- Email: [email protected]
Photoluminescence and excited states dynamics in PbWO4:Pr3+ crystals
Journal of Luminescence 154 (2014) 381–386
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and excited states dynamics in PbWO4: Pr3 þ crystals E. Auffray a, M. Korjik b, T. Shalapska c, S. Zazubovich c,n a
CERN, Geneva 23, Geneva, Switzerland Institute for Nuclear Problems, 11 Bobruiskaya, 220020 Minsk, Belarus c Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2014 Received in revised form 5 May 2014 Accepted 7 May 2014 Available online 21 May 2014
Luminescence and photo-thermally stimulated defects creation processes are studied for a Pr3 þ -doped PbWO4 crystal at 4.2–400 K under excitation in the band-to-band, exciton, and charge-transfer transitions regions, as well as in the Pr3 þ -related absorption bands. Emission spectra of Pr3 þ centers depend on the excitation energy, indicating the presence of Pr3 þ centers of two types. The origin of these centers is discussed. The 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions of Pr3 þ ions, is found to be effectively excited in a broad intense absorption band peaking at 4.2 K at 3.92 eV. By analogy with some other Pr3 þ -doped compounds, this band is suggested to arise from an electron transfer from an impurity Pr3 þ ion to the crystal lattice W6 þ or Pb2 þ ions. The dynamics of the Pr3 þ related excited states is clarified. In the PbWO4:Pr crystal studied, the concentration of single oxygen and lead vacancies as traps for electrons and holes is found to be negligible. & 2014 Elsevier B.V. All rights reserved.
Keywords: Pr3 þ -doped lead tungstate Photoluminescence Thermoluminescence Charge-transfer state
1. Introduction Single crystals of lead tungstate attracted a wide interest as promising scintillation materials for calorimeters used in the high energy physics experiments owing to their high density, fast luminescence decay, strong radiation hardness, short radiation length, high chemical stability, and low cost (see, e.g., Refs. [1–5]). Co-doping of PbWO4 crystals with large and stable trivalent rare-earth ions (A3þ : La3 þ , Lu3þ , Y3þ , and Gd3þ ) was found to be an effective method for an improvement of their scintillation characteristics owing to the significant suppression of the concentration of traps for electrons and holes (see, e.g., reviews [4,6,7]). The PbWO4:A3 þ crystals, having the absorption and emission bands in the visible and infrared spectral regions, arising from the transitions between the energy levels of a rare-earth ion (e.g., Pr3 þ ), are promising materials also for lasers and mercury-free luminescence lamps (see, e.g., Ref. [8] and references therein). Heavily Pr3 þ -doped PbWO4 crystals were also proposed for application for dual scintillation/Cherenkov light detector [9]. In the luminescence spectrum of Pr3þ -doped PbWO4 single crystals, besides the intrinsic blue emission characteristic for the PbWO4 host, several groups of narrow emission lines appear in the energy region above 2.9 eV (see, e.g., Refs. [8–17]). In the samples studied in Refs. [10,11], about 50% of radioluminescence light was
n
Corresponding author. Tel.: þ 372 7374766; fax: þ 372 7383033. E-mail address: svet@fi.tartu.ee (S. Zazubovich).
http://dx.doi.org/10.1016/j.jlumin.2014.05.012 0022-2313/& 2014 Elsevier B.V. All rights reserved.
emitted from Pr3þ -related transitions at room temperature (RT) which suggests efficient energy capture at Pr3þ centers. It was concluded that such luminescence centers can effectively increase the scintillation light output. In Refs. [9,11,13], a strong Pr3þ -induced reduction of the intrinsic blue emission of PbWO4 was reported and explained by the energy transfer from the host lattice to the impurity Pr3 þ ions. As a result, in the crystals with sufficiently large Pr3 þ content, mainly the Pr3þ -related narrow emission bands, arising from the 4f-4f transitions of the Pr3þ ions, were observed in the emission spectrum. The detailed spectroscopic study of Pr3 þ -related centers in PbWO4 was carried out in many papers (see, e.g., Refs. [8,9,11–18] and references therein). The absorption bands of PbWO4:Pr, located in the 1–3 eV energy range, were assigned to the transitions from the 3H4 ground state to the excited multiplets 3P0,1,2, 1I6, 1D2, and 1G4 belonging to the 4f2 configuration. The luminescence bands in this energy range were ascribed to the electronic transitions from the above-mentioned excited levels to the 3H4,5,6 and 3F2,3,4 levels. The positions of the Pr3 þ -related absorption and emission bands are shown in Table 1. A quantum cutting process [15] and up-conversion luminescence [16,17] were also observed in PbWO4:Pr crystals. However, an influence of Pr3 þ ions on the origin and concentration of traps for electrons and holes in PbWO4 crystals was not studied in detail. In Ref. [9], the suppression of deep oxygen-vacancy-related electron traps with increasing Pr3 þ content was only reported. In this paper, we consider the luminescence characteristics, excited state dynamics, energy transfer and charge transfer processes in a PbWO4:Pr crystal in the 4.2–400 K temperature range. By analogy
382
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Table 1 Positions of absorption and emission bands of PbWO4:Pr located in the 1.9–2.8 eV energy range and arising from the 4f-4f transitions of Pr3 þ (for notations of the bands, the data of Ref. [17] are used). Absorption bands (295 K) 4f–4f transitions 3
3
H4- P2 H4-3P1 H4-3P0 3 H4-1D2 3 3
nm 447.58; 450.83 473.74 487.53 585.36; 592.63; 602.72
Emission bands (4.2 K) eV
4f–4f transitions
nm
eV
2.77; 2.75 2.617 2.543 2.118; 2.092; 2.057
3
451; 461.06 475.93; 477.94; 479.67 486.95; 495.92 529.82 553.48 601.55; 607.44; 610.43; 616.50 615.89; 619.9 644.05; 648.43
2.749; 2.689 2.605; 2.594; 2.583 2.546; 2.5 2.34 2.24 2.061; 2.041; 2.031; 2.011 2.013; 2.00 1.925; 1.912
3
P2- H4 P1-3H4 P0-3H4 3 P1-3H5 3 P0-3H5 1 D2-3H4 3 P0-3H6 3 P0-3F2 3 3
with our previous works (see, e.g., Refs. [7,19–22]), we also investigate defects creation processes under selective UV irradiation of the PbWO4:Pr crystal in the 3.5–5.0 eV energy range at the temperatures from 80 K to 200 K.
2. Experimental details A single crystal of PbWO4:Pr3 þ studied in the present paper was grown by the Czochralski method in Ar atmosphere at Bogoroditsk Technical Chemical Plant from the raw materials with purity 5 N. The concentration of Pr3 þ ions in the melt was 1 at%. The absorption spectrum was measured with the spectrophotometer JASCO V660. The steady-state emission and excitation spectra in the 80–400 K temperature range were measured using a setup, consisting of a deuterium DDS-400 lamp, two monochromators (SF-4 and SPM-1), and a vacuum nitrogen cryostat. The luminescence was detected by a photomultiplier (FEU-39 or FEU-79) with an amplifier and recorder. In the 4.2–300 K temperature range, the spectra were measured with the use of the computer-controlled setup consisting of the LOT-ORIEL xenon lamp (150 W), two monochromators (MDR-3 and ORIEL Corner Stone 1/8 m), a Hamamatsu 6240 photon counting system, and an immersion helium cryostat. The spectra were corrected for the spectral distribution of the excitation light, the transmission and dispersion of the monochromators, and the spectral sensitivity of the detectors. Thermally stimulated luminescence (TSL) glow curves were obtained with a constant heating rate of 0.2 K/s after selective irradiation of the crystal, located in the nitrogen cryostat, in the energy range 3.5–5.0 eV for 1 h at different temperatures from 80 K to 200 K with the deuterium DDS-400 lamp through a monochromator SF-4. The integrated emission of the crystal was detected with the photomultiplier FEU-39.
