On the annealing mechanism in PbWO4 crystals

Physica B 324 (2002) 53–58

On the annealing mechanism in PbWO4 crystals Wenliang Zhua, Xiqi Fenga,*, Zhonghua Wub, Zhenyong Mana a

The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, People’s Republic of China b Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Bin2-7, Beijing, 100039, People’s Republic of China Received 24 March 2002; received in revised form 19 May 2002; accepted 20 May 2002

Abstract To elucidate the annealing process in air, optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) measurements are conducted in PbWO4 crystals annealed in air at different temperatures. The annealing exhibits a complex phenomenon, but the essence is the exchange of oxygen components between the crystal and environment to modify the intrinsic defects in the crystal. Interstitial oxygen ions are found to play an important role in the process for the influence on the 350 nm absorption band. The mechanism of annealing is discussed in this paper. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Ff; 61.72.Ji Keywords: Lead tungstate; Annealing; Defects

1. Introduction Since the choice of PbWO4 single crystal (PWO) as the scintillator in detectors at the Large Hadron Collider in CERN because of its high density, short radiation length and fast decay time [1,2], world-wide extensive attentions have been drawn on how to improve the radiation hardness of the crystals under a severe environment and to identify related possible defect structures begot during crystal growth or produced by radiation during use. For this aim, annealing in air exhibits an effective feature [3–5]: (1) It can improve macro*Corresponding author. Tel.: +86-21-52412661; Fax: +8652413903. E-mail address: [email protected] (X. Feng).

character of the crystals through eliminating the defects caused by local unbalance of temperatures during crystal growth [6]; (2) it can exert a significant influence on the optical absorption band at 350 nm or 420 nm, which plays an important role in the scintillation performance of this material [7]; (3) it can help to acquire defects knowledge for the defects transformation, e.g. [8]. However, the annealing process itself presents a complex phenomenon: Burachas et al. [9] discovered that radiation stability of PWO crystal deteriorates after annealing at 7001C in air while Baccaro [10] demonstrated that annealing in oxygen atmosphere gives a crystal radiation resistance. Further systematic studies of sequential annealing treatments [7,11] show that, as the temperature rises, the 350 nm optical absorption

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 2 6 9 - 3

W. Zhu et al. / Physica B 324 (2002) 53–58

2. Experiments The PWO crystal is grown from 5N PbO and WO3 powders in stoichiometric ratio by the modified Bridgeman method. Samples with 10
10
2 mm3 dimension are taken from the same crystal, including as-grown PWO, and three are annealed in air at 7401C, 8401C and 10401C for 6 h, respectively. XPS measurements are carried out on ESCALAB MK-II XPS instrument produced by VG Scientific Ltd., using Mg Ka X-ray sources. By a hemispherical analyzer and a multichannel detector, energy spectra of the emitted electrons are obtained. In order to avoid or minimize contamination, it is critical for all samples to be kept fresh in this experiment. Optical absorption spectra of the samples for XPS and an additional one, taken from the same crystal and annealed at 6401C for 6 h, are recorded by Shimadzu UVPC spectrophotometer before the XPS measurement. EXAFS spectra of W-L3 absorption for the PWO as-grown and annealed

at 8401C and 10401C were collected in transmission mode on beamline 4W1B XAFS experimental station in Beijing Synchrotron Radiation Facility (BSRF). The powdery samples were homogeneously smeared on Scotch adhesive tape to reach the optimum absorption thickness (DmdE1:0; Dm is the absorption edge step, d is the physical thickness of the sample).

3. Results The optical absorption spectra of the samples are given in Fig. 1. The experimental results demonstrate that annealing at a temperature

ABSORPTION COEFFICIENT (cm-1)

band increases first and then decreases obviously even to disappear at further annealing to 10401C but the mechanism still remains fuzzy, because the optical absorption or transmission spectroscopy cannot provide enough information about the related defects. Recently, in La doped PWO, a dielectric relaxation, ascribed to [2LaPb-Oi], was observed after annealing in air, indicating a new annealing mechanism [12]. It is worth noting that in the PWO: La crystal, interstitial oxygen ions might be stabilized by the introduced LaPb defect and the 350 nm color center has been suppressed by the dopants. However, the emergence and/or origin of Oi might be similar in the doped and undoped PWO, because the oxygen components in this material take an active part in the annealing process for (1) the participation in charges compensation caused by nonstoichiometric deviation and (2) the high rates of diffusion in the crystals. It is the aim of this paper to study the oxygen-ion-related defects by Extended X-ray absorption fine structure (EXAFS) and X-ray photoelectron Spectroscopy (XPS) in PWO annealed at different temperatures.

