21 , respectively. The emission spectrum (solid curve) of the sample excited by VUV at 147 nm was similar with Fig. 2, the f–d transition of Eu 21 with a maximum at about 410 nm and the f–f transition of Eu 31 peaks, respectively, at 588, 594, 614, 620, 652, 686 and 698 nm.
3.1.3. VUV excitation spectra Fig. 3 showed VUV excitation spectra of Eu 21 (dash– dot line) and Eu 31 (dash curve) emission in Ca 12x Eu x BPO 5 (x50.03) prepared in air. In the VUV excitation spectrum of Eu 21 , there were two bands centered around 113 nm (112 nm in Eu 21 curve and 114 nm in Eu 31 curve) and 158 nm (162 nm in Eu 21 and 155 nm in Eu 31 ). Because the bands were present when monitored at either the emission of Eu 21 or the emission of Eu 31 , they were most probably due to the absorption of the host lattice. The bands at 246 and 320 nm were obviously the charge transfer band of Eu 31 and the f–d transition of
3.2. XANES at Eu–L3 edge
Fig. 2. The UV excited emission spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at room temperature: (a) emission spectrum of Eu 21 , lex 5312 nm; (b) emission spectrum of Eu 31 , lex 5396 nm.
Fig. 4. XANES of Ca 0.97 Eu 0.03 BPO 5 at Eu–L 3 edge (solid line), the fitted curve (dot line) and deconvolution lines: (a) L 3 edge of Eu 21 ; (b) L 3 edge of Eu 31 ; (c) step function.
By XANES investigations of some rare earth compounds, it has been reported [23] that L 3 edge of RE 21 and RE 31 ions locate at different peak positions, and the discrepancy is about 7.5 eV. So the existence of Eu 21 or Eu 31 ions in MBPO 5 :Eu (M5Ca,Sr) could be probed by XANES.
3.2.1. XANES of Ca0.97 Eu0.03 BPO5 at Eu–L3 edge The result of XANES at Eu–L 3 edge in Ca 0.97 Eu 0.03 BPO 5 is shown in Fig. 4. Fitted curves and deconvolution lines were also depicted. Double peaks
Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136
appeared in XANES of the sample at Eu–L 3 edge. The 21 lower energy edge was corresponding to Eu ions, while 31 the higher one was corresponding to Eu ions. The 31 21 positions of L 3 edge of Eu and Eu ions were at 6972.8 eV and 6964.8 eV, respectively. The energy of L 3 edge of Eu 21 was 8.0 eV lower than that of Eu 31 . Just as the luminescent results, the XANES results also indicated that Eu 31 ions could be reduced into divalent Eu 21 in Ca 0.97 Eu 0.03 BPO 5 prepared in air.
3.2.2. XANES of SrBPO5 : Eu at Eu–L3 edge The result of XANES at Eu–L 3 edge in Sr 0.99 Eu 0.01 BPO 5 was shown in Fig. 5. Fitted curve and deconvolution lines were also depicted. Double peaks appeared in XANES of the sample at Eu–L 3 edge. The lower energy edge was corresponding to Eu 21 ions, while the higher one was corresponding to Eu 31 ions. The positions of L 3 edge of Eu 31 and Eu 21 ions were at 6972.9 and 6964.9 eV, respectively. The energy of L 3 edge of Eu 21 was 8.0 eV lower than that of Eu 31 . The results indicated that Eu 31 ions could be reduced into divalent Eu 21 in Sr 0.99 Eu 0.01 BPO 5 prepared in air. We think the mechanism of this abnormal reduction of Eu 31 in M 12x Eu x BPO 5 (M5Ca,Sr) in oxygenating atmosphere is connected with two reasons. First, aliovalent substitution brings about reduction, and second, the rigid tetrahedron structure of anions BPO 4 guarantees the reduced valence state steadiness. When Eu 31 were built into the matrix, they would first replace M 21 . In order to keep the electroneutrality of the compound, two Eu 31 ions would substitute for three M 21 ions. Therefore, two positive defects of [Eu M ] ? and one negative M 21 vacancy of [VM ]0 would be created by each substitution. By thermal stimulation, electrons of the [VM ]0 vacancies would be transferred to doped Eu 31 ions and reduce them. The similar process had been suggested for the reduction of Eu 31 in Ca 2 B 5 O 9 Cl:Eu by our group [12].