Fig. 1. Absorption spectrum of PbWO4:Pr measured at 295 K.
3. Results and discussion In the absorption spectrum of the PbWO4:Pr3 þ crystal measured at 295 K, besides the host lattice absorption in the E4 3.6 eV energy range, the narrow Pr3 þ -related absorption bands, located in the E o2.8 eV range and arising from the 4f-4f transitions of the Pr3 þ ions, are observed (Fig. 1). Under excitation in the band-to-band (Eexc ¼4.6 eV) and exciton (Eexc ¼ 4.18 eV) absorption regions at 4.2 K, the broad intrinsic blue (B) emission band of PbWO4 is observed around 2.9 eV (Fig. 2). The low-energy side of this emission band is strongly distorted due to the reabsorption in the Pr3 þ -related absorption bands located in the 2.5–2.8 eV energy range. No green G(I) emission, arising in the undoped PbWO4 crystals from lead-deficient crystal regions and excited around 3.8–3.9 eV (see, e.g., Ref. [23]), is observed in the
Fig. 2. Emission spectra of PbWO4:Pr measured at 4.2 K under excitation in the exciton absorption region (Eexc ¼4.18 eV, solid line) and in the band-to-band transitions region (Eexc ¼ 4.6 eV, dashed line).
PbWO4:Pr3 þ crystal. However, a weak UV emission band located at about 3 eV is excited in this energy range (see the insets in Fig. 3 (a) and Fig. 4(a)). Similar emission was detected in other A3 þ doped PbWO4 crystals [19] and ascribed to the radiative decay of an exciton localized near a trivalent rare-earth A3 þ ion [20]. No tunneling electron recombination G(II) emission, whose intensity should be maximum at 220 K under photostimulation around
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Fig. 3. Emission spectra of PbWO4:Pr measured at 4.2 K under excitation with (a) Eexc ¼ 3.8 eV and (b) Eexc ¼2.76 eV. In the inset of (a), the E3 eV emission band measured at 80 K is shown. In the inset of (b), the emission spectra measured at 4.2 K under excitation with Eexc ¼ 3.8 eV (solid line) and Eexc ¼ 4.18 eV (dashed line) are compared in the 1.9–2.1 eV energy range.
4.1 eV (see, e.g., Refs. [7,23]), is detected in the crystal studied. As this intrinsic emission arises from the recombination of the electrons trapped at oxygen vacancies with the holes trapped at lead vacancies [22], this result indicates a negligible number of lead and oxygen vacancies in the crystal studied. In the excitation spectrum of the B emission measured at 4.2 K, the lowest-energy exciton-related band is located at 4.18 eV (Fig. 4 (a)). Besides the intrinsic 2.9 eV emission, the lower-energy narrow emission bands of Pr3 þ -related centers, arising mainly from the radiative decay of the excited 3P2, 3P1, 3P0 levels (see Table 1), are all effectively excited in the band-to-band and exciton absorption regions of the PbWO4 host (Fig. 2) (mainly at Eexc 44.0 eV at 4.2 K, see, e.g., Fig. 4(b)). This indicates an effective energy transfer from the PbWO4 crystal lattice to Pr3 þ ions, which becomes possible due to an overlap of the B emission band of PbWO4 with the 3H4-3P2,1,0 absorption bands of Pr3 þ centers. Unlike these emission bands, the 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions, is most effectively excited not in the band-to-band and exciton absorption regions of the PbWO4 crystal lattice (see Fig. 2 and the inset in Fig. 3(b)) but in an intense absorption band located at 4.2 K around 3.92 eV (Fig. 4(c)). The shape of the 3.92 eV excitation band indicates a very large optical density in this energy range. The Pr3 þ -related emissions, arising from the excited 3P0 level (see Table 1), are also excited in the 3.92 eV band (see Figs. 3(a) and 4(b)) but with much smaller relative efficiency as compared with the 2.03–2.06 eV emission. The latter emission is excited also in the 3H4-3P2,1,0 absorption
383
Fig. 4. Excitation spectra of PbWO4:Pr measured at 4.2 K for different emission spectra ranges: (a) Eem ¼2.9 eV, (b) Eem ¼2.53 eV, and (c) Eem ¼2.05 eV. The inset of (a) shows the excitation spectrum of the 3 eV emission (measured at Eem ¼ 3.2 eV) at 80 K. The inset of (c) shows the normalized excitation spectra of the 2.05 eV emission at 4.2 K and 295 K.
bands but with smaller efficiency as compared with the 3.92 eV band (Fig. 4(c)). Under excitation in the 3.92 eV band region, the positions of the emission bands, arising from the 1D2-3H4 transitions, depend on the excitation energy (Fig. 5(a)). A similar effect was observed in Ref. [24] for Pr3 þ -doped CaMoO4. The positions and relative intensities of the Pr3 þ -related emission bands depend on the excitation energy (see Figs. 2, 3, and 5) which allows the suggestion on the presence of at least two types of Pr3 þ centers in PbWO4:Pr crystals (see also Refs. [13,14]). One can assume that the centers of the first type can be single Pr3 þ ions, substituting for Pb2 þ ions without any defect in their surroundings. This assumption is supported by the EPR data [25–27], which indicate that the local crystal field at the paramagnetic A3 þ related center in PbWO4 has the S4 tetragonal symmetry, i.e., an A3 þ ion can substitute for a Pb2 þ lattice cation with the charge compensation at a distance. The centers of the second type are most probably the associates of two close Pr3 þ ions with a lead vacancy VPb [28,29] (the complexes of the type of {2Pr3 þ -VPb}), where the excess positive charge of a Pr3 þ ion is completely compensated. Indeed, the observation of the up-conversion in Refs. [16,17] indicates the presence of pairs of Pr3 þ ions in PbWO4:Pr crystals. Temperature dependences of the intrinsic B emission intensity are measured under excitation in the exciton (Fig. 6(a), open circles) and band-to-band (filled circles) absorption regions coincide. Thermal quenching of this emission in PbWO4:Pr takes place at lower temperatures (Tq E150 K) as compared with the other
384
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Fig. 5. Emission spectra of PbWO4 measured under excitation (a) in the charge transfer absorption region with Eexc ¼ 3.7 eV (solid line) and Eexc ¼ 4.0 eV (dashed line) at 80 K; (b) with Eexc ¼ 4.18 eV (solid line), Eexc ¼ 3.8 eV (dashed line), and Eexc ¼2.76 eV (dotted line) at 4.2 K.
