Influence of stoichiometry on the optical properties of lead tungstate crystals

3 October 1997

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CHEMICAL PHYSICS LETTERS

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ELSEVIER

Chemical Physics Letters 277 (1997) 65-70

Influence of stoichiometry on the optical properties of lead tungstate crystals A.N. Belsky a, S.M. Klimov a, V.V. Mikhailin ~, A.N. Vasil'ev ~, E. Auffray b, P. Lecoq b, C. Pedrini c, M.V. Korzhik d, A.N. Annenkov e, p. Chevallier f'*, P. Martin f, J.C. Krupa g a SRL Physics Faculty, Moscow State University, 118998 Moscow, Russia b CERN, 1211 Geneva 23, Switzerland c LPCML Universitd Lyon I, 43, Bd. du 11 Novembre 1918, 69622 ViUeurbanne, France d Institute of Nuclear Problems, 11 Bobruiskaya, Minsk 220050, Belarus • Spectr Co, Research Division, Bogoroditsk. Russia f LURE, Bat. 209D, Centre Universitaire Paris-Sua~ 91405 Orsay, France 8 IPN, Centre Universitaire Paris-Sud, 91405 Orsay, France

Received 23 May 1997; in final form 31 July 1997

Abstract

A series of PbWO4 crystals, grown consistently from one melt, were studied under excitation by VUV and X-ray synchrotron radiation. We observed a systematic change in the spectroscopic data (emission spectra, luminescence decay, thermolumiuescence, radiation hardness) from the first grown to the last. In the reflectivity spectrum, the intensity in the 4 - 6 eV absorption region, which is assigned to the Pb 2+ 6s-6p transitions, decreases relative to the intensity in the 7-10 eV region which is due to (WO4) 2- absorption. We connect these changes with the variation in stoichiometry of the melt during crystal growth. © 1997 Elsevier Science B.V.

1. I n t r o d u c t i o n Recently, papers have been published describing the properties of P b W O 4 (PWO) crystals and discussing the different models for the luminescence centers in such crystals (see e.g. Refs. [1-5]). Three years ago we started [6] investigations on lead tungstate luminescence and scintillation properties using synchrotron radiation (SR), this Letter proceeds from there. For this examination we chose

* Corresponding author.

several crystals from the same batch grown consecutively from one melt by the Czochralsld method with an initial P b W O 4 stoichiometric composition fit. This allowed us to omit both growth technology and impurities and to concentrate on effects bound up with crystal composition peculiarities and intrinsic defects. According to previous investigations [7], in such a batch o f crystals, the deficiency of oxygen and lead increases and leads to the formation o f a new spatial structure different from that of the P b W O 4 crystal (scheelite). Luminescence spectra, luminescence decay kinetics and luminescence intensity depending on irradia-

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A.N. Belsky et aL / Chemical Physics Letters 277 (1997) 65-70

tion time were measured under VUV and X-ray SR excitation. These properties characterize the luminescence centers and the mechanisms of electron excitation transfers to such centers; they also determine the scintillation properties of the crystal. In 4 - 1 0 eV range reflection spectra were obtained which characterize the low excited states in the crystal. Systematic changes in the above-mentioned properties were discovered for a series of chosen crystals. These changes are clearly visible on comparing the first and last samples of the batch. In this Letter we present results for two such samples (PWO1 and PWO2). The samples were cleaved before the measurements and placed into an experimental chamber pumped to 10 -8 Tort in order to reduce the contribution of surface defects introduced by the polishing procedure and surface photochemical reactions. The temperature of the samples during luminescence and reflection measurements was 300 K. The luminescence from the irradiated side of the crystals was detected by a Philips XP2020Q photomultiplier tube through a Jobin-Yvon 10D monochromator.

2. Experimental results and discussion

2.1. Reflection spectra Reflection spectra were measured in the 4 - 1 0 eV energy range using SR the beamline of a SuperACO storage ring (LURE, Orsay, France). Reflected radiation was registered with a solar blind PM Hamamatsu R1120. The registered signal was normalized taking into account the relative changes of the incident SR intensity and the spectral sensitivity of the PM. Therefore, the reftectivity in Fig. 1 is given in arbitrary units. The first two maxima at 4.35 and 5 eV in the PbWO 4 reflection spectrum indicated in Fig. l with arrows A i, were previously assigned to transitions between the 6s and 6p levels of Pb 2+ and the wide non-elementary band in the 6 - 1 0 eV region (arrows B2-B 4) - to transitions in molecular anion (WO4) 2[6]. This interpretation of the PbWO 4 reflection spectrum needs some more accurate definition. Reflection in the 6 - 1 0 eV region from the PWO1 sample (dashed line in Fig. l) is similar to the reflection from CaWO 4 [8]. In this region three bands can be

