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EPR and optical studies of Cr-doped PbWO4 single crystals
Optical Materials 29 (2007) 457–461 www.elsevier.com/locate/optmat
EPR and optical studies of Cr-doped PbWO4 single crystals Weifeng Li a
a,b
, Xiqi Feng
a,*
The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, Shanghai 200050, China b Graduate School of Chinese Academy of Science, Beijing, China Received 24 January 2005; accepted 22 July 2005 Available online 4 January 2006
Abstract Electron paramagnetic resonance (EPR) studies were performed on Cr ions in PbWO4 crystals using an X-band spectrometer. With the help of Fourier-transform infrared (FTIR) and thermoluminescence (TSL) data, it was proposed that Cr3+ impurities enter PbWO4 lattice in the Pb-site and result in large relaxation of crystal lattice, which was illustrated by the significant distortion of WO4 tetrahedron and the rapid decrease of blue luminescence band. Further, for heavily doped samples some new EPR signals, which did not appear in the lightly doped sample, were detected. The new EPR lines were ascribed to the exchange-coupled effect of Cr3+-ion pairs. Luminescence spectra provide support to the idea about defect clusters. The presence of Cr3+-ion pairs or Cr3+-ion associations in heavily doped PWO is the reason for the luminescence quench of PWO host and Cr3+ ion. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 76.30.Fc; 76.30.v; 78.20.e Keywords: A. PbWO4 crystal; B. Doping; C. Point defects; E. EPR; E. Photoluminescence
1. Introduction Trivalent chromium in various materials has been extensively studied due to their potential applications as active media for tunable laser, and to the in-depth understanding of the interaction between impurity and host lattice [1–4]. Cr3+ has rich spectroscopic features and these spectroscopies, such as optical and EPR, depend strongly on the site location and local symmetry. Therefore, Cr3+ serves as a sensitive probe for surveying the coordination environment [4–6]. It is the intention of this paper to present a series of spectra to investigate behaviors of chromium impurities and the interplay among defects in PbWO4 (abbreviated *
Corresponding author. Address: The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, Shanghai 200050, China. Tel.: +86 21 52412416; fax: +86 21 52413122. E-mail address: [email protected] (X. Feng). 0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.07.014
PWO) host, which is recognized as a good scintillator in the future generation of colliders in high-energy physics [7]. Although such crystals have been under investigation for several decades, some problems, especially about defect structures, are still debated. PWO is a defect system having the remarkable property that large concentration of aliovalent ions can be accommodated within its structure with no loss of structural integrity [8–10]. Since the contribution to scintillation efficiency and scintillation kinetics is related to the presence of the structural defects and the energy-transfer processes between them, the defect structures play a very important role in PWO. It is thus necessary to clarify behaviors of intrinsic defects and impurities in PWO. In this paper, a series of Cr-doped PWO crystals have been studied by EPR, FTIR, TSL and luminescence methods. The doping mechanism was consequently discussed and a satisfactory interpretation was presented on the luminescence-quenching phenomena in Cr-doped PWO.
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2. Experimental All the investigated PWO crystals were grown using Czochralski method. The concentrations of Cr3+ in the melt were 100 (light yellow) and 2000 ppm (brown), respectively. The content of impurities in PWO:Cr,La samples were: (a) 0.3 mol% Cr3+ + 0.6 mol% La3+, (b) 0.4 mol% Cr3+ + 0.8 mol% La3+. The crystal axes were checked by X-ray analysis. Samples were prepared as follows: (1) samples of 8.0 · 2.0 · 2.0 mm3 for EPR experiments, with their longer dimension parallel to the a-axis, (2) plates of 8 · 8 · 2 mm3 for optical measurements, with their large faces normal to the crystal c-axis, (3) powder sample from the same as-grown crystals for XRD and FT-IR examination. The structure of the crystals were analyzed by powder X-ray diffraction (XRD) method using Rigaku D/max 2550 V diffractometer, CuKa. The EPR spectra were obtained with an ER 200D-SRC spectrometer in the X band with a field modulation frequency of 100 kHz at room temperature. A Digilab-FTS-80 spectrophotometer was employed to record the FTIR data. The TSL investigations after UV light irradiation were performed by FJ427A thermo-luminescence spectrometer with a heating rate of 2 K/s. A 500 W high-pressure mercury lamp was used for irradiating the samples for 15 min. X-ray excited luminescence (XEL) spectra were measured using an X-ray-excited spectrometer, FluorMain, where an F-30 movable X-ray tube (W anticathode target) was used as the X-ray source, and operated under the same condition (80 kV, 4 mA). Photoluminescence (PL) spectra were taken with a Perkin–Elmer LS-55 fluorescence spectrophotometer.