PbWO4 crystals studied (see, e.g., Ref. [30]), and at RT the B emission is not practically observed. Under the same excitation, temperature dependences of intensities of all the Pr3 þ -related emission bands are similar to those of the B emission, and Tq is around 150 K as well (see, e.g., Fig. 6(b)). This result indicates the dominating radiative energy transfer between the PbWO4 host lattice and Pr3 þ ions, i.e., to the reabsorption of the PbWO4 emission by Pr3 þ ions. In Ref. [31], the shorter (1.76 ns) dominating decay time was obtained at RT for the X-ray excited B emission in PbWO4:Pr as compared with that in the undoped PbWO4 crystal (3.31 ns), which could indicate a contribution of the nonradiative energy transfer process as well. However, the observed shortening of the radioluminescence decay time can be caused by the abovementioned stronger thermal quenching of the B emission in PbWO4:Pr as compared with the undoped PbWO4 crystal and by the absence of the slow G(II) emission in the crystal studied. Under 2.76 eV excitation, the redistribution of the emission intensities is observed around 20 K between E2.55 eV and 2.50 eV emission bands, both arising from the 3P0-3H4 transitions (Fig. 6(c)), indicating the thermally stimulated transitions between the corresponding energy levels. The same effect is observed under excitation in the PbWO4 host absorption region as well. Under 2.76 eV excitation, the intensities of the other emission bands of Pr3 þ (e.g., 2.24 eV and 1.92 eV bands), arising from the 3 P0 state, also slightly increase up to 50–70 K and then gradually
Fig. 6. Temperature dependences of the maximum emission intensities measured: (a) for the intrinsic B emission (Eem ¼ 2.85 eV) under excitation in the exciton (Eexc ¼ 4.18 eV, open circles) and band-to-band (Eexc ¼5.2 eV, filled circles) absorption regions; (b) for the Pr3 þ -related emissions under excitation in the exciton (Eexc ¼ 4.18 eV, open circles) and band-to-band (Eexc ¼4.6 eV, filled circles) absorption regions; (c) for the Pr3 þ -related 2.54 eV (filled circles), 2.50 eV (open circles), 2.24 eV (open triangles), and 1.92 eV (filled triangles) emissions under Eexc ¼2.76 eV; (d) for 2.5 eV and 1.92 eV emissions (open circles) and the 2.06 eV emission (closed circles) under Eexc ¼3.8 eV. Tq is the temperature where the emission intensity decreases twice.
decrease about 2 times as the temperature increases up to 250– 300 K (Fig. 6(c), triangles). Under excitation in the 3.92 eV band, the intensities of all the emissions, arising from the radiative decay of the 3P0 state, are temperature-independent in the 4.2–200 K range and then gradually decrease only twice as the temperature increases up to 400 K. For the 2.50 eV emission, this temperature dependence is shown in Fig. 6(d) (open circles). However, the intensity of the 2.06 eV emission noticeably decreases with increasing temperature (filled circles). This decrease is not accompanied with the enhancement of the other emission bands also observed under this excitation. Thus, the obtained data indicate that the intense 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions, is observed not only under resonant pumping of the 1D2 level with Eexc E2.1 eV (as it was concluded in Refs. [16,17]). This emission is excited also in the 3H4-3P2,1,0 absorption bands, but mainly in the 3.92 eV band, while the emission bands arising from the 3P2,1-3H4 transitions are not excited around 3.92 eV (Fig. 3(a)). The maximum of the 3.92 eV excitation band is gradually shifted to lower
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
energies with the increasing temperature (up to E 3.7 eV at RT, see the inset in Fig. 4(c)), while the positions of all the other Pr3 þ related bands are practically independent of temperature. Such temperature dependence of the 3.92 eV band is characteristic for the bands of the charge-transfer or exciton origin. Just the presence at RT of the intense separate Pr3 þ -related 3.7 eV absorption band explains the low-energy shift of the absorption edge of PbWO4:Pr as compared with the undoped PbWO4. As the above-mentioned shift depends on the Pr3 þ content [9], the 3.92 eV band is surely connected with Pr3 þ ions. We suggest that this band is of charge-transfer origin (see also Ref. [9]). An analogous band was detected in Pr3 þ -doped molybdates [24]. By analogy with some other Pr3 þ -doped compounds (see, e. g., Refs. [24,32–34] and references therein), we assume that under excitation in the 3.92 eV band region, the following photoinduced electron transfer processes from a Pr3 þ ion to a close W6 þ or Pb2 þ crystal lattice ions can take place: {Pr3þ þW6 þ }-{Pr4þ þW þ 5} or
3P 2 3P 1
CTS
3P 0 1D
2
3F 2 3H 6 3H
5 3H 4
Fig. 7. Configurational coordinate diagram proposed for the description of the features observed in PbWO4:Pr under excitation in the charge-transfer band. Full arrows indicate absorption and emission processes and dotted arrows indicate vibronic relaxation processes.
385
{Pr3þ þPb2 þ }-{Pr4þ þPb þ }, resulting in the formation of the charge-transfer state (CTS). Configurational coordinate diagram proposed for the description of the features observed in PbWO4:Pr under 3.92 eV excitation is presented in Fig. 7. This diagram takes into account that under the 3.92 eV excitation, the above-mentioned charge-transfer transition takes place. The vibrational relaxation from the CTS results in the population of only the 3P0 and 1D2 excited states of a Pr3 þ ion followed by the weaker emission at 2.50 eV and E2.55 eV, arising from the 3P0-3H4 transitions, and the stronger emissions around 2.03–2.06 eV and 1.92 eV, arising from 1D2-3H4 and 3P0-3F2 transitions, respectively. The relaxation from the CTS into the 3P2,1 levels does not occur. The observation of the separate broad Pr3 þ -related 3.92 eV absorption band in PbWO4:Pr crystals allows proposing a new explanation also for some other features observed earlier in this material. For example, the appearance of the bands arising from 1 S0-1D2 and 1D2-3H4 radiative transitions in the luminescence spectrum of PbWO4:Pr under excitation around 6.08 eV (the 4f 3 H4-5d 1S0 transition) observed in Ref. [15] can be explained not only by the quantum cutting but also by the reabsorption of the 5d 1 S0-4f 1D2 emission of Pr3 þ , peaking at about 3.8 eV, in the 3.92 eV absorption band. Indeed, the same Pr3 þ -related emission bands appear under 3.92 eV excitation (see Fig. 3(a)) as those observed under 6.08 eV excitation in Ref. [15]. By analogy with our previous works (see, e.g., Refs. [7,19–22]), the creation of defects under UV irradiation is studied by the TSL method also in the PbWO4:Pr crystal. However, in this case, the TSL intensity, measured even without a monochromator or filters after long irradiation of the crystal at different temperatures (up to 200 K) in the 3.5–5.0 eV energy range by the light of the DDS-400 lamp with the maximum monochromator slits, is negligible. This result correlates well with the above-mentioned absence of the photostimulated G(II) emission in the crystal studied. Our estimations show that in the PbWO4:Pr crystal studied, the TSL intensity is at least by three orders of magnitude smaller as compared with that observed at the same UV irradiation and TSL measurement conditions in the best A3þ -doped PbWO4 crystals prepared by Furukawa Co. (Japan) and studied in Refs. [19,20]. It means that the concentration of single oxygen and lead vacancies as traps for electrons and holes in the PbWO4:Pr crystal studied in the present paper is negligible due to an effective filling of lead vacancies by Pr3 þ ions and formation of the associates of two Pr3þ ions with a lead vacancy. It is also interesting to note that unlike Refs. [19,20], no TSL peak, arising from the thermal destruction of the electron {(WO4)3 –Pr3 þ } centers, appears after long irradiation of the PbWO4:Pr crystal at 80 K.