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distinguished, despite their strong overlapping. These bands are identified in Ref. [8] as transitions in the molecular anion and their energies in the spectra of both crystals are similar. The non-bonding orbital t I built from oxygen 2 p ( ~ ) atomic orbitals is the highest occupied molecular orbital (HOMO) in this complex. Two bands indicated with arrows the B 2 and B 3 in the reflection spectrum are assigned to transitions from the HOMO to the 4t 2 orbital built mainly from the 5d(o') tungsten electrons, and arrow B 4 indicates the transition from the next oxygen 3t 2 orbital. Arrow B~ (5.9 eV) in Fig. 1 indicates the energy of the lowest energy band in the CaWO 4 reflection spectrum. This band corresponds to the transition from the HOMO to the 2e(~r*) lowest unoccupied molecular orbital (LUMO) made of 5d(~r) tungsten states. As seen in Fig. l, the reflection spectrum of PbWO 4 has a minimum at this energy. The reflection band A 2 at 5 eV can be accounted for by that transition. The almost 1 eV red shift of this maximum in lead tungstate relative to its energy in calcium tungstate can be explained by the difference in anion exciton states. The reflectivity maximum A 1(4.35 eV) of PbWO 4 can be ascribed to

A.N. Belsky et al. / Chemical Physics Letters 277 (1997) 65-70

the creation of a cation exciton (Pb6s-Pb6p), similar sharp exciton maxima being observed in many lead ionic compounds. Then the PbWO 4 absorption in the 4 - 6 eV range can be roughly represented as the superposition of transitions in lead and tungsten exciton subsystems. In previously published reflection spectra of PbWO 4 different ratios of the reflectivity values in the 4 - 6 eV range and in the 6 - 1 0 eV range were observed. According to Ref. [6], the first reflection region was much more intense than the second one, although the reverse was observed in Ref. [2]. As explained below, this may be due to even a slight difference in stoiehiometry between the samples. As seen in Fig. 1, reflection spectra of PWO1 and PWO2 exhibit a surprisingly large amplitude difference in the A~ reflection maximum. The PWO1 sample (with composition close to PbWO 4 stoichiometry) shows an essentially greater reflectivity in the energy range near the fundamental absorption edge. Moreover investigations of lead tungstate crystal structures have shown [7] that, under conditions of a PbO deficiency in the raw materials, crystallization into a structure with symmetry lower than that of scheelite becomes possible. A crystal with such a structure can be defined a s Pb7WsO(32_x), where the lead and oxygen sites are not all filled in (i.e. there are vacancies with local charge compensation) and Pb can occupy two non-equivalent sites. Hence, with decreasing PbO concentration in the melt, first crystals with the scheelite structure and point defects such as oxygen and lead vacancies as obtained, then structures with clusters of the second phase and finally, the second phase. All the crystals, except those of defect-free scheelite type, are expected to have a large set of electron states in the band gap. In such crystals, the probability of creation of a lead exciton may decrease. In order to explain this behavior we performed a simulation of the refiectivity in the following way. The absorption spectrum was assumed to be represented as a sum of oscillators with the energies 4.25, 5.2, 7.5, 8.2 and 9 eV with Gaussian broadenings of 0.1-0.3, 0.6, 0.5, 0.5 and 0.5 eV and oscillator strength values of 1, 1.4, 0.6, 1 and 0.4, respectively, multiplied by the number of electrons in each level. This absorption was converted into reflection using the Kramers-Kronig technique. Two broad absorption peaks were added

67

at 15 and 20 eV in order to satisfy the sum rules for the total number of valence electrons and the refraction index in the transparent region. The first peak (4.25 eV) is supposed to represent the 6s-6p Pb transition. Four peaks with higher energies correspond to transitions in the (WO4) z- group and their energies are close to that in CaWO 4 (except for the lowest transition). Variations in the crystal composition should result not only in the modification of the coordination number of lead sites and thus in a shift in energy of 6s-6p Pb transition but also in a shift of the relative energies of the Pb 2+ and (WO4) 2electronic levels. This earl account for the erosion and narrowing of the band gap which leads to the overlapping of the lead exciton energy levels with the states of the anion complex. Also we can suggest for non-stoichiometric compounds an increase in the probability of indirect transitions. In our model we supposed that this inhomogeneous broadening was chosen to be about 0.1 eV for good crystals and 0.3 eV for crystals with defects. The results plotted in Fig. 1 (insert panel), are in satisfactory agreement with the experimental data. Electron states living in the band gap of nonstoichiometric crystals can change drastically the luminescence properties since they act as traps for charge carriers. This leads to the possibility of a modification or redistribution of different mechanisms of luminescence excitation and hence, of changes in the luminescence spectra. Electron and hole trapping should produce effects like an increase in the slow component intensity, a luminescence yield dependence on the excitation flux and in the balance of traps filling additional maxima in the thermoluminescence spectrum. 2.2. Luminescence properties