The scheelite-type PWO crystal has a tetragonal structure belonging to space group I41/a. Each Pb ion is surrounded by eight oxygen ions, which come from eight WO4 tetrahedrons. W ions are four-coordinated by O ions. Fig. 2a shows the EPR spectra of Cr:PWO samples measured at room temperature. A number of EPR signals in Cr-doped crystals are observed. These signals have not been found in pure PWO crystals and moreover, their intensities enhance with the increase of dopant concentration. Therefore, the EPR signals here are ascribed to Cr impurities. The other proof comes from the EPR results of PWO:Cr,La crystals, in which all Cr3+ ions have been transformed into Cr4+ ions and the used raw Cr2O3 in the process of crystal growth is the same as that of PWO:Cr samples [11]. Usually, the EPR signals of Cr4+ in X-band at room temperature are difficult to be detected [12] and in our experiments, no EPR signal related to Cr4+ was found in Fig. 2b. Fig. 2b also indicates that no other unwanted
3. Results and discussion The X-ray diffraction patterns in Fig. 1 reveal that all the investigated crystals keep the scheelite-style structure.
Fig. 1. X-ray diffraction patterns of Cr-doped PWO crystals.
Fig. 2. EPR spectra, measured at room temperature: (a) of Cr-doped, (b) Cr,La-co-doped PWO crystals with the magnetic field parallel and normal to the c-axis. Arrows denote lines that were not detected in lightly doping sample.
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impurities, such as Fe3+ or Gd3+ ions, could make contribution to the EPR signals in our samples. In PWO, there are two possible sites for paramagnetic impurities, Pb site and W site. In theory, the EPR spectra cannot be experimentally obtained if Cr3+ resides at tetrahedral coordination (here, W-site) [13]. One should also note that the Cr3+ ion has not been reported in tetrahedral symmetry previously. This is contrary to our experimental results. The properties of Cr3+ in octahedral coordination have been widely studied. Since the excited states are far from the fundamental state, the EPR spectra can be obtained at room temperature [14]. However, it seems that this is not yet our case because there is no octahedral coordination site in PWO. Let us see an example of Cr3+ in fluorite-type crystals [15], which can evolve into tetragonal scheelite-type structure of PWO. Cr3+ usually enters these matrices occupying cation position with an unusual eightfold co-ordination and produces EPR signal. It is also expected that Cr3+ ions go into Pb-sites with an unusual eightfold co-ordination in PWO host. An approximate octahedron for Cr3+ is produced by a displacement of two oxygen ions away from the Cr3+ ion and an inward movement of the other six oxygen ions. It can thus conclude that Cr3+ ions locating at distorted octahedra contribute to the EPR lines of the Fig. 2a. Taking into account the misfit of radii and effective ˚ ) and Pb2+ (rCN = 8 = charge between Cr3+ (rCN = 6 = 0.62 A ˚ ), the local relaxation of crystal lattice is necessary 1.20 A to reduce defect-induced lattice strains when the substitution reaction of Cr3+ in Pb2+ occurs. The hypothesis is supported by FTIR data. The theoretical analysis gives that PWO has four infrared active vibration modes [16], among which W–O vibrations correspond to the strong absorption band of 400–1000 cm1. Accordingly, FTIR spectra can be employed to monitor the change of WO4 group. Fig. 3 exhibits the FTIR spectra of pure and Cr-doped PWO crystal. With the increase of dopant, the shape of the char-