4. Conclusions In the PbWO4:Pr crystal, the reabsorption of the intrinsic blue emission in the absorption bands of Pr3 þ centers results in a strong reduction of this emission and appearance of the Pr3 þ related emission bands under excitation in the PbWO4 host absorption region. The presence of two types of Pr3 þ -related centers (the single Pr3 þ ions with non-compensated positive charge and the dimer {2Pr3 þ -VPb}-type centers with totally compensated charge) is suggested. The broad intense absorption (excitation) band located at 4.2 K at 3.92 eV is ascribed to the electron transfer from a Pr3 þ ion to the nearest-neighboring host lattice W6 þ or Pb2 þ ion. In this charge-transfer band, the red emission, arising from the 4f 1D2-4f 3H4 transitions of Pr3 þ ions, is effectively excited. The relaxation from the charge-transfer state takes place also into the 4f 3P0 excited level, while the 4f 3P2,1 levels are not populated under the 3.92 eV excitation. The absence of the photo- and thermally stimulated tunneling electron recombination G(II) luminescence in the PbWO4:Pr
386
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
crystal studied allows the conclusion that the concentration of single oxygen and lead vacancies as traps for electrons and holes, respectively, in this crystal is negligible. Acknowledgments The work was supported by Estonian Research Council—Institutional Research Funding IUT02-26 and the Estonian Science Foundation (Grant no. 8678). The work of T. Shalapska was supported by the Marie Curie Actions Postdoctoral Research under Grant no. 0057J. References [1] V.G. Baryshevski, M. Korzhik, V.I. Moroz, V.B. Pavlenko, A.F. Lobko, A. A. Fedorov, V.A. Kachanov, V.L. Solovjanov, B.I. Zadneprovsky, V.A. Nefyodov, P.V. Nefyodov, B.A. Dorogovin, L.L. Nagornaja, Nucl. Instrum. Methods Phys. Res. A 322 (1992) 231. [2] M. Kobayashi, M. Ishii, Y. Usuki, H. Yahagi, Nucl. Instrum. Methods Phys. Res. A 333 (1993) 429. [3] P. Lecoq, I. Dafinei, E. Auffray, M. Schneegans, M.V. Korzhik, O.V. Missevitch, V.B. Pavlenko, A.A. Fedorov, A.N. Annenkov, V.L. Kostylev, V.D. Ligun, Nucl. Instrum Methods Phys. Res. A 365 (1995) 291. [4] M. Nikl, Phys. Status Solidi A 178 (2000) 595. [5] A.A. Annenkov, M.V. Korzhik, P. Lecoq, Nucl. Instrum. Methods Phys. Res. A 490 (2002) 30. [6] M. Nikl, V.V. Laguta, A. Vedda, Phys. Status Solidi B 245 (2008) 1701. [7] V.V. Laguta, M. Nikl, S. Zazubovich, IEEE Trans. Nucl. Sci. 55 (2008) 1275. [8] F.B. Xiong, W.Z. Zhu, X.G. Meng, H.F. Lin, J. Lumin. 132 (2012) 337. [9] M. Nikl, P. Bohacek, A. Vedda, M. Fasoli, J. Pejchal, A. Beitlerova, M. Fratemali, M. Livan, J. Appl. Phys. 104 (2008) 093514. [10] M. Nikl, P. Bohacek, E. Mihokova, J. Rosa, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, V. Laguta, M. Kobayashi, M. Ishii, Y. Usuki, D. Zimmermann, S. Baccaro, A. Cecilia, Radiat. Meas. 33 (2001) 705. [11] M. Nikl, P. Bohacek, E. Mihokova, N. Solovieva, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, M. Kobayashi, M. Ishii, Y. Usuki, D. Zimmermann, J. Cryst. Growth 229 (2001) 312.
[12] F. Xiong, Z. Luo, Y. Huang, Appl. Phys. B: Laser Opt. (2005)321. [13] O. Antonenko, O. Chukova, Yu. Hizhnyi, S. Nedilko, V. Scherbatskyi, Opt. Mater. 28 (2006) 643. [14] O. Chukova, S. Nedilko, V. Scherbatskyi, in: A. Gektin, B. Grinyov (Eds.), Proceedings of the Eighth International Conference on Inorganic Scintillators and Their Use in Scientific and Industrial Applications, Ukraine, Kharkov, 2006, pp. 354–357. [15] F. Xiong, Y. Lin, Y. Chen, Chem. Phys. Lett. 429 (2006) 410. [16] Y. Huang, K.H. Jang, K. Jang, H.J. Seo, J. Lumin. 122–123 (2007) 47. [17] Y. Huang, T. Tsuboi, H.J. Seo, J. Phys. Chem. A 112 (2008) 5839. [18] B.P. Singh, K.K. Sharma, I.S. Minhas, J. Phys. C: Solid State Phys. 19 (1986) 6655. [19] A. Krasnikov, T. Kärner, V. Kiisk, V.V. Laguta, M. Nikl, I. Sildos, Y. Usuki, S. Zazubovich, J. Phys. C: Conf. Ser. 249 (2010) 012001. [20] P. Fabeni, A. Krasnikov, T. Kärner, V.V. Laguta, M. Nikl, G.P. Pazzi, S. Zazubovich, J. Lumin. 136 (2013) 42. [21] A. Krasnikov, M. Nikl, S. Zazubovich, Phys. Status Solidi B 243 (2006) 1727. [22] V.V. Laguta, M. Nikl, S. Zazubovich, Phys. Status Solidi B 249 (2012) 2161. [23] V. Babin, P. Bohacek, A. Krasnikov, M. Nikl, A. Stolovits, S. Zazubovich, J. Lumin. 124 (2007) 113. [24] E.G. Reut, A.I. Ryskin, Phys. Status Solidi A 17 (1973) 47. [25] S.V. Nistor, M. Stefan, E. Goovaerts, M. Nikl, P. Bohacek, J. Phys.: Condens. Matter 18 (2006) 719. [26] F.F. Popescu, V. Bercu, J.N. Barascu, M. Martinelli, C.A. Massa, L.A. Pardi, M. Stefan, S.V. Nistor, M. Nikl, P. Bohacek, J. Chem. Phys. 131 (2009) 034505. [27] F.F. Popescu, V. Bercu, J.N. Barascu, M. Martinelli, C.A. Massa, L.A. Pardi, M. Stefan, S.V. Nistor, M. Nikl, Opt. Mater. 32 (2010) 570. [28] B.G. Han, X.Q. Feng, G.Q. Hu, P.C. Wang, Z.W. Yin, J. Appl. Phys. 84 (1998) 2831. [29] Q.S. Li, X.Q. Feng, J. Phys.: Condens. Matter 15 (2003) 1963. [30] A. Krasnikov, M. Nikl, S. Zazubovich, in: A. Gektin, B. Grinyov (Eds.), Proceedings of the Eighth International Conference on Inorganic Scintillators and Their Use in Scientific and Industrial Applications, Ukraine, Kharkov, 2006, pp. 362–365. [31] M. Batenchuk, I. Konstankevych, L. Limarenko, Z. Moroz, M. Pashkovsky Yu. Zorenko, in: Proceedings of the International Workshop on Tungstate Crystals, Roma, Universita degli Studi La Sapienza, October 12–14, 1998, pp. 49–54. [32] P. Boutinaud, R. Mahiou, E. Cavalli, M. Bettinelli, J. Lumin. 122–123 (2007) 430. [33] W. Jia, D. Jia, T. Rodriguez, D.R. Evans, R.S. Meltzer, W.M. Yen, J. Lumin. 119–120 (2006) 13. [34] E. Cavalli, F. Angiuli, P. Boutinaud, R. Mahiou, J. Solid State Chem. 185 (2012) 136.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and excited states dynamics in PbWO4: Pr3 þ crystals E. Auffray a, M. Korjik b, T. Shalapska c, S. Zazubovich c,n a
CERN, Geneva 23, Geneva, Switzerland Institute for Nuclear Problems, 11 Bobruiskaya, 220020 Minsk, Belarus c Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2014 Received in revised form 5 May 2014 Accepted 7 May 2014 Available online 21 May 2014
Luminescence and photo-thermally stimulated defects creation processes are studied for a Pr3 þ -doped PbWO4 crystal at 4.2–400 K under excitation in the band-to-band, exciton, and charge-transfer transitions regions, as well as in the Pr3 þ -related absorption bands. Emission spectra of Pr3 þ centers depend on the excitation energy, indicating the presence of Pr3 þ centers of two types. The origin of these centers is discussed. The 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions of Pr3 þ ions, is found to be effectively excited in a broad intense absorption band peaking at 4.2 K at 3.92 eV. By analogy with some other Pr3 þ -doped compounds, this band is suggested to arise from an electron transfer from an impurity Pr3 þ ion to the crystal lattice W6 þ or Pb2 þ ions. The dynamics of the Pr3 þ related excited states is clarified. In the PbWO4:Pr crystal studied, the concentration of single oxygen and lead vacancies as traps for electrons and holes is found to be negligible. & 2014 Elsevier B.V. All rights reserved.