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It had been reported that all anions are in the form of the rigid tetrahedron structure of BO 4 and PO 4 in the host CaBPO 5 [26]. We think that this factor is suitable to stabilize the Eu 21 ions. Eu 21 ions occupy the substituted lattice sites of M 21 ions, this rigid tetrahedral structure can efficiently enclose Eu 21 ion, protect the Eu 21 from the attack of the oxygen in air, so the reduced state is easy to be stabilized. This is supported by the facts that the reduction of rare earth ions occurs in the matrices such as SrB 4 O 7 , SrB 6 O 10 and BaB 8 O 13 [1,8,17]. In host SrB 4 O 7 , 31 all anions exist in the form of tetrahedron BO 4 , the Eu can be reduced in air. While in host BaB 8 O 13 , the anions consist of triangular structure BO 3 and tetrahedron BO 4 , the Eu 31 can also be reduced in air, but fewer Eu 31 ions can be reduced in BaB 8 O 13 :Eu than in SrB 4 O 7 :Eu [16]. And in Sr 3 B 2 O 6 , Sr 2 B 2 O 5 and SrB 2 O 4 , the anions contain only triangular BO 3 group, Eu 31 ion cannot be reduced in air by aliovalent substitution [1].
4. Conclusions VUV/ UV excitation and emission spectra of CaBPO 5 :Eu and XANES of CaBPO 5 :Eu and SrBPO 5 :Eu suggested that Eu 31 could be reduced to Eu 21 in air at high temperature. (1) The excitation and emission spectra of CaBPO 5 :Eu presented the characteristic d–f transition band of Eu 21 . The results implied that the trivalent europium ions were reduced. From the VUV excitation spectra, it could be observed that the material has absorption with the maximum around 113 and 158 nm, respectively. (2) For the XANES of both the CaBPO 5 :Eu and SrBPO 5 :Eu, at Eu–L 3 edge, the absorption peak for Eu 21 is at 6964.9 eV and that for Eu 31 is at 6972.9 eV, respectively. It also indicated the existence of Eu 21 in the sample, and is consistent with the luminescent results. Divalent europium ions can stably exist in the matrix when synthesized in oxygenating atmosphere, which are probably due to aliovalent replacement and the existence of the rigid tetrahedron structure of BPO 4 .
Acknowledgements The work is supported by State Key Project of Basic Research of China, the National Natural Science Foundation of China and the Foundation of Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences.
References Fig. 5. XANES of Sr 0.99 Eu 0.01 BPO 5 at Eu–L 3 edge (solid line), fitted curves (dot line) and deconvolution lines (a, L 3 edge of Eu 21 , b, L 3 edge of Eu 31 , c, step function).
[1] Z. Pei, Q. Su, J. Zhang, J. Alloys Comp. 198 (1993) 51. [2] Q. Zeng, Z. Pei, Q. Su, in: Proceedings of The Third International
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[3]
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136 Conference on Rare Earth Development and Applications, Baotou, China, 1, 1995, p. 270. Q. Zeng, Z. Pei, Q. Su, in: Proceedings of The Third International Conference on Rare Earth Development and Applications, Baotou, China, 1, 1995, p. 274. Q. Zeng, Z. Pei, S. Wang, Q. Su, S. Lu, Mater. Res. Bull. 34 (1999) 1837. Q. Zeng, Z. Pei, Q. Su, S. Lu, J. Luminesc. 82 (1999) 241. Q. Zeng, Z. Pei, S. Wang, Q. Su, Chinese J. Luminesc. 17 (Suppl.) (1996) 111. Q. Zeng, Z. Pei, S. Wang, Q. Su, J. Alloys Comp. 275–277 (1998) 238. Q. Zeng, Z. Pei, S. Wang, Q. Su, Chem. Mater. 11 (1999) 605. Q. Zeng, Z. Pei, Q. Su, S. Huang, Phys. Stat. Sol. (b) 212 (1999) 207. Q. Zeng, Z. Pei, S. Wang, Q. Su, J. Mater. Sci. Technol. 15 (1999) 449. Q. Zeng, Z. Pei, S. Wang, Q. Su, Spectrosc. Lett. 32 (1999) 895. Z. Pei, Q. Zeng, Q. Su, J. Solid State Chem. 145 (1999) 212. Z. Pei, Q. Zeng, Q. Su, J. Phys. Chem. Solids 61 (2000) 9. Q. Zeng, T. Zhang, Z. Pei, Q. Su, J. Mater. Sci. Technol. 15 (1999) 281.