Let us now consider the difference in luminescence properties of the two examined samples. Luminescence measurements were made under X-ray SR pulsed excitations of 15 keV photon energy. The pulses were separeted by 316 ns and the excitation flux was about 1012 photons/s. The luminescence spectra are shown in Fig. 2. Both of them are normalized by intensity in the wavelength range 500-550 rim. The PWO1 sample emission in the blue region of the spectrum prevails over that of the

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A.N. Belsky et al. / Chemical Physics Letters 277 (1997) 65- 70

PWO2 sample. The difference between the two spectra (shown in Fig. 2, bottom panel) has a band peaking near 420 nm. The band profile coincides with that of the fast luminescence in PbWO 4 [6]. The decay curves of the 430 nm luminescence obtained under X-ray SR excitation are shown in Fig. 3. A strong slow component is observed for PWO2, but not for PWO1. Most authors ascribe this slow component to an electron localized near an oxygen vacancy or, in other words, to luminescence of the (WO3)- group [5]. Our data confirm this suggestion, since the PWO2 sample is supposed to contain a large number of oxygen vacancies. It must be noted that the whole luminescence yield of PWO2 is about 10 times larger than that of PWO1, due to a strong contribution of the slow emission component. Thus, luminescence of stoiehiometric PbWO 4 crystals consists mainly of a fast blue component. For a long time, two models concerning the nature of this emission have existed and ben discussed. One attributes the emission to recombination in (WO4) 2-, the other one to the 6 p - 6 s radiative transition in the lead ion. The main problem is that the blue emission exhibits a strong temperature quenching in the interval 150-300 K. Our re-

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suits do not give a definite answer to the question of the nature of the blue emission. Nevertheless, for a sample with prevailing blue emission, more intense reflection is observed in the range of lead excitons. This allows us to suggest that the creation of such an exciton is significant for the excitation of PbWO 4 fluorescence. The low scintillation yield of PbWO 4 is a difficulty for its practical use. Doping with ions, which stabilize the cation exciton at room temperature, can be considered as a possible way of improving the fluorescence yield. For example, we obtained good results when doping with magnesium, where the phosphorescence was suppressed without any considerable decrease in the light yield [9]. Recent results of doping by La 3+ and Lu 3+ [10] are also interesting. 2.3. Radiation hardness

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One of the decisive parameters for the practical use of scintillators is the light yield dependence on irradiation dose and dose rate. It was found that in some PWO crystals, the light yield already decreases at doses of a few Gy only [11]. For different PWO samples, we have investigated the light yield as a function of the VUV and X-ray SR irradiation dose [ 12]. The irradiation rate under such excitation conditions (small penetration depth and great flux of SR) can reach 1 M G y / s . Fig. 4 shows the dependence of the luminescence intensity on the irradiation dose for the samples PWOI and PWO2 under constant illu-

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A.N. Belsky et al. / Chemical Physics Letters 277 (1997) 65-70

ruination by a flux of photons (109 photons/s) of 90 eV from the Super ACO storage ring and irradiation rate of about 1 k G y / s . For both samples a slow decrease in intensity is observed. After a 5130 KGy dose this decrease is about 5% for both crystals. On the other hand, for PWO2 the decrease in the light yield is more rapid at the beginning of the irradiation. After a 100 kGy dose it is about 10%. We notice the quick recovery of the light yield of the second sample as after only a 100 s pause in irradiation, about 3% is restored while the slow component does not change. We propose two different mechanisms to explain these two behaviors. One is related to a slow accumulation of radiation defects; the probability of their creation is low but such defects are stable. The second mechanism is characterized by a strong dose sensitivity and fast recovery of the light yield; this seems to be connected with changes in the charge state of the local levels in the band gap, i.e. with the filling of traps. Therefore, its contribution to the radiation sensitivity may be decreased on diminishing the density of appropriate defect levels. To obtain additional information about the traps responsible for the high irradiation sensibility of PbWO4 we measured the thermoluminescence spectra shown in Fig. 5. These spectra were measured

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after X-ray SR irradiation at 80 K. Low-temperature peaks of the same intensity are observed around 130 K in the spectra of both crystals. This is probably related to the release of electrons trapped on F or F + centers [13]. The glow peak with maximum at 165 K (the most intense in PWO2) is totally absent in the PWO1 spectrum. This and other weaker peaks in the 2130-300 K region can be associated with traps that arise when violating the PbWO 4 stoiehiometry. It is important to note that in the 100-300 K region, both thermoluminescence and phosphorescence are observed only in the long-wavelength emission band. Thus, the sequential capture of electrons and holes, or the recombination mechanism of excitation, is not typical of the blue emission of PbWO 4 at room temperature.