Keywords: Pr3 þ -doped lead tungstate Photoluminescence Thermoluminescence Charge-transfer state
1. Introduction Single crystals of lead tungstate attracted a wide interest as promising scintillation materials for calorimeters used in the high energy physics experiments owing to their high density, fast luminescence decay, strong radiation hardness, short radiation length, high chemical stability, and low cost (see, e.g., Refs. [1–5]). Co-doping of PbWO4 crystals with large and stable trivalent rare-earth ions (A3þ : La3 þ , Lu3þ , Y3þ , and Gd3þ ) was found to be an effective method for an improvement of their scintillation characteristics owing to the significant suppression of the concentration of traps for electrons and holes (see, e.g., reviews [4,6,7]). The PbWO4:A3 þ crystals, having the absorption and emission bands in the visible and infrared spectral regions, arising from the transitions between the energy levels of a rare-earth ion (e.g., Pr3 þ ), are promising materials also for lasers and mercury-free luminescence lamps (see, e.g., Ref. [8] and references therein). Heavily Pr3 þ -doped PbWO4 crystals were also proposed for application for dual scintillation/Cherenkov light detector [9]. In the luminescence spectrum of Pr3þ -doped PbWO4 single crystals, besides the intrinsic blue emission characteristic for the PbWO4 host, several groups of narrow emission lines appear in the energy region above 2.9 eV (see, e.g., Refs. [8–17]). In the samples studied in Refs. [10,11], about 50% of radioluminescence light was
n
Corresponding author. Tel.: þ 372 7374766; fax: þ 372 7383033. E-mail address: svet@fi.tartu.ee (S. Zazubovich).
http://dx.doi.org/10.1016/j.jlumin.2014.05.012 0022-2313/& 2014 Elsevier B.V. All rights reserved.
emitted from Pr3þ -related transitions at room temperature (RT) which suggests efficient energy capture at Pr3þ centers. It was concluded that such luminescence centers can effectively increase the scintillation light output. In Refs. [9,11,13], a strong Pr3þ -induced reduction of the intrinsic blue emission of PbWO4 was reported and explained by the energy transfer from the host lattice to the impurity Pr3 þ ions. As a result, in the crystals with sufficiently large Pr3 þ content, mainly the Pr3þ -related narrow emission bands, arising from the 4f-4f transitions of the Pr3þ ions, were observed in the emission spectrum. The detailed spectroscopic study of Pr3 þ -related centers in PbWO4 was carried out in many papers (see, e.g., Refs. [8,9,11–18] and references therein). The absorption bands of PbWO4:Pr, located in the 1–3 eV energy range, were assigned to the transitions from the 3H4 ground state to the excited multiplets 3P0,1,2, 1I6, 1D2, and 1G4 belonging to the 4f2 configuration. The luminescence bands in this energy range were ascribed to the electronic transitions from the above-mentioned excited levels to the 3H4,5,6 and 3F2,3,4 levels. The positions of the Pr3 þ -related absorption and emission bands are shown in Table 1. A quantum cutting process [15] and up-conversion luminescence [16,17] were also observed in PbWO4:Pr crystals. However, an influence of Pr3 þ ions on the origin and concentration of traps for electrons and holes in PbWO4 crystals was not studied in detail. In Ref. [9], the suppression of deep oxygen-vacancy-related electron traps with increasing Pr3 þ content was only reported. In this paper, we consider the luminescence characteristics, excited state dynamics, energy transfer and charge transfer processes in a PbWO4:Pr crystal in the 4.2–400 K temperature range. By analogy
382
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Table 1 Positions of absorption and emission bands of PbWO4:Pr located in the 1.9–2.8 eV energy range and arising from the 4f-4f transitions of Pr3 þ (for notations of the bands, the data of Ref. [17] are used). Absorption bands (295 K) 4f–4f transitions 3
3
H4- P2 H4-3P1 H4-3P0 3 H4-1D2 3 3
nm 447.58; 450.83 473.74 487.53 585.36; 592.63; 602.72
Emission bands (4.2 K) eV
4f–4f transitions
nm
eV
2.77; 2.75 2.617 2.543 2.118; 2.092; 2.057
3
451; 461.06 475.93; 477.94; 479.67 486.95; 495.92 529.82 553.48 601.55; 607.44; 610.43; 616.50 615.89; 619.9 644.05; 648.43
2.749; 2.689 2.605; 2.594; 2.583 2.546; 2.5 2.34 2.24 2.061; 2.041; 2.031; 2.011 2.013; 2.00 1.925; 1.912
3
P2- H4 P1-3H4 P0-3H4 3 P1-3H5 3 P0-3H5 1 D2-3H4 3 P0-3H6 3 P0-3F2 3 3
with our previous works (see, e.g., Refs. [7,19–22]), we also investigate defects creation processes under selective UV irradiation of the PbWO4:Pr crystal in the 3.5–5.0 eV energy range at the temperatures from 80 K to 200 K.
2. Experimental details A single crystal of PbWO4:Pr3 þ studied in the present paper was grown by the Czochralski method in Ar atmosphere at Bogoroditsk Technical Chemical Plant from the raw materials with purity 5 N. The concentration of Pr3 þ ions in the melt was 1 at%. The absorption spectrum was measured with the spectrophotometer JASCO V660. The steady-state emission and excitation spectra in the 80–400 K temperature range were measured using a setup, consisting of a deuterium DDS-400 lamp, two monochromators (SF-4 and SPM-1), and a vacuum nitrogen cryostat. The luminescence was detected by a photomultiplier (FEU-39 or FEU-79) with an amplifier and recorder. In the 4.2–300 K temperature range, the spectra were measured with the use of the computer-controlled setup consisting of the LOT-ORIEL xenon lamp (150 W), two monochromators (MDR-3 and ORIEL Corner Stone 1/8 m), a Hamamatsu 6240 photon counting system, and an immersion helium cryostat. The spectra were corrected for the spectral distribution of the excitation light, the transmission and dispersion of the monochromators, and the spectral sensitivity of the detectors. Thermally stimulated luminescence (TSL) glow curves were obtained with a constant heating rate of 0.2 K/s after selective irradiation of the crystal, located in the nitrogen cryostat, in the energy range 3.5–5.0 eV for 1 h at different temperatures from 80 K to 200 K with the deuterium DDS-400 lamp through a monochromator SF-4. The integrated emission of the crystal was detected with the photomultiplier FEU-39.