[15] Q. Zeng, Z. Pei, S. Wang, Q. Su, S. Lu, J. Phys. Chem. Solids 60 (1999) 515. [16] H.B. Liang, T.D. Hu, S.B. Wang, Q.H. Zeng, Z.W. Pei, Q. Su, Chinese J. Chem. 18 (2000) 294. [17] J.R. Peterson, W. Xu, S. Dai, Chem. Mater. 7 (1995) 1686. [18] W. Xu, J.R. Peterson, J. Alloys Comp. 249 (1997) 213. [19] J.R. Peterson, W. Xu, J. Radioanal. Nucl. Chem. 203 (1996) 301. [20] K.-I. Machida, D. Ueda, S. Inoue, G.-Y. Adachi, Electrochem. Solid State Lett. 2 (1999) 597. [21] I. Tale, P. Kulis, V. Kronghauz, J. Luminesc. 20 (1979) 343. [22] P. Ramamoorthy, T.J. Rockett, J. Am. Ceram. Soc. 57 (1974) 501. [23] A. Levesseur, R. Olazcuaga, M. Kbala, M. Zahir, P. Hagenmuller, M. Couzi, Sol State Ionics 2 (1981) 205. [24] M. Scagliotti, M. Villa, G. Chiodelli, J. Non-Cryst. Solid. 93 (1987) 350. [25] A.A. Voronkov, Yu.A. Pyatenko, Kristallografiva 12 (1967) 258. [26] R. Kniep, G. Gozel, B. Eisenmann, C. Rohr, M. Asbrand, M. Kizilyalli, Angew. Chem. Int. Ed. Engl. 33 (1994) 749. [27] Y. Shi, J. Liang, H. Zhang, J. Yang, W. Zhuang, G. Rao, J. Solid State Chem. 129 (1997) 45. [28] G. Blasse, A. Bril, J. De Vries, J. Inorg. Nucl. Chem. 31 (1969) 568.