1.5

PHOTON ENERGY (eV) 43.63.2 2.8 2.4 2 1.6 asgrown 0 640 C 0 740 C

1.2 0.9 0.6 0.3 0.0 300

400

500

600

700

800

WAVELENGTH (nm)

PHOTON ENERGY (eV) 43.63.2 2.8 2.4 2 1.6 ABSORPTION COEFFICIENT (cm-1)

54

1.5 0

740 C 0 840 C 0 1040 C

1.2 0.9 0.6 0.3 0.0 300

400

500

600

700

800

WAVELENGTH (nm)

Fig. 1. The optical absorption spectra of as-grown PbWO4 crystal and samples annealed at 6401C, 7401C, 8401C and 10401C, respectively.

W. Zhu et al. / Physica B 324 (2002) 53–58

2

524

526

528

530

3

532

534

530

3

534

536

0

1040 C

524 526 528 530 532 534 536

Binding Energy (eV)

532

1

Counts (a.u.)

Counts (a.u.)

528

840 C

a 2

(c)

526

3

Binding Energy (eV)

(b) 0

1

0

740 C

a 2

524

536

Binding Energy (eV)

(a)

1

asgrown

Counts (a.u.)

Counts (a.u.)

1

55

3 a 2

524

(d)

526

528

530

532

534

536

Binding Energy (eV)

Fig. 2. The XP spectra of O1s electrons of as-grown PbWO4 crystal (a), and samples annealed for 6 h at 7401C (b), 8401C (c) and 10401C, respectively (d). Dot lines denote the fitted Gaussian curves.

Table 1 Curve-fitted parameters of O1s XP spectra in as-grown PWO and samples annealed at 7401C, 8401C and 10401C, respectively Peak position (eV)

FWHM (eV)

Intensity (%)

Samples

P1

Pa

P2

P3

P1

Pa

P2

P3

P1

Pa

P2

P3

As-grown 7401C 8401C 10401C

530.01 529.99 529.98 529.97

— 530.95 530.98 530.97

531.75 531.75 531.80 531.75

532.60 532.60 532.60 532.65

1.52 1.49 1.50 1.49

— 1.30 1.30 1.30

1.45 1.45 1.45 1.45

2.30 2.30 2.30 2.30

79.3 78.6 52.2 76.2

— 4.5 11.9 2.5

10.3 8.7 11.3 7.7

10.4 8.2 24.6 13.6

below 7401C will enhance the absorption at 350 nm in the process but begin to wash it out above 7401C. The turning point of the 350 nm absorption in the annealing process indicates a new defect formation or transformation occurs at 7401C. For further studies, Fig. 2 shows the O1s XP spectra of four samples, which are decomposed by Gaussian curves, with the fitted data in Table 1. The spectrum of as-grown PWO in Fig. 2(a) can be defined as a superposition of three Gaussian components, peaking at 530.0, 531.8 and 532.6 eV, respectively. Peak 1 (at 530.0 eV), with an overwhelming intensity, is deemed to be caused by the oxygen ion in WO2 4 because of its high concen-

tration, while the others, peak 2 (at 531.8 eV) and peak 3 (at 532.6 eV), can be ascribed to chemically adsorbed water and adsorbed oxygen, respectively, for the approximation to their normal values [13]. Coinciding with the result in the absorption spectra, after annealing at around 7401C, a new XPS peak appears at about 531.0 eV in Fig. 2(b). It reveals that there exists some kind of oxygen atom, which deviates a little from the normal O lattice and stays in a different coordinate environment or in a different valence state. In pure PWO crystals, it could be O related hole center or interstitial oxygen ion (O2 i ) for the absence of EPR signal of O single hole center [14]. In general, formation of hole centers in crystals will result in the