acteristic absorption band of WO4 deforms and its FWHM widens. It indicates that the amount of distorted WO4 tetrahedrons increases. Thus our suggestion above, that is, the doped Cr3+ will cause the distortion of the WO4 tetrahedrons near it, is demonstrated. The TSL glow curves also help us to well understand the substitution reaction of Cr3+ in Pb2+. In Fig. 4, the TSL glow curves of pure and Cr-doped PWO samples are reported. One can see that the presence of Cr3+ ions suppresses strongly TSL peaks above room temperature, which are typical for pure PWO. Such behavior can be explained by the presence of competitive processes in carrier capture with traps related to RT—200 C peaks (the concentration of the deep traps being efficiently suppressed by Cr doping). The same behavior was also found in La-, Y-, Gd- and Lu-doped PWO, in which the trivalent cations were supposed to occupy Pb-site [17,18]. For heavily Cr-doped sample some new EPR lines, which does not appear in the lightly doped one, have been detected. Who should be responsible for these new strong lines? The absorption spectra of PWO:Cr in the NIR wavelength region reveal that Cr4+ in a tetrahedral site was formed in the heavily doped sample, as shown in Fig. 5. However, the weak absorption in Fig. 5 is inconsistent with the strong signal in Fig. 2a, and according to above discussion, Cr4+ in PWO host could not give rise EPR signal. On the other hand, it is noticed that in heavily Cr-doped material, besides isolated Cr3+ ion, the close exchange-coupled Cr3+–Cr3+ pairs also make a measurable contribution to optical and EPR spectra [19,20]. Moreover, the La dimmers or small aggregates have been reported in heavily La-doped PWO [21,22]. Hence, Cr3+-ion-pairs or Cr3+ion-associations could form in the heavily Cr-doped PWO and its close exchange-coupled effect results in the new EPR signal. Fig. 6 presents the XEL spectra of samples. With increasing Cr-doping concentration, the host luminescence of PWO (at the range of 350–600 nm) decreases significantly.
Fig. 3. FT-IR spectra of pure and Cr-doped PWO crystals.
Fig. 4. TSL glow curves of pure and Cr-doped PWO crystals above RT after UV light irradiation for 15 min.
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Fig. 5. NIR optical absorption spectra of pure and Cr-doped PWO crystals.
Fig. 6. XEL spectra of pure and Cr-doped PWO crystals.
Finally, the blue luminescence band disappears in heavily doped sample. For 100 ppm Cr-doped PWO, a new luminescence band centered about 760 nm appears, which possibly comes from the electronic transition of 4T2 ! 4A2 of Cr3+. However, the band disappears in 2000 ppm Cr-doped one. It indicates that a new nonradiative center related to Cr3+ ion forms due to the increase of Cr3+. In order to minimize the self-absorption effects of bulk crystals, the standard 45 geometry photoluminescence measurements under 310 nm UV light and excitation spectra by monitoring 390 nm emission are carried out, as shown in Figs. 7 and 8. The results are similar to that of XEL. On the emission spectra, the blue band dominates the spectra of pure PWO and a weak green band overlaps it. The addition of Cr3+ reduces the luminescence intensity and divides distinctly the band into two parts; one centered at around 390 and the other 512 nm. It is also found that the blue band diminishes more rapidly than that of the green one. So we could imagine that there might be some
Fig. 7. The emission spectra (excited by 310 nm) of pure and Cr-doped PWO crystals.