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Preparation of divalent rare earth ions in air by aliovalent substitution and spectroscopic properties of Ln 21 Qiang Su
a,b ,
*, Hongbin Liang a,b , Tiandou Hu c , Ye Tao c , Tao Liu c
a
Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b State Key Laboratory of Ultrafast Laser Spectroscopy, School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, PR China c Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China
Abstract When alkaline earth ions in borates, phosphates or borophosphates [SrB 4 O 7 , SrB 6 O 10 , BaB 8 O 13 , MBPO 5 (M5Ca,Sr)] are substituted partially and aliovalently by trivalent rare earth ions such as Sm 31 , Eu 31 , these rare earth ions can be reduced to divalent state by the produced negative charge vacancy V 99 M . The matrices must have appropriate structure containing a rigid three-dimensional network of tetragonal AO 4 groups (A5B,P). These groups can surround and isolate the produced divalent RE 21 ions from the reaction with oxygen. Therefore, this reduction reaction can be carried out even in air at high temperature. The produced divalent rare earth ions can be detected by luminescence and XANES methods and their spectroscopic properties are discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Divalent rare earth ions; Reduction; Luminescence; Borate; Borophosphate
1. Introduction In 1993, we provided an aliovalent substitution method to reduce trivalent rare earth ions Sm 31 , Eu 31 or Yb 31 into divalent ions Sm 21 , Eu 21 or Yb 21 even in air when these ions were doped in alkaline earth borate SrB 4 O 7 [1]. One of the necessary conditions for this method is that the alkaline earth borate must contains tetrahedral BO 4 group, as we know that structures of alkaline earth borates such as BaB 8 O 13 , SrB 6 O 10 and Sr 2 B 5 O 9 Cl all contain a tetrahedral BO 4 group. According to the principles proposed by us [1], we achieved reduction of Sm 31 [2,4,5], Eu 31 [3,6] and Yb 31 [4] into Ln 21 (Ln 21 5Sm,Eu,Yb) in BaB 8 O 13 ; reduction of Eu 31 [7,11], Sm 31 [8–11] and Tm 31 [11] 21 21 into Ln (Ln 5Sm,Eu,Tm) in SrB 6 O 10 ; and reduction of Eu 31 into Eu 21 in Sr 2 B 5 O 9 Cl [12,13] in air at high temperature. It is interesting to note that not only the rare earth ions mentioned above can be reduced into divalent state in these alkaline earth borates containing tetrahedral 31 BO 4 group, but Bi ion can also be reduced into divalent ion Bi 21 in BaB 8 O 13 even in air at high temperature [14]. Luminescent methods [1,15], ESR [3,7] and XANES *Corresponding author. Tel. / fax: 186-20-8411-1038. E-mail address: [email protected] (Q. Su).
[16] can be used to determine qualitatively the divalent state of rare earth ions. Peterson et al. used our method to reduce Tm 31 into Tm 21 [17] in 1995 and to stabilize the divalent Nd 21 in 1997 [18] in the same matrix SrB 4 O 7 in air. They extended our method to the actinide and to stabilize the divalent actinide Cf 21 in SrB 4 O 7 [19] in 1996. In 1999, Machida et al. also observed that most of the Eu 31 ions in the starting materials are reduced during the formation of crystalline BaB 8 O 13 host lattice by the simple solid state reaction even in air [20] as we did in 1995 [3]. They also found that the Eu 21 ions in the crystalline BaB 8 O 13 host lattice are oxidized into Eu 31 ion in the vitreous form derived from the melting of BaB 8 O 13 as the BO 4 units in the crystalline BaB 8 O 13 transform into the BO 3 units in the vitreous one. The valence change is structural modification-induced. From the results mentioned above, it is obvious that the tetrahedral BO 4 unit in the alkaline earth borate is an important structural factor for stabilization of the divalent rare earth ions formed in the solid state reaction in air at high temperature. Because phosphate anion exists also in the tetrahedral PO 4 unit, it is possible that the divalent rare earth ions such as Eu 21 or Sm 21 can be stabilized in the alkaline earth phosphate containing the tetrahedral PO 4
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00351-1
Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136
units, even when the synthesis is carried out in air at high temperature. It is proved that Eu 31 can be reduced in host (Ba) 3 (PO 4 ) 2 by solid state reaction at high temperature in air [21]. According to these facts, some rare earth ions such as Eu 31 or Sm 31 can also be reduced by solid state reaction in air at high temperature in the crystalline alkaline earth borophosphate hosts, which contain both tetrahedral BO 4 and PO 4 units. In this paper, we report the preparation of divalent Eu 21 ions in air by aliovalent substitution method in the crystalline alkaline earth borophosphate hosts CaBPO 5 and SrBPO 5 . The research on the phase relationship of the system CaO–B 2 O 3 –P2 O 5 indicates only one ternary compound CaBPO 5 could form in the system [22]. It is suggested by Raman spectra that the silica-like network anion units BPO 4 , where B and P are fourcoordinated exist in borophosphates [23,24]. And by crystal structural study, it is confirmed that CaBPO 5 crystallizes in stillwellite type [25] and contains central three single chains of BO 4 tetrahedron and link to terminal PO 4 tetrahedron to form loop-branched chains [26]. In recent years, the spectral properties of rare earth ions in different hosts in vacuum ultraviolet range (VUV, l, 200 nm) have been intensively studied. There are two reasons compelling the developments in research on rare earth ions in this VUV region. The first reason is to study the high-energy states of rare earth ions. Although energy levels up to 50 000 cm 21 for most rare earth ions have been observed, the reports on 4f n energy levels in VUV region are scarce. The second reason is to explore efficient VUV excited luminescent materials, which could be used in mercury-free fluorescent lamps or plasma display panels.