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Stoichiometric PbWO 4 crystals are characterized by an intense maximum in the reflection spectra at the fundamental absorption edge. This maximum can be associated with the creation of lead (Pb6s-Pb6p) excitons. In such crystals where blue fluorescence prevails, the light yield is insensitive to the irradiation dose. The results obtained in the present study, together with data on the excitation spectra [6],

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A.N. Belsky et al. / Chemical Physics Letters 277 (1997) 65-70

confirm the hypothesis about the excitonic origin of the blue luminescence in P b W O 4. Violation of the PbWO 4 stoichiometry decreases the intensity of the reflection maximum connected with the lead excitons. Such crystals exhibit a slow long-wavelength emission with a light yield about 10 times larger than that of the blue luminescence. Furthermore, the light yield of this emission is more sensitive to the irradiation dose. Moreover, in nonstoichiometric crystals, several types of traps arise decreasing the radiation hardness by recombination mechanisms. A detailed study of such defects is now in progress.

Acknowledgements

This work was partially supported by two programs of the European Economic Community: "Human Capital and Mobility" contract number CHRXCT93-0108 and INTAS Program contract number INTAS-93-2554, and by the French program "Formarion Recherche". It was performed in the framework of the "Crystal Clear" Collaboration, CERN Research and Development project RD-18.

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[2] V. Kolobanov, J. Becker, M. Runne, A. Schroeder, G. Zimmerer, V. Mikhailin, P. Orekhanov, I. Spinkov, P. Denes, D. Renker, B. Red'kin, N. Klassen and S. Shmurak, Proe. Int. Conf. on Inorganic Scintillators and Their Applications, SCINT95 (Delft University Press, Delft, The Netherlands, 1996) p. 249. [3] D. Millers, L. Grigorijeva, S. Chernov, A. Popov, P. Leeoq and E. Auffray, Dependence of luminescence intensity on temperature and transient absorption of PbWO4 under pulsed electron beam irradiation, CCC meeting, CERN, Geneva, 18 Oct. (1996). [4] V. Murk, M. Nikl, E. Mihokova and K. Nitseh, J. Phys.: Cond. Mat., accepted for publication. [5] M.V. Korzhik, V.B. Pavlenko, T.N. Timosehenko, V.A. Katehanov, A.V. Singovskii, A.N. Annenkov, V.A. Ligun, I.M. Solskii, J.-P. Peigneux, Phys. Status Solidi (a) 154 (1996) 779. [6] A.N. Belsky, V.V. Mikhailin, A.N. Vasil'ev, I. Dafinei, P. Leeoq, C. Pedrini, P. Chevallier, P. Dhez, P. Martin, Chem. Phys. Lett. 243 (1995) 552. [7] J.M. Moreau, Ph. Galez, J.P. Peineux, M.V. Korzhik, J. Compos. Alloys 238 (1996)46. [8] R. Grasser, E. Pitt, A. Seharmann, G. Zimmerer, Phys. Status Solidi (b) 69 (1975) 359. [9] A.N. Belsky, V.V. Mikhailin, M.V. Korzhik and P. Leeoq, CCC Meeting, CERN, Geneve, May (1995). [10] E. Auffray, P. Lecoq, M. Korzhik, O. Jarolimek, M. Nikl, S. Baecaro, A. Cecilia, M. Diemoz and I. Dafinei, Nuel. Instr. Meth., to he published. [1 I] E. Affray, I. Daffinei, F. Gautheron, O. Lafond-Puyet, P. Lecoq and M. Sehneegans, Proe. Int. Conf. on Inorganic Scintillators and Their Applications, SClNT95 (Delft University Press, Delft, The Netherlands, 1996) p. 282. [12] A.N. Belsky, S.M. Klimov, V.V. Mikhailin, P. Leeoq, E. Anffray and P. Chevallier, Nuel. Instr. Meth., to he published. [13] M. Nikl, J. Rosa, K. Nitseh, H.R. Asatryan, S. Baeearo, A. Cecilia, M. Monteeehi, B. Borgia, I. Dafinei, M. Diemoz, P. Leeoq, Mater. Sei. Forum 239-241 (1997) 271.