Fig. 8. The excitation spectra (monitoring 390 nm emission) of pure and Cr-doped PWO crystals.
changes to regular [WO4]2 groups, which are assumed to be the blue luminescence centers in tungsten scheelites [23]. It is consistent with the previous EPR and FTIR results. At highly doping level, the luminescence is so weak that it can hardly be detected. On excitation spectra of 390 nm emission, excitation peak at 315 nm turns weaker and weaker and almost disappears on that of PWO doped by Cr3+ at 2000 ppm. This implies that heavy Cr-doping annihilates intrinsic lattice defects and intensively suppresses exciton luminescence efficiency. The excitation mechanism might be changed due to the formation of new trap centers, which are involved in the energy transfer and storage process in these crystals [22]. The aforementioned Cr3+-ion pairs or Cr3+ion associations could act as ‘‘luminescence killing centers’’ where effective nonradiative recombination of free electrons and holes occurs and hence intensively suppress the luminescence of PWO.
W. Li, X. Feng / Optical Materials 29 (2007) 457–461
4. Conclusions EPR spectroscopy in the X band has been applied to chromium-doped PWO crystals at various concentrations. In lightly doping case, all EPR lines are attributed to Cr3+ ion in an approximate octahedron, which comes from the distortion of Pb2+ site. The ðCr3þ Pb Þ exerts great influence on WO4 group nearby, which is confirmed by FTIR data. Thereby, the blue band of PL rapidly decreases. At highly doping level, some new lines, which do not appear in the weakly doped sample, can be discerned. It is sug gested that Cr3+-ion pairs in addition to ðCr3þ Pb Þ contribute 3+ to the EPR spectroscopy. The formation of Cr -ion pairs or Cr3+-ion associations could be responsible for the luminescence quench of PWO and Cr3+ ion. Acknowledgement This work is supported by the National Science Foundation of China. (Grant No. 50172054). References [1] J.C. Walling, O.G. Peterson, H.P. Jenssen, R.C. Morris, E.W. O’Dell, IEEE J. Quantum Elect. QE-16 (1980) 1302. [2] B. Struve, G. Huber, Appl. Phys. B 36 (1985) 195. [3] S.A. Basun, A.A. Kaplyanskii, A.B. Kutsenko, V. Dierolf, T. Troester, S.E. Kapphan, K. Polgar, Appl. Phys. B 73 (2001) 453. [4] T.H. Yeom, I.G. Kim, S.H. Lee, S.H. Choh, Y.M. Yu, J. Appl. Phys. 93 (2003) 3315. [5] M. Haouari, H.B. Ouada, H. Maaref, H. Hommel, A.P. Lefrand, J. Phys.: Condens. Matter 9 (1997) 6711. [6] M. Casalboni, A. Luci, U.M. Grassamo, B.V. Mill, A.A. Kaminskii, Phys. Rev. B 49 (1994) 3781.
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[7] M. Kobayashi, M. Ishii, Y. Usuki, H. Yahagi, Nucl. Instr. Methods A 333 (1993) 429. [8] M. Kobayashi, Y. Usuki, M. Ishii, N. Senguttuvan, K. Tanji, M. Chiba, K. Hara, H. Takano, M. Nikl, P. Bohacek, S. Baccaro, M. Diemoz, Nucl. Instr. Methods A 434 (1999) 412. [9] M. Nikl, K. Nitsch, S. Baccaro, A. Cecilia, M. Montecchi, B. Borgia, I. Dafinei, M. Diemoz, M. Martini, E. Rosetta, G. Spinolo, A. Vedda, M. Kobayashi, M. Ishii, Y. Usuki, O. Jarolimek, P. Reiche, J. Appl. Phys. 82 (1997) 5758. [10] Q.S. Lin, X.Q. Feng, J.T. Chen, J. Alloys Comp. 307 (2000) 245. [11] W.F. Li, X.Q. Feng, Y.M. Kan, Phys. Rev. B 71 (2005) 104101-1. [12] D.E. Budil et al., J. Chem. Phys. 101 (1994) 3538. [13] F.J. Lo´pez, E. Moya, C. Zaldo, Solid State Commun. 76 (1990) 1169. [14] J.E. Wertz, J.R. Bolton, Electron Spin Resonance: Elementary Theory and Practical Applications, McGraw-Hill, New York, 1972. [15] R. Alcala, P.J. Alonso, V.M. Orera, H.W.D. Hartog, Phys. Rev. B 32 (1985) 4185. [16] P. Tarte, M. Liegeois-Duyckaerts, Spectrochim. Acta Part A 28 (1972) 2029. [17] S. Baccro, P. Bohacek, B. Borgia, A. Cecilia, I. Dafinei, M. Diemoz, M. Ishii, O. Jarolimek, M. Kobayashi, M. Martini, M. Montecchi, M. Nikl, K. Nitsch, Y. Usuki, A. Vedda, Phys. Status Solidi A 160 (1997) R5. [18] S. Baccro, P. Bohacek, A. Cecilia, I. Dafinei, M. Diemoz, P. Fabeni, M. Ishii, M. Kobayashi, M. Martini, E. Mihokova, M. Nikl, G.P. Pazzi, J. Rosa, Y. Usuki, A. Vedda, Radiat. Eff. Def. Solids 