W. Zhu et al. / Physica B 324 (2002) 53–58

56

Fourier transform spectra (a.u.)

appearance of optical absorption band. However, no obvious absorption band except that at 350 nm was found in the spectrum (see Fig. 1). Furthermore, the same peak at 531.0 eV is also observed in the O1 s XP spectrum of heavily La3+-doped PbWO4 crystal [15], where existence of Oi seems to

1040 0C 840 0C

asgrown -1 0

1

2

3

4

5

6

7

8

R(Å)

(a)

EXA FTN

3

k X

1040 0C 840 0C

asgrown 2

4

6

(b)

8

10 12 14 16 -1

K (Å )

Fig. 3. The Fourier transforms spectra (a) and k3 -weighted W L3-edge EXAFS (b) of the PWO as-grown and annealed at 8401C and 10401C, respectively. EXA and FTN represent experimental curve and fitted curve, respectively.

be credible [16,17] and hole centers are dramatically suppressed for the introduction of excess positive charges. Therefore, the new peak is reasonable to be ascribed to interstitial oxygen ion. The interstitial O may occupy any of the any empty vertexes of the cube, which contains a WO2 4 tetrahedron [15,18]. At a higher temperature (8401C), the concentration of lattice oxygen ion, peak 1, decreases, whereas that of interstitial oxygen (peak a) or adsorbed oxygen (peak 3) increases. Nevertheless, when temperatures rise up, it is the contrary: The intensity of either peak a or peak 3 drops back pronouncedly, while that of peak 1 increases, shown in Figs. 2(c) and (d). From all the spectra, we can find that the concentration of chemically adsorbed H2O on the surface remains little changed, as peak 2 shows, which corresponds to the same condition of crystal growth and sample keeping. To research into the problem, EXAFS measurements of W-L3 absorption for the pure PWO asgrown and annealed at 8401C and 10401C, respectively, were employed to investigate the local atomic structures around W atoms. Fig 3(a) shows the radial structure function (RDF) of W atom obtained by Fourier transformation of the fine structure signals. There is one strong peak in the RDF for the samples, corresponding to the nearest-neighbor atoms around W ion. After Fourier filtering of the peak, the extracted EXAFS function is curve fitted using parameters calculated with the software FEFF [19], with the results given in Table 2. The coordination number (4.0) of the first shell surrounding W atom in as-grown PWO demonstrates W–O keeps well a tetrahedron structure. After annealing at 8401C, the coordination number increases obviously to 4.6, indicating that the

Table 2 Structural parameters obtained by EXAFS in PWO as-grown and annealed at 8401C and 10401C, respectively Sample

Coordination number N

( Bond length (A)

( 2) Debye–Waller factor s2 (A

As-grown PWO PWO annealed at 8401C PWO annealed at 10401C

4.0 4.6 4.2

1.774 1.773 1.771

0.0019 0.0025 0.0024

W. Zhu et al. / Physica B 324 (2002) 53–58

local atomic structures around the W6+ ion have changed. However, it drops back to about 4.2 at further annealing. The extra oxygen coordination can be attributed to the contribution of interstitial oxygen. In comparison, despite a different turning point, the results of EXAFS, XPS and optical absorption spectroscopy present a similarity in the annealing process, which suggests a potential relation between the pertinent defects in the crystal.

4. Discussion In fact, for the evaporation of PbO and WO3 during crystal growth, the nonstoichiometric deviation is caused, which results in the existence of intrinsic defects of oxygen vacancies Vo ; casually distributed cation vacancies Vc and regular cation vacancies Vc [20–22]. Therefore, when the crystal is annealed in air below 7401C, it is really possible for oxygen to diffuse into the lattice to fill oxygen vacancies, and induce two holes into the crystals. PWO þ Vo þ 12O2 -O
o þ 2 h;