133
mined at room temperature and performed on a Hitachi F-4500 fluorescence spectrophotometer (resolution 0.2 nm) and a xenon lamp as excitation source. VUV spectra and XANES at Eu–L 3 edge of the samples were performed at Beijing Synchrotron Radiation Facilities (BSRF) on beam 3B1B at VUV spectral experimental station or on beam 4W1B at XAFS experimental station under normal operating conditions (2.2 GeV, |80 mA), respectively. For the determination of VUV spectra, an ARC-502 monochromator was used for the excitation spectrum, an ARC-308 monochromator was used for the emission spectrum and the signal was detected by a H7421-50 photomultiplier. The relative VUV excitation intensities of the sample were corrected by comparing the measured excitation intensities of the samples with the excitation intensities of sodium salicylate at the same excitation condition. For XANES measurements, X-ray photon was defined by a slit of size 1.0(H)310(V) mm 2 and monochromatized by a Si(111) double crystal monochromator whose energy resolution was 1.2310 24 DE /E. All VUV and XANES experimental data were collected at 293 K. In this work, the reduced behavior of europium in CaBPO 5 :Eu and SrBPO 5 :Eu prepared in air was checked by XANES at Eu–L 3 edge and luminescence of europium. In addition, the VUV excitation and VUV excited luminescent spectra of the material were reported.
3. Results and discussion
3.1. The reduced behavior of europium in CaBPO5 : Eu prepared in air
2. Experimental
3.1.1. Excitation spectra The excitation curves for Eu 21 and Eu 31 in the sample Ca 0.97 Eu 0.03 BPO 5 are presented in Fig. 1. They were obtained by f–d transition emission band of divalent
Powder samples, Sr 0.99 Eu 0.01 BPO 5 and Ca 0.97 Eu 0.03 BPO 5 , were synthesized by solid state reaction at high temperature in air. The powder sample Ca 0.97 Eu 0.03 BPO 5 was prepared by the method of solid phase reaction in air atmosphere. The mixed reactants, analytical-grade calcium carbonate, boric acid (excess 3 mol% to compensate the evaporation), ammonium dihydrophosphate and europium oxide (99.99%) were heated at 400 8C for 4 h, then firing them at 1000 8C for another 4 h in air atmosphere. The powder X-ray diffraction patterns of SrBPO 5 showed the prepared samples were single phases, corresponding to JCPDS 18-1270 (SrBPO 5 ) or Ref. [27]. The XRD data of CaBPO 5 indicated all of the synthesized samples were single hexagonal phase, and which were coincident with JCPDS 18-283. The constants of matrix crystal cell which calculated from d values were ˚ and c56.6234 A. ˚ a56.6772 A The emission and UV excitation spectra were deter-
Fig. 1. The UV excitation spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at room temperature: (a) excitation spectrum of Eu 21 , lem 5402 nm; (b) excitation spectrum of Eu 31 , lem 5592 nm.
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Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136
europium at 402 nm and f–f transition emission line of trivalent europium at 592 nm, respectively. The excitation spectrum exhibited a broad band with a maximum at 312 nm and which was caused by the permitted 4f 6 5d– 4f 7 ( 8 S 7 / 2 ) transition of Eu 21 . The f–f transition of Eu 31 led to the sharp lines, and the peaking of the prominent one was at 396 nm.