150 (1999) 15. [19] G.G. Siu, M.G. Zhao, Phys. Rev. B 43 (1991) 13575. [20] V.G. Grachev, G.I. Malovichko, V.V. Troitskii, Sov. Phys. Solid State 29 (1987) 349. [21] Q.S. Lin, X.Q. Feng, Ferroelectrics 251 (2001) 85. [22] M. Nikl, P. Bohacek, E. Mihokova, M. Martini, A. Vedda, S. Croci, G.P. Pazzi, P. Fabeni, S. Baccaro, B. Borgia, I. Dafinei, M. Diemoz, G. Organtini, E. Auffray, P. Lecoq, M. Kobayashi, M. Ishii, Y. Usuki, Appl. Phys. Lett. 71 (1997) 3755. [23] W. Van Loo, Phys. Status Solidi A 27 (1975) 565.
EPR and optical studies of Cr-doped PbWO4 single crystals Weifeng Li a
a,b
, Xiqi Feng
a,*
The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, Shanghai 200050, China b Graduate School of Chinese Academy of Science, Beijing, China Received 24 January 2005; accepted 22 July 2005 Available online 4 January 2006
Abstract Electron paramagnetic resonance (EPR) studies were performed on Cr ions in PbWO4 crystals using an X-band spectrometer. With the help of Fourier-transform infrared (FTIR) and thermoluminescence (TSL) data, it was proposed that Cr3+ impurities enter PbWO4 lattice in the Pb-site and result in large relaxation of crystal lattice, which was illustrated by the significant distortion of WO4 tetrahedron and the rapid decrease of blue luminescence band. Further, for heavily doped samples some new EPR signals, which did not appear in the lightly doped sample, were detected. The new EPR lines were ascribed to the exchange-coupled effect of Cr3+-ion pairs. Luminescence spectra provide support to the idea about defect clusters. The presence of Cr3+-ion pairs or Cr3+-ion associations in heavily doped PWO is the reason for the luminescence quench of PWO host and Cr3+ ion. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 76.30.Fc; 76.30.v; 78.20.e Keywords: A. PbWO4 crystal; B. Doping; C. Point defects; E. EPR; E. Photoluminescence
1. Introduction Trivalent chromium in various materials has been extensively studied due to their potential applications as active media for tunable laser, and to the in-depth understanding of the interaction between impurity and host lattice [1–4]. Cr3+ has rich spectroscopic features and these spectroscopies, such as optical and EPR, depend strongly on the site location and local symmetry. Therefore, Cr3+ serves as a sensitive probe for surveying the coordination environment [4–6]. It is the intention of this paper to present a series of spectra to investigate behaviors of chromium impurities and the interplay among defects in PbWO4 (abbreviated *
Corresponding author. Address: The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramic, Chinese Academy of Science, Shanghai 200050, China. Tel.: +86 21 52412416; fax: +86 21 52413122. E-mail address: [email protected] (X. Feng). 0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.07.014
PWO) host, which is recognized as a good scintillator in the future generation of colliders in high-energy physics [7]. Although such crystals have been under investigation for several decades, some problems, especially about defect structures, are still debated. PWO is a defect system having the remarkable property that large concentration of aliovalent ions can be accommodated within its structure with no loss of structural integrity [8–10]. Since the contribution to scintillation efficiency and scintillation kinetics is related to the presence of the structural defects and the energy-transfer processes between them, the defect structures play a very important role in PWO. It is thus necessary to clarify behaviors of intrinsic defects and impurities in PWO. In this paper, a series of Cr-doped PWO crystals have been studied by EPR, FTIR, TSL and luminescence methods. The doping mechanism was consequently discussed and a satisfactory interpretation was presented on the luminescence-quenching phenomena in Cr-doped PWO.