Fig. 1. Absorption spectrum of PbWO4:Pr measured at 295 K.
3. Results and discussion In the absorption spectrum of the PbWO4:Pr3 þ crystal measured at 295 K, besides the host lattice absorption in the E4 3.6 eV energy range, the narrow Pr3 þ -related absorption bands, located in the E o2.8 eV range and arising from the 4f-4f transitions of the Pr3 þ ions, are observed (Fig. 1). Under excitation in the band-to-band (Eexc ¼4.6 eV) and exciton (Eexc ¼ 4.18 eV) absorption regions at 4.2 K, the broad intrinsic blue (B) emission band of PbWO4 is observed around 2.9 eV (Fig. 2). The low-energy side of this emission band is strongly distorted due to the reabsorption in the Pr3 þ -related absorption bands located in the 2.5–2.8 eV energy range. No green G(I) emission, arising in the undoped PbWO4 crystals from lead-deficient crystal regions and excited around 3.8–3.9 eV (see, e.g., Ref. [23]), is observed in the
Fig. 2. Emission spectra of PbWO4:Pr measured at 4.2 K under excitation in the exciton absorption region (Eexc ¼4.18 eV, solid line) and in the band-to-band transitions region (Eexc ¼ 4.6 eV, dashed line).
PbWO4:Pr3 þ crystal. However, a weak UV emission band located at about 3 eV is excited in this energy range (see the insets in Fig. 3 (a) and Fig. 4(a)). Similar emission was detected in other A3 þ doped PbWO4 crystals [19] and ascribed to the radiative decay of an exciton localized near a trivalent rare-earth A3 þ ion [20]. No tunneling electron recombination G(II) emission, whose intensity should be maximum at 220 K under photostimulation around
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Fig. 3. Emission spectra of PbWO4:Pr measured at 4.2 K under excitation with (a) Eexc ¼ 3.8 eV and (b) Eexc ¼2.76 eV. In the inset of (a), the E3 eV emission band measured at 80 K is shown. In the inset of (b), the emission spectra measured at 4.2 K under excitation with Eexc ¼ 3.8 eV (solid line) and Eexc ¼ 4.18 eV (dashed line) are compared in the 1.9–2.1 eV energy range.
4.1 eV (see, e.g., Refs. [7,23]), is detected in the crystal studied. As this intrinsic emission arises from the recombination of the electrons trapped at oxygen vacancies with the holes trapped at lead vacancies [22], this result indicates a negligible number of lead and oxygen vacancies in the crystal studied. In the excitation spectrum of the B emission measured at 4.2 K, the lowest-energy exciton-related band is located at 4.18 eV (Fig. 4 (a)). Besides the intrinsic 2.9 eV emission, the lower-energy narrow emission bands of Pr3 þ -related centers, arising mainly from the radiative decay of the excited 3P2, 3P1, 3P0 levels (see Table 1), are all effectively excited in the band-to-band and exciton absorption regions of the PbWO4 host (Fig. 2) (mainly at Eexc 44.0 eV at 4.2 K, see, e.g., Fig. 4(b)). This indicates an effective energy transfer from the PbWO4 crystal lattice to Pr3 þ ions, which becomes possible due to an overlap of the B emission band of PbWO4 with the 3H4-3P2,1,0 absorption bands of Pr3 þ centers. Unlike these emission bands, the 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions, is most effectively excited not in the band-to-band and exciton absorption regions of the PbWO4 crystal lattice (see Fig. 2 and the inset in Fig. 3(b)) but in an intense absorption band located at 4.2 K around 3.92 eV (Fig. 4(c)). The shape of the 3.92 eV excitation band indicates a very large optical density in this energy range. The Pr3 þ -related emissions, arising from the excited 3P0 level (see Table 1), are also excited in the 3.92 eV band (see Figs. 3(a) and 4(b)) but with much smaller relative efficiency as compared with the 2.03–2.06 eV emission. The latter emission is excited also in the 3H4-3P2,1,0 absorption
383
Fig. 4. Excitation spectra of PbWO4:Pr measured at 4.2 K for different emission spectra ranges: (a) Eem ¼2.9 eV, (b) Eem ¼2.53 eV, and (c) Eem ¼2.05 eV. The inset of (a) shows the excitation spectrum of the 3 eV emission (measured at Eem ¼ 3.2 eV) at 80 K. The inset of (c) shows the normalized excitation spectra of the 2.05 eV emission at 4.2 K and 295 K.
bands but with smaller efficiency as compared with the 3.92 eV band (Fig. 4(c)). Under excitation in the 3.92 eV band region, the positions of the emission bands, arising from the 1D2-3H4 transitions, depend on the excitation energy (Fig. 5(a)). A similar effect was observed in Ref. [24] for Pr3 þ -doped CaMoO4. The positions and relative intensities of the Pr3 þ -related emission bands depend on the excitation energy (see Figs. 2, 3, and 5) which allows the suggestion on the presence of at least two types of Pr3 þ centers in PbWO4:Pr crystals (see also Refs. [13,14]). One can assume that the centers of the first type can be single Pr3 þ ions, substituting for Pb2 þ ions without any defect in their surroundings. This assumption is supported by the EPR data [25–27], which indicate that the local crystal field at the paramagnetic A3 þ related center in PbWO4 has the S4 tetragonal symmetry, i.e., an A3 þ ion can substitute for a Pb2 þ lattice cation with the charge compensation at a distance. The centers of the second type are most probably the associates of two close Pr3 þ ions with a lead vacancy VPb [28,29] (the complexes of the type of {2Pr3 þ -VPb}), where the excess positive charge of a Pr3 þ ion is completely compensated. Indeed, the observation of the up-conversion in Refs. [16,17] indicates the presence of pairs of Pr3 þ ions in PbWO4:Pr crystals. Temperature dependences of the intrinsic B emission intensity are measured under excitation in the exciton (Fig. 6(a), open circles) and band-to-band (filled circles) absorption regions coincide. Thermal quenching of this emission in PbWO4:Pr takes place at lower temperatures (Tq E150 K) as compared with the other
384
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
Fig. 5. Emission spectra of PbWO4 measured under excitation (a) in the charge transfer absorption region with Eexc ¼ 3.7 eV (solid line) and Eexc ¼ 4.0 eV (dashed line) at 80 K; (b) with Eexc ¼ 4.18 eV (solid line), Eexc ¼ 3.8 eV (dashed line), and Eexc ¼2.76 eV (dotted line) at 4.2 K.