3.1.2. Emission spectra As shown in Fig. 2, when the sample was excited with wavelength 312 nm, a broad emission band appeared, it was caused by 4f 7 ( 8 S 7 / 2 )–4f 6 5d transition of divalent europium and the maximum centered at 402 nm with half width about 25 nm. When the sample was excited with wavelength 396 nm, the emission narrow lines of trivalent europium could be observed, which were due to the 5 D 0 – 7 FJ (J50, 1, 2) transitions of Eu 31 ions and were situated at 586, 592, 610 and 619 nm, respectively. The luminescent spectrum of Eu 21 in CaBPO 5 :Eu prepared in air is coincident with that prepared in H 2 / N 2 reducing atmosphere [28]. From excitation and emission spectra, it could be observed that parts of trivalent europium ions had been reduced to the divalent in the sample Ca 12x Eu x BPO 5 (x50.03) prepared in air at high temperature.
Fig. 3. The VUV spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at 293 K: solid line, emission spectrum, lex 5147 nm; dash–dot line, excitation spectrum of Eu 21 , lem 5410 nm; dash line, excitation spectrum of Eu 31 , lem 5594 nm.
Eu 21 , respectively. The emission spectrum (solid curve) of the sample excited by VUV at 147 nm was similar with Fig. 2, the f–d transition of Eu 21 with a maximum at about 410 nm and the f–f transition of Eu 31 peaks, respectively, at 588, 594, 614, 620, 652, 686 and 698 nm.
3.1.3. VUV excitation spectra Fig. 3 showed VUV excitation spectra of Eu 21 (dash– dot line) and Eu 31 (dash curve) emission in Ca 12x Eu x BPO 5 (x50.03) prepared in air. In the VUV excitation spectrum of Eu 21 , there were two bands centered around 113 nm (112 nm in Eu 21 curve and 114 nm in Eu 31 curve) and 158 nm (162 nm in Eu 21 and 155 nm in Eu 31 ). Because the bands were present when monitored at either the emission of Eu 21 or the emission of Eu 31 , they were most probably due to the absorption of the host lattice. The bands at 246 and 320 nm were obviously the charge transfer band of Eu 31 and the f–d transition of
3.2. XANES at Eu–L3 edge
Fig. 2. The UV excited emission spectra of Ca 0.97 Eu 0.03 BPO 5 prepared in air at room temperature: (a) emission spectrum of Eu 21 , lex 5312 nm; (b) emission spectrum of Eu 31 , lex 5396 nm.
Fig. 4. XANES of Ca 0.97 Eu 0.03 BPO 5 at Eu–L 3 edge (solid line), the fitted curve (dot line) and deconvolution lines: (a) L 3 edge of Eu 21 ; (b) L 3 edge of Eu 31 ; (c) step function.
By XANES investigations of some rare earth compounds, it has been reported [23] that L 3 edge of RE 21 and RE 31 ions locate at different peak positions, and the discrepancy is about 7.5 eV. So the existence of Eu 21 or Eu 31 ions in MBPO 5 :Eu (M5Ca,Sr) could be probed by XANES.
3.2.1. XANES of Ca0.97 Eu0.03 BPO5 at Eu–L3 edge The result of XANES at Eu–L 3 edge in Ca 0.97 Eu 0.03 BPO 5 is shown in Fig. 4. Fitted curves and deconvolution lines were also depicted. Double peaks
Q. Su et al. / Journal of Alloys and Compounds 344 (2002) 132–136
appeared in XANES of the sample at Eu–L 3 edge. The 21 lower energy edge was corresponding to Eu ions, while 31 the higher one was corresponding to Eu ions. The 31 21 positions of L 3 edge of Eu and Eu ions were at 6972.8 eV and 6964.8 eV, respectively. The energy of L 3 edge of Eu 21 was 8.0 eV lower than that of Eu 31 . Just as the luminescent results, the XANES results also indicated that Eu 31 ions could be reduced into divalent Eu 21 in Ca 0.97 Eu 0.03 BPO 5 prepared in air.