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2. Experimental All the investigated PWO crystals were grown using Czochralski method. The concentrations of Cr3+ in the melt were 100 (light yellow) and 2000 ppm (brown), respectively. The content of impurities in PWO:Cr,La samples were: (a) 0.3 mol% Cr3+ + 0.6 mol% La3+, (b) 0.4 mol% Cr3+ + 0.8 mol% La3+. The crystal axes were checked by X-ray analysis. Samples were prepared as follows: (1) samples of 8.0 · 2.0 · 2.0 mm3 for EPR experiments, with their longer dimension parallel to the a-axis, (2) plates of 8 · 8 · 2 mm3 for optical measurements, with their large faces normal to the crystal c-axis, (3) powder sample from the same as-grown crystals for XRD and FT-IR examination. The structure of the crystals were analyzed by powder X-ray diffraction (XRD) method using Rigaku D/max 2550 V diffractometer, CuKa. The EPR spectra were obtained with an ER 200D-SRC spectrometer in the X band with a field modulation frequency of 100 kHz at room temperature. A Digilab-FTS-80 spectrophotometer was employed to record the FTIR data. The TSL investigations after UV light irradiation were performed by FJ427A thermo-luminescence spectrometer with a heating rate of 2 K/s. A 500 W high-pressure mercury lamp was used for irradiating the samples for 15 min. X-ray excited luminescence (XEL) spectra were measured using an X-ray-excited spectrometer, FluorMain, where an F-30 movable X-ray tube (W anticathode target) was used as the X-ray source, and operated under the same condition (80 kV, 4 mA). Photoluminescence (PL) spectra were taken with a Perkin–Elmer LS-55 fluorescence spectrophotometer.
The scheelite-type PWO crystal has a tetragonal structure belonging to space group I41/a. Each Pb ion is surrounded by eight oxygen ions, which come from eight WO4 tetrahedrons. W ions are four-coordinated by O ions. Fig. 2a shows the EPR spectra of Cr:PWO samples measured at room temperature. A number of EPR signals in Cr-doped crystals are observed. These signals have not been found in pure PWO crystals and moreover, their intensities enhance with the increase of dopant concentration. Therefore, the EPR signals here are ascribed to Cr impurities. The other proof comes from the EPR results of PWO:Cr,La crystals, in which all Cr3+ ions have been transformed into Cr4+ ions and the used raw Cr2O3 in the process of crystal growth is the same as that of PWO:Cr samples [11]. Usually, the EPR signals of Cr4+ in X-band at room temperature are difficult to be detected [12] and in our experiments, no EPR signal related to Cr4+ was found in Fig. 2b. Fig. 2b also indicates that no other unwanted
3. Results and discussion The X-ray diffraction patterns in Fig. 1 reveal that all the investigated crystals keep the scheelite-style structure.
Fig. 1. X-ray diffraction patterns of Cr-doped PWO crystals.
Fig. 2. EPR spectra, measured at room temperature: (a) of Cr-doped, (b) Cr,La-co-doped PWO crystals with the magnetic field parallel and normal to the c-axis. Arrows denote lines that were not detected in lightly doping sample.