ð1Þ

where ‘PWO’ denotes the whole crystal. In crystals, the atom on a lattice does not keep static, but hops around the center of balance position. When a vacancy appears in a normal lattice, the atoms in the vicinity will relax to some extent to the vacancy and form an expanded elastic lattice distortion area. Similar phenomenon occurs for an interstitial atom, but the deformation is by far greater, resulting in larger formation energy. That is why oxygen will fill the lattice position instead of the interstitial one. The annealing process will modulate the microstructure of as-grown PWO by decreasing the concentration of Vc ; and then aggravate the absorption at 350 nm in Fig. 1. 2 h þ 2Vc þ Vo -½VF  Vo  VF :

ð2Þ ½VF

Here, we consider the associated defect  Vo  VF  as the origin of 350-nm absorption band [23]. According to our computer simulation of PWO [24], Frenkel defects reaction can occur when the

57

annealing is performed at elevated temperature [25]. O
o þ PWO-Oi þ Vo :

ð3Þ

Above 7401C, some oxygen ion in the sublattice may achieve the energy to jump over potential bulwark around the atom to form an interstitial one, but still not enough to diffuse into the air. At the same time, the oxygen vacancy can move isotropically in the crystal [24]. As the annealing temperature increases, the concentration of Vo also augments. Meanwhile, the positive mobile Vo may decompose the 350 nm associated defects into even stable neutral one ½Vc 2Vo  [24], which is optically inactive, thus inhibiting the 350 nm absorption. Vo þ ½VF  Vo  VF -2½Vc  Vo  þ 2 h:

ð4Þ

And some of Oi may get the two holes and move to be adsorbed on the surface, which increases the intensity of peak 3 in Fig. 2(c). Oi þ 2 h-12O2 ðad:Þ-12O2 ðgasÞ:

ð5Þ

Reactions (1), (3) and (5) compete. In this temperature range, Reaction (3) prevails [24], but the infusion of oxygen ion from the air continues. As a result, the concentration of Oi still increases, though part of Oi diffuse because of decomposition of the intrinsic defect. This also coincides with the result in EXAFS experiments and that in Ref. [12]. That is why the intensity of lattice oxygen ion decreases, while that of interstitial oxygen ion and adsorbed oxygen increase from the as-grown, the annealed at 7401C to the annealed at 8401C in Figs. 2(a)–(c). At even higher temperature, e.g. above 8801C, Schottky defects reaction will occur for the evaporation of PbO. PWO-Vc þ Vo þ PbO ðgasÞ:

ð6Þ

The Vc and Vo may form stable dipole ½Vc 2Vo : The interstitial oxygen has achieved enough energy to diffuse to the surface and to overcome surfaceadsorption energy, so the adsorbed oxygen will diffuse into the air. When at 10401C, reactions (4) and (5) tend to be complete, which decreases the concentration of 350 nm intrinsic defects even to vanish. At this time, most of the interstitial oxygen

58

W. Zhu et al. / Physica B 324 (2002) 53–58

ions formed and adsorbed on the surface in the annealing process diffuse into the air. As a result, the O1s XP spectrum changes back to be similar to that of as-grown PWO and the coordination of W also drops back simultaneously.

[3] [4]

[5]

5. Conclusion In lead tungstate crystals, Vo and VPb are ineluctable for the evaporation of PbO and WO3 during crystal growth. When annealing at a lower temperature, oxygen in the air may diffuse into the crystals, filling the lattice. However, at higher temperature beyond 7401C, the O1s XP spectra of PWO reveal a new peak, indicating interstitial oxygen is formed for the Frenkel reaction, which indirectly affect the color center at 350-nm absorption band. As the temperature is elevated to 8401C, the concentration of Oi increases, with more oxygen induced and adsorbed on the surface. But at the further annealing to 10401C, the interstitial oxygen may escape out of the crystals and cause the 350-nm absorption band to disappear indirectly. In short, the essence of the annealing in air is an exchange of oxygen components between the crystal and environment resulting in the concentration change of oxygenrelated point defects in PbWO4 crystal.

[6]

[7] [8] [9]

[10] [11]

[12] [13] [14]

[15]

[16]

Acknowledgements The EXAFS experiments are conducted in BSRL. The authors are grateful to Ms Y.X. Zhang (SIC) for her help in sample preparations and Prof. M.R. Ji (USTC) for his help in XPS experiments. This work is financially supported by the National Science Foundation of China (Grant No. 50172054).