PbWO4 crystals studied (see, e.g., Ref. [30]), and at RT the B emission is not practically observed. Under the same excitation, temperature dependences of intensities of all the Pr3 þ -related emission bands are similar to those of the B emission, and Tq is around 150 K as well (see, e.g., Fig. 6(b)). This result indicates the dominating radiative energy transfer between the PbWO4 host lattice and Pr3 þ ions, i.e., to the reabsorption of the PbWO4 emission by Pr3 þ ions. In Ref. [31], the shorter (1.76 ns) dominating decay time was obtained at RT for the X-ray excited B emission in PbWO4:Pr as compared with that in the undoped PbWO4 crystal (3.31 ns), which could indicate a contribution of the nonradiative energy transfer process as well. However, the observed shortening of the radioluminescence decay time can be caused by the abovementioned stronger thermal quenching of the B emission in PbWO4:Pr as compared with the undoped PbWO4 crystal and by the absence of the slow G(II) emission in the crystal studied. Under 2.76 eV excitation, the redistribution of the emission intensities is observed around 20 K between E2.55 eV and 2.50 eV emission bands, both arising from the 3P0-3H4 transitions (Fig. 6(c)), indicating the thermally stimulated transitions between the corresponding energy levels. The same effect is observed under excitation in the PbWO4 host absorption region as well. Under 2.76 eV excitation, the intensities of the other emission bands of Pr3 þ (e.g., 2.24 eV and 1.92 eV bands), arising from the 3 P0 state, also slightly increase up to 50–70 K and then gradually
Fig. 6. Temperature dependences of the maximum emission intensities measured: (a) for the intrinsic B emission (Eem ¼ 2.85 eV) under excitation in the exciton (Eexc ¼ 4.18 eV, open circles) and band-to-band (Eexc ¼5.2 eV, filled circles) absorption regions; (b) for the Pr3 þ -related emissions under excitation in the exciton (Eexc ¼ 4.18 eV, open circles) and band-to-band (Eexc ¼4.6 eV, filled circles) absorption regions; (c) for the Pr3 þ -related 2.54 eV (filled circles), 2.50 eV (open circles), 2.24 eV (open triangles), and 1.92 eV (filled triangles) emissions under Eexc ¼2.76 eV; (d) for 2.5 eV and 1.92 eV emissions (open circles) and the 2.06 eV emission (closed circles) under Eexc ¼3.8 eV. Tq is the temperature where the emission intensity decreases twice.
decrease about 2 times as the temperature increases up to 250– 300 K (Fig. 6(c), triangles). Under excitation in the 3.92 eV band, the intensities of all the emissions, arising from the radiative decay of the 3P0 state, are temperature-independent in the 4.2–200 K range and then gradually decrease only twice as the temperature increases up to 400 K. For the 2.50 eV emission, this temperature dependence is shown in Fig. 6(d) (open circles). However, the intensity of the 2.06 eV emission noticeably decreases with increasing temperature (filled circles). This decrease is not accompanied with the enhancement of the other emission bands also observed under this excitation. Thus, the obtained data indicate that the intense 2.03–2.06 eV emission, arising from the 1D2-3H4 transitions, is observed not only under resonant pumping of the 1D2 level with Eexc E2.1 eV (as it was concluded in Refs. [16,17]). This emission is excited also in the 3H4-3P2,1,0 absorption bands, but mainly in the 3.92 eV band, while the emission bands arising from the 3P2,1-3H4 transitions are not excited around 3.92 eV (Fig. 3(a)). The maximum of the 3.92 eV excitation band is gradually shifted to lower
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
energies with the increasing temperature (up to E 3.7 eV at RT, see the inset in Fig. 4(c)), while the positions of all the other Pr3 þ related bands are practically independent of temperature. Such temperature dependence of the 3.92 eV band is characteristic for the bands of the charge-transfer or exciton origin. Just the presence at RT of the intense separate Pr3 þ -related 3.7 eV absorption band explains the low-energy shift of the absorption edge of PbWO4:Pr as compared with the undoped PbWO4. As the above-mentioned shift depends on the Pr3 þ content [9], the 3.92 eV band is surely connected with Pr3 þ ions. We suggest that this band is of charge-transfer origin (see also Ref. [9]). An analogous band was detected in Pr3 þ -doped molybdates [24]. By analogy with some other Pr3 þ -doped compounds (see, e. g., Refs. [24,32–34] and references therein), we assume that under excitation in the 3.92 eV band region, the following photoinduced electron transfer processes from a Pr3 þ ion to a close W6 þ or Pb2 þ crystal lattice ions can take place: {Pr3þ þW6 þ }-{Pr4þ þW þ 5} or
3P 2 3P 1
CTS
3P 0 1D
2
3F 2 3H 6 3H
5 3H 4
Fig. 7. Configurational coordinate diagram proposed for the description of the features observed in PbWO4:Pr under excitation in the charge-transfer band. Full arrows indicate absorption and emission processes and dotted arrows indicate vibronic relaxation processes.
385
{Pr3þ þPb2 þ }-{Pr4þ þPb þ }, resulting in the formation of the charge-transfer state (CTS). Configurational coordinate diagram proposed for the description of the features observed in PbWO4:Pr under 3.92 eV excitation is presented in Fig. 7. This diagram takes into account that under the 3.92 eV excitation, the above-mentioned charge-transfer transition takes place. The vibrational relaxation from the CTS results in the population of only the 3P0 and 1D2 excited states of a Pr3 þ ion followed by the weaker emission at 2.50 eV and E2.55 eV, arising from the 3P0-3H4 transitions, and the stronger emissions around 2.03–2.06 eV and 1.92 eV, arising from 1D2-3H4 and 3P0-3F2 transitions, respectively. The relaxation from the CTS into the 3P2,1 levels does not occur. The observation of the separate broad Pr3 þ -related 3.92 eV absorption band in PbWO4:Pr crystals allows proposing a new explanation also for some other features observed earlier in this material. For example, the appearance of the bands arising from 1 S0-1D2 and 1D2-3H4 radiative transitions in the luminescence spectrum of PbWO4:Pr under excitation around 6.08 eV (the 4f 3 H4-5d 1S0 transition) observed in Ref. [15] can be explained not only by the quantum cutting but also by the reabsorption of the 5d 1 S0-4f 1D2 emission of Pr3 þ , peaking at about 3.8 eV, in the 3.92 eV absorption band. Indeed, the same Pr3 þ -related emission bands appear under 3.92 eV excitation (see Fig. 3(a)) as those observed under 6.08 eV excitation in Ref. [15]. By analogy with our previous works (see, e.g., Refs. [7,19–22]), the creation of defects under UV irradiation is studied by the TSL method also in the PbWO4:Pr crystal. However, in this case, the TSL intensity, measured even without a monochromator or filters after long irradiation of the crystal at different temperatures (up to 200 K) in the 3.5–5.0 eV energy range by the light of the DDS-400 lamp with the maximum monochromator slits, is negligible. This result correlates well with the above-mentioned absence of the photostimulated G(II) emission in the crystal studied. Our estimations show that in the PbWO4:Pr crystal studied, the TSL intensity is at least by three orders of magnitude smaller as compared with that observed at the same UV irradiation and TSL measurement conditions in the best A3þ -doped PbWO4 crystals prepared by Furukawa Co. (Japan) and studied in Refs. [19,20]. It means that the concentration of single oxygen and lead vacancies as traps for electrons and holes in the PbWO4:Pr crystal studied in the present paper is negligible due to an effective filling of lead vacancies by Pr3 þ ions and formation of the associates of two Pr3þ ions with a lead vacancy. It is also interesting to note that unlike Refs. [19,20], no TSL peak, arising from the thermal destruction of the electron {(WO4)3 –Pr3 þ } centers, appears after long irradiation of the PbWO4:Pr crystal at 80 K.