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impurities, such as Fe3+ or Gd3+ ions, could make contribution to the EPR signals in our samples. In PWO, there are two possible sites for paramagnetic impurities, Pb site and W site. In theory, the EPR spectra cannot be experimentally obtained if Cr3+ resides at tetrahedral coordination (here, W-site) [13]. One should also note that the Cr3+ ion has not been reported in tetrahedral symmetry previously. This is contrary to our experimental results. The properties of Cr3+ in octahedral coordination have been widely studied. Since the excited states are far from the fundamental state, the EPR spectra can be obtained at room temperature [14]. However, it seems that this is not yet our case because there is no octahedral coordination site in PWO. Let us see an example of Cr3+ in fluorite-type crystals [15], which can evolve into tetragonal scheelite-type structure of PWO. Cr3+ usually enters these matrices occupying cation position with an unusual eightfold co-ordination and produces EPR signal. It is also expected that Cr3+ ions go into Pb-sites with an unusual eightfold co-ordination in PWO host. An approximate octahedron for Cr3+ is produced by a displacement of two oxygen ions away from the Cr3+ ion and an inward movement of the other six oxygen ions. It can thus conclude that Cr3+ ions locating at distorted octahedra contribute to the EPR lines of the Fig. 2a. Taking into account the misfit of radii and effective ˚ ) and Pb2+ (rCN = 8 = charge between Cr3+ (rCN = 6 = 0.62 A ˚ ), the local relaxation of crystal lattice is necessary 1.20 A to reduce defect-induced lattice strains when the substitution reaction of Cr3+ in Pb2+ occurs. The hypothesis is supported by FTIR data. The theoretical analysis gives that PWO has four infrared active vibration modes [16], among which W–O vibrations correspond to the strong absorption band of 400–1000 cm1. Accordingly, FTIR spectra can be employed to monitor the change of WO4 group. Fig. 3 exhibits the FTIR spectra of pure and Cr-doped PWO crystal. With the increase of dopant, the shape of the char-
acteristic absorption band of WO4 deforms and its FWHM widens. It indicates that the amount of distorted WO4 tetrahedrons increases. Thus our suggestion above, that is, the doped Cr3+ will cause the distortion of the WO4 tetrahedrons near it, is demonstrated. The TSL glow curves also help us to well understand the substitution reaction of Cr3+ in Pb2+. In Fig. 4, the TSL glow curves of pure and Cr-doped PWO samples are reported. One can see that the presence of Cr3+ ions suppresses strongly TSL peaks above room temperature, which are typical for pure PWO. Such behavior can be explained by the presence of competitive processes in carrier capture with traps related to RT—200 C peaks (the concentration of the deep traps being efficiently suppressed by Cr doping). The same behavior was also found in La-, Y-, Gd- and Lu-doped PWO, in which the trivalent cations were supposed to occupy Pb-site [17,18]. For heavily Cr-doped sample some new EPR lines, which does not appear in the lightly doped one, have been detected. Who should be responsible for these new strong lines? The absorption spectra of PWO:Cr in the NIR wavelength region reveal that Cr4+ in a tetrahedral site was formed in the heavily doped sample, as shown in Fig. 5. However, the weak absorption in Fig. 5 is inconsistent with the strong signal in Fig. 2a, and according to above discussion, Cr4+ in PWO host could not give rise EPR signal. On the other hand, it is noticed that in heavily Cr-doped material, besides isolated Cr3+ ion, the close exchange-coupled Cr3+–Cr3+ pairs also make a measurable contribution to optical and EPR spectra [19,20]. Moreover, the La dimmers or small aggregates have been reported in heavily La-doped PWO [21,22]. Hence, Cr3+-ion-pairs or Cr3+ion-associations could form in the heavily Cr-doped PWO and its close exchange-coupled effect results in the new EPR signal. Fig. 6 presents the XEL spectra of samples. With increasing Cr-doping concentration, the host luminescence of PWO (at the range of 350–600 nm) decreases significantly.
Fig. 3. FT-IR spectra of pure and Cr-doped PWO crystals.
Fig. 4. TSL glow curves of pure and Cr-doped PWO crystals above RT after UV light irradiation for 15 min.
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Fig. 5. NIR optical absorption spectra of pure and Cr-doped PWO crystals.