3.2.2. XANES of SrBPO5 : Eu at Eu–L3 edge The result of XANES at Eu–L 3 edge in Sr 0.99 Eu 0.01 BPO 5 was shown in Fig. 5. Fitted curve and deconvolution lines were also depicted. Double peaks appeared in XANES of the sample at Eu–L 3 edge. The lower energy edge was corresponding to Eu 21 ions, while the higher one was corresponding to Eu 31 ions. The positions of L 3 edge of Eu 31 and Eu 21 ions were at 6972.9 and 6964.9 eV, respectively. The energy of L 3 edge of Eu 21 was 8.0 eV lower than that of Eu 31 . The results indicated that Eu 31 ions could be reduced into divalent Eu 21 in Sr 0.99 Eu 0.01 BPO 5 prepared in air. We think the mechanism of this abnormal reduction of Eu 31 in M 12x Eu x BPO 5 (M5Ca,Sr) in oxygenating atmosphere is connected with two reasons. First, aliovalent substitution brings about reduction, and second, the rigid tetrahedron structure of anions BPO 4 guarantees the reduced valence state steadiness. When Eu 31 were built into the matrix, they would first replace M 21 . In order to keep the electroneutrality of the compound, two Eu 31 ions would substitute for three M 21 ions. Therefore, two positive defects of [Eu M ] ? and one negative M 21 vacancy of [VM ]0 would be created by each substitution. By thermal stimulation, electrons of the [VM ]0 vacancies would be transferred to doped Eu 31 ions and reduce them. The similar process had been suggested for the reduction of Eu 31 in Ca 2 B 5 O 9 Cl:Eu by our group [12].
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It had been reported that all anions are in the form of the rigid tetrahedron structure of BO 4 and PO 4 in the host CaBPO 5 [26]. We think that this factor is suitable to stabilize the Eu 21 ions. Eu 21 ions occupy the substituted lattice sites of M 21 ions, this rigid tetrahedral structure can efficiently enclose Eu 21 ion, protect the Eu 21 from the attack of the oxygen in air, so the reduced state is easy to be stabilized. This is supported by the facts that the reduction of rare earth ions occurs in the matrices such as SrB 4 O 7 , SrB 6 O 10 and BaB 8 O 13 [1,8,17]. In host SrB 4 O 7 , 31 all anions exist in the form of tetrahedron BO 4 , the Eu can be reduced in air. While in host BaB 8 O 13 , the anions consist of triangular structure BO 3 and tetrahedron BO 4 , the Eu 31 can also be reduced in air, but fewer Eu 31 ions can be reduced in BaB 8 O 13 :Eu than in SrB 4 O 7 :Eu [16]. And in Sr 3 B 2 O 6 , Sr 2 B 2 O 5 and SrB 2 O 4 , the anions contain only triangular BO 3 group, Eu 31 ion cannot be reduced in air by aliovalent substitution [1].
4. Conclusions VUV/ UV excitation and emission spectra of CaBPO 5 :Eu and XANES of CaBPO 5 :Eu and SrBPO 5 :Eu suggested that Eu 31 could be reduced to Eu 21 in air at high temperature. (1) The excitation and emission spectra of CaBPO 5 :Eu presented the characteristic d–f transition band of Eu 21 . The results implied that the trivalent europium ions were reduced. From the VUV excitation spectra, it could be observed that the material has absorption with the maximum around 113 and 158 nm, respectively. (2) For the XANES of both the CaBPO 5 :Eu and SrBPO 5 :Eu, at Eu–L 3 edge, the absorption peak for Eu 21 is at 6964.9 eV and that for Eu 31 is at 6972.9 eV, respectively. It also indicated the existence of Eu 21 in the sample, and is consistent with the luminescent results. Divalent europium ions can stably exist in the matrix when synthesized in oxygenating atmosphere, which are probably due to aliovalent replacement and the existence of the rigid tetrahedron structure of BPO 4 .
Acknowledgements The work is supported by State Key Project of Basic Research of China, the National Natural Science Foundation of China and the Foundation of Laboratory of Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences.
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