4. Conclusions In the PbWO4:Pr crystal, the reabsorption of the intrinsic blue emission in the absorption bands of Pr3 þ centers results in a strong reduction of this emission and appearance of the Pr3 þ related emission bands under excitation in the PbWO4 host absorption region. The presence of two types of Pr3 þ -related centers (the single Pr3 þ ions with non-compensated positive charge and the dimer {2Pr3 þ -VPb}-type centers with totally compensated charge) is suggested. The broad intense absorption (excitation) band located at 4.2 K at 3.92 eV is ascribed to the electron transfer from a Pr3 þ ion to the nearest-neighboring host lattice W6 þ or Pb2 þ ion. In this charge-transfer band, the red emission, arising from the 4f 1D2-4f 3H4 transitions of Pr3 þ ions, is effectively excited. The relaxation from the charge-transfer state takes place also into the 4f 3P0 excited level, while the 4f 3P2,1 levels are not populated under the 3.92 eV excitation. The absence of the photo- and thermally stimulated tunneling electron recombination G(II) luminescence in the PbWO4:Pr
386
E. Auffray et al. / Journal of Luminescence 154 (2014) 381–386
crystal studied allows the conclusion that the concentration of single oxygen and lead vacancies as traps for electrons and holes, respectively, in this crystal is negligible. Acknowledgments The work was supported by Estonian Research Council—Institutional Research Funding IUT02-26 and the Estonian Science Foundation (Grant no. 8678). The work of T. Shalapska was supported by the Marie Curie Actions Postdoctoral Research under Grant no. 0057J. References [1] V.G. Baryshevski, M. Korzhik, V.I. Moroz, V.B. Pavlenko, A.F. Lobko, A. A. Fedorov, V.A. Kachanov, V.L. Solovjanov, B.I. Zadneprovsky, V.A. Nefyodov, P.V. Nefyodov, B.A. Dorogovin, L.L. Nagornaja, Nucl. Instrum. Methods Phys. Res. A 322 (1992) 231. [2] M. Kobayashi, M. Ishii, Y. Usuki, H. Yahagi, Nucl. Instrum. Methods Phys. Res. A 333 (1993) 429. [3] P. Lecoq, I. Dafinei, E. Auffray, M. Schneegans, M.V. Korzhik, O.V. Missevitch, V.B. Pavlenko, A.A. Fedorov, A.N. Annenkov, V.L. Kostylev, V.D. Ligun, Nucl. Instrum Methods Phys. Res. A 365 (1995) 291. [4] M. Nikl, Phys. Status Solidi A 178 (2000) 595. [5] A.A. Annenkov, M.V. Korzhik, P. Lecoq, Nucl. Instrum. Methods Phys. Res. A 490 (2002) 30. [6] M. Nikl, V.V. Laguta, A. Vedda, Phys. Status Solidi B 245 (2008) 1701. [7] V.V. Laguta, M. Nikl, S. Zazubovich, IEEE Trans. Nucl. Sci. 55 (2008) 1275. [8] F.B. Xiong, W.Z. Zhu, X.G. Meng, H.F. Lin, J. Lumin. 132 (2012) 337. [9] M. Nikl, P. Bohacek, A. Vedda, M. Fasoli, J. Pejchal, A. Beitlerova, M. Fratemali, M. Livan, J. Appl. Phys. 104 (2008) 093514. [10] M. Nikl, P. Bohacek, E. Mihokova, J. Rosa, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, V. Laguta, M. Kobayashi, M. Ishii, Y. Usuki, D. Zimmermann, S. Baccaro, A. Cecilia, Radiat. Meas. 33 (2001) 705. [11] M. Nikl, P. Bohacek, E. Mihokova, N. Solovieva, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, M. Kobayashi, M. Ishii, Y. Usuki, D. Zimmermann, J. Cryst. Growth 229 (2001) 312.
[12] F. Xiong, Z. Luo, Y. Huang, Appl. Phys. B: Laser Opt. (2005)321. [13] O. Antonenko, O. Chukova, Yu. Hizhnyi, S. Nedilko, V. Scherbatskyi, Opt. Mater. 28 (2006) 643. [14] O. Chukova, S. Nedilko, V. Scherbatskyi, in: A. Gektin, B. Grinyov (Eds.), Proceedings of the Eighth International Conference on Inorganic Scintillators and Their Use in Scientific and Industrial Applications, Ukraine, Kharkov, 2006, pp. 354–357. [15] F. Xiong, Y. Lin, Y. Chen, Chem. Phys. Lett. 429 (2006) 410. [16] Y. Huang, K.H. Jang, K. Jang, H.J. Seo, J. Lumin. 122–123 (2007) 47. [17] Y. Huang, T. Tsuboi, H.J. Seo, J. Phys. Chem. A 112 (2008) 5839. [18] B.P. Singh, K.K. Sharma, I.S. Minhas, J. Phys. C: Solid State Phys. 19 (1986) 6655. [19] A. Krasnikov, T. Kärner, V. Kiisk, V.V. Laguta, M. Nikl, I. Sildos, Y. Usuki, S. Zazubovich, J. Phys. C: Conf. Ser. 249 (2010) 012001. [20] P. Fabeni, A. Krasnikov, T. Kärner, V.V. Laguta, M. Nikl, G.P. Pazzi, S. Zazubovich, J. Lumin. 136 (2013) 42. [21] A. Krasnikov, M. Nikl, S. Zazubovich, Phys. Status Solidi B 243 (2006) 1727. [22] V.V. Laguta, M. Nikl, S. Zazubovich, Phys. Status Solidi B 249 (2012) 2161. [23] V. Babin, P. Bohacek, A. Krasnikov, M. Nikl, A. Stolovits, S. Zazubovich, J. Lumin. 124 (2007) 113. [24] E.G. Reut, A.I. Ryskin, Phys. Status Solidi A 17 (1973) 47. [25] S.V. Nistor, M. Stefan, E. Goovaerts, M. Nikl, P. Bohacek, J. Phys.: Condens. Matter 18 (2006) 719. [26] F.F. Popescu, V. Bercu, J.N. Barascu, M. Martinelli, C.A. Massa, L.A. Pardi, M. Stefan, S.V. Nistor, M. Nikl, P. Bohacek, J. Chem. Phys. 131 (2009) 034505. [27] F.F. Popescu, V. Bercu, J.N. Barascu, M. Martinelli, C.A. Massa, L.A. Pardi, M. Stefan, S.V. Nistor, M. Nikl, Opt. Mater. 32 (2010) 570. [28] B.G. Han, X.Q. Feng, G.Q. Hu, P.C. Wang, Z.W. Yin, J. Appl. Phys. 84 (1998) 2831. [29] Q.S. Li, X.Q. Feng, J. Phys.: Condens. Matter 15 (2003) 1963. [30] A. Krasnikov, M. Nikl, S. Zazubovich, in: A. Gektin, B. Grinyov (Eds.), Proceedings of the Eighth International Conference on Inorganic Scintillators and Their Use in Scientific and Industrial Applications, Ukraine, Kharkov, 2006, pp. 362–365. [31] M. Batenchuk, I. Konstankevych, L. Limarenko, Z. Moroz, M. Pashkovsky Yu. Zorenko, in: Proceedings of the International Workshop on Tungstate Crystals, Roma, Universita degli Studi La Sapienza, October 12–14, 1998, pp. 49–54. [32] P. Boutinaud, R. Mahiou, E. Cavalli, M. Bettinelli, J. Lumin. 122–123 (2007) 430. [33] W. Jia, D. Jia, T. Rodriguez, D.R. Evans, R.S. Meltzer, W.M. Yen, J. Lumin. 119–120 (2006) 13. [34] E. Cavalli, F. Angiuli, P. Boutinaud, R. Mahiou, J. Solid State Chem. 185 (2012) 136.