Fig. 6. XEL spectra of pure and Cr-doped PWO crystals.
Finally, the blue luminescence band disappears in heavily doped sample. For 100 ppm Cr-doped PWO, a new luminescence band centered about 760 nm appears, which possibly comes from the electronic transition of 4T2 ! 4A2 of Cr3+. However, the band disappears in 2000 ppm Cr-doped one. It indicates that a new nonradiative center related to Cr3+ ion forms due to the increase of Cr3+. In order to minimize the self-absorption effects of bulk crystals, the standard 45 geometry photoluminescence measurements under 310 nm UV light and excitation spectra by monitoring 390 nm emission are carried out, as shown in Figs. 7 and 8. The results are similar to that of XEL. On the emission spectra, the blue band dominates the spectra of pure PWO and a weak green band overlaps it. The addition of Cr3+ reduces the luminescence intensity and divides distinctly the band into two parts; one centered at around 390 and the other 512 nm. It is also found that the blue band diminishes more rapidly than that of the green one. So we could imagine that there might be some
Fig. 7. The emission spectra (excited by 310 nm) of pure and Cr-doped PWO crystals.
Fig. 8. The excitation spectra (monitoring 390 nm emission) of pure and Cr-doped PWO crystals.
changes to regular [WO4]2 groups, which are assumed to be the blue luminescence centers in tungsten scheelites [23]. It is consistent with the previous EPR and FTIR results. At highly doping level, the luminescence is so weak that it can hardly be detected. On excitation spectra of 390 nm emission, excitation peak at 315 nm turns weaker and weaker and almost disappears on that of PWO doped by Cr3+ at 2000 ppm. This implies that heavy Cr-doping annihilates intrinsic lattice defects and intensively suppresses exciton luminescence efficiency. The excitation mechanism might be changed due to the formation of new trap centers, which are involved in the energy transfer and storage process in these crystals [22]. The aforementioned Cr3+-ion pairs or Cr3+ion associations could act as ‘‘luminescence killing centers’’ where effective nonradiative recombination of free electrons and holes occurs and hence intensively suppress the luminescence of PWO.
W. Li, X. Feng / Optical Materials 29 (2007) 457–461
4. Conclusions EPR spectroscopy in the X band has been applied to chromium-doped PWO crystals at various concentrations. In lightly doping case, all EPR lines are attributed to Cr3+ ion in an approximate octahedron, which comes from the distortion of Pb2+ site. The ðCr3þ Pb Þ exerts great influence on WO4 group nearby, which is confirmed by FTIR data. Thereby, the blue band of PL rapidly decreases. At highly doping level, some new lines, which do not appear in the weakly doped sample, can be discerned. It is sug gested that Cr3+-ion pairs in addition to ðCr3þ Pb Þ contribute 3+ to the EPR spectroscopy. The formation of Cr -ion pairs or Cr3+-ion associations could be responsible for the luminescence quench of PWO and Cr3+ ion. Acknowledgement This work is supported by the National Science Foundation of China. (Grant No. 50172054). References [1] J.C. Walling, O.G. Peterson, H.P. Jenssen, R.C. Morris, E.W. O’Dell, IEEE J. Quantum Elect. QE-16 (1980) 1302. [2] B. Struve, G. Huber, Appl. Phys. B 36 (1985) 195. [3] S.A. Basun, A.A. Kaplyanskii, A.B. Kutsenko, V. Dierolf, T. Troester, S.E. Kapphan, K. Polgar, Appl. Phys. B 73 (2001) 453. [4] T.H. Yeom, I.G. Kim, S.H. Lee, S.H. Choh, Y.M. Yu, J. Appl. Phys. 93 (2003) 3315. [5] M. Haouari, H.B. Ouada, H. Maaref, H. Hommel, A.P. Lefrand, J. Phys.: Condens. Matter 9 (1997) 6711. [6] M. Casalboni, A. Luci, U.M. Grassamo, B.V. Mill, A.A. Kaminskii, Phys. Rev. B 49 (1994) 3781.
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