References

[17] [18]

[19] [20] [21]

[22] [23]

[1] Compact Muon Solenoid Technical Proposal, CERN/ LHCC 94-38, LHCC, 1994, P1. [2] P. Lecoq, I. Dafinei, E. Auffray, M. Schneegans, M.V. Korzhik, O.V. Missevitch, V.B. Pavlenko, A.A. Fedorov,

[24] [25]

A.N. Annenkov, V.L. Kostylev, Nucl. Instrum. Methods A 365 (1995) 291. R.Y. Zhu, Q. Deng, H. Newman, C.L. Woody, J.A. Kiestead, S.P. Stoll, IEEE Trans. Nucl. Sci. 45 (1998) 686. R.Y. Zhu, C.L. Woody, Proceedings of the International Conference on Calorimetry in High Energy Physics, 1996, p. 577. J.Y. Liao, B.F. Shen, P.F. Shao, Z.W. Yin, J. Inorg. Mater. 14 (1999) 352. E. Auffray, I. Dafinei, F. Gantheron, O. Lafond-Puyet, PLecoq, M. Schneegans, Proceedings of the Conference on Inorganic Scintillators and their Applications, SCINT95, Delft, University Press, The Netherlands, 1996, p. 490. B.G. Han, X.Q. Feng, G.Q. Hu, Y.X. Zhang, Z.W. Yin, J. Appl. Phys. 86 (1999) 3571. M. Nikl, K. Nitsch, J. Hybler, J. Chval, P. Reiche, Phys. Stat. Sol. (B) 96 (1996) K7. S. Burachas, V. Bondar, Yu. Borodenko, K. Katrunov, V. Martinov, L. Nagornaya, V. Ryzhikov, G. Tamulaitis, H. Gutbrod, V. Manko, J. Crystal Growth 199 (1999) 881. S. Baccaro, IEEE Trans. Nucl. Sci. 46 (1999) 292. N. Senguttuvan, M. Ishii, K. Tanji, T. Kittaka, Y. Usuki, M. Kobayashi, M. Nikl, Jpn. J. Appl. Phys. 39 (2000) 5134. W.S. Li, T.B. Tang, H.W. Huang, X.Q. Feng, Jpn. J. Appl. Phys. 40 (2001) 6893. Y.G. Cui, J. Instrum. Anal. 15 (1996) 17. S. Baccaro, B. Borgia, I. Dafinei, E. Longo, Proceedings of the International Workshop on Tungstate Crystals, Roma, October 12–14, University degli Studi La Sapienza, 1998, p. 129. W.L. Zhu, X.Q. Feng, Y.L. Huang, Q.S. Lin, Z.H. Wu, Doping Mechanism of Heavily Doped PbWO4:La3+, Phys. Stat. Sol. (A), in press. T. Esaka, T. Mina-ai, H. Iwahara, Solid State Ionics 57 (1992) 319. T. Esaka, Solid State Ionics 136–137 (2000) 1. Y.H. Chen, X.L. Ye, D.F. Zhou, C.S. Shi, H.F. Chen, Proceedings of the Conference on Inorganic Scintillators and their Applications, SCINT 99, Moscow, 1999, p. 609. S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Phys. Rev. B 52 (1995) 2995. J.M. Moreau, Ph. Galez, J.P. Peigneux, M.V. Korzhik, J. Alloys Compounds 238 (1996) 46. M. Nikl, K. Nitsch, S. Baccarro, 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. A. Annenkov, E. Auffray, M. Korzhik, P. Lecoq, J.-P. Peigneux, Phys. Stat. Sol. (A) 170 (1998) 47. Q.S. Lin, X.Q. Feng, Z.Y. Man, C.S. Shi, Q.R. Zhang, Phys. Stat. Sol. (A) 181 (2000) R1. Q.S. Lin, X.Q. Feng, Z.Y. Man, Phys. Rev. B 63 (2001) 134105. W. Van Loo, J. Solid State Chem. 147 (1975) 359.