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Influence in the absorption spectrum of PbWO4 crystal by K+ doping
ARTICLE IN PRESS
Physica B 355 (2005) 427–431 www.elsevier.com/locate/physb
Influence in the absorption spectrum of PbWO4 crystal by K+ doping Feiwu Zhang, Qiren Zhang, Yuanyuan Sun, Kun Tao College of Science, University of Shanghai for Science and Technology, Box 251, 516 Jungong Road, Shanghai 200093, PR China Received 26 September 2004; received in revised form 7 November 2004; accepted 11 November 2004
Abstract The positions of the impurity K ion in the crystal are simulated by computer technology. The various kinds of defects corresponding in the K:PbWO4 are calculated. The defect chemistry and the defect reactions with the different impurity concentration doping PWO crystal have been studied. The origin of the decrease of the 350 nm absorption band of K:PbWO4 under low concentration is discussed, and the previous experimental results recur by computer simulation. The calculated results show that the 420 nm absorption band will be eliminated and the property of the light-induced refractive will be enhanced under the condition of heavily doped K:PbWO4. r 2004 Elsevier B.V. All rights reserved. PACS: 74.62.Dh; 61.72.Ji Keywords: PWO; Doping; Computer simulation; GULP
1. Introduction Although the luminescence of lead tungstate crystal, PbWO4 (PWO), has been reported [1] for a half-century since 1948, it has not been intensively investigated until the design and construction of a Large Hadron Collider (LHC) at CERN, which requires scintillators to make an Electromagnetic Calorimeter (ECAL) of CMS experiment [2,3]. Corresponding author. Tel.: +86 21 65694943;
fax: +86 21 65684057. E-mail address: [email protected] (F. Zhang).
PWO crystal has been chosen for the scintillation detector due to its high density, short radiation length, and fast decay time [3]. Apart from most investigations mainly focused on the factors that affect spectroscopy and scintillation properties of PWO, the nature and role of various defects in PWO crystal became a subject of debate. Extensive researches have been performed by doping or codoping in the PWO with the impurity ions for effectively improving the luminescence and scintillation characteristics [4–6]. In 1996, Nikl et al. [7] doped K+ ion in PWO, and observed some improvements in the 350 nm absorption band.
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.072
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F. Zhang et al. / Physica B 355 (2005) 427–431
Unfortunately, the research of K+:PWO was not developed in depth. Even in the PWO crystal computer simulation [8], the situation of heavily doped K+:PbWO4 was not considered. As some theory models were put out [9–12], we could explain the microcosmic mechanism of different density doping in the crystal and foretell some experimental results. The most recent defect theory model [12] refers the 350 nm absorption band to þ the Vþ K and VF color centers and the 420 nm band 2þ to the lead and oxygen vacancies pair V2 Pb 2VO : All of the proposed defect microstructure is unfortunately based on hypotheses, none of which was directly confirmed by experiments [8,9–12]. Due to the limited theory knowledge and experimental evidence about the complex defects in PWO, the computer simulation techniques that have seldom been used in the PWO study will effectively compensate the insufficiency of the present experiment results and explore the defects in PWO crystal. In this paper, we used computer simulation technology to investigate the defect chemistry in the different density doping crystal K+:PWO and discussed their microcosmic mechanism.
2. Methods of simulation The lattice simulations were performed using the freeware general utility lattice program (GULP) program [13] that is based upon the Mott–Littleton methodology for accurate modeling of defective lattices. The program GULP [13–16] optimizes the structure with respect to the asymmetric unit fractional coordinates and cell strains, using analytical symmetry-adapted first and second derivatives within a Newton–Raphson procedure starting from the exact Hessian matrix. An important feature of these calculations is the modeling of defects. The simplification of the Mott–Littleton method is to divide the crystal lattice that surrounds the defect into three regions known as 1, 2a, and 2b [14,17,18]. In the inner region, all interactions are treated at an atomistic level and the ions are explicitly allowed to relax in response to the defect, while the remainder of the crystal, where the defect forces are relatively weak,
is treated by more approximate quasi-continuum methods. In this way, local relaxation is effectively modeled and the crystal is not considered simply as a rigid lattice through which ion species diffuse. In this study, the inner defect region was set at 7.5 A˚, which was found to be adequate for the convergence of the computed energies, and the result almost approximates the one when the inner defect region was set at 15.0 A˚. Both perfect- and defect-lattice calculations are formulated within the framework of the Born-like model [21]. In this approximation, the potentials describing the interatomic interactions between two ions, with distance r, are presented as follows: Z i Z j e2 r C U ij ðrÞ ¼ þ A exp 6; r r r where the first part is the long-range Coulombic term and the latter are the short-range term described by the two-body Buckingham form [17], which is composed of the short-range Pauli repulsion and the leading term of the dispersion energy. Therefore, Z i is the formal charge of atom i, and A, r; and C are the adjustable potential parameters, respectively. Because charged defects will polarize other ions in the lattice, ionic polarizability (a) is also incorporated into the potential model. A shellmodel [22] treatment of such effects is described in terms of a shell with charge Y connected via an isotropic harmonic spring of force constant k to a massive core of charge ZY, namely, a¼
Y2 : k
In the calculations, all ions are treated as polarizable. Table 1 gives the short-range potential parameters and shell-model parameters, where the maximum short-range cutoff is 15.0 A˚.
3. Simulation results and discussion Table 2 presents the calculated energies of isolated point defects and cluster defect in the PWO crystal. Some of the computed results coincide with previous work [8,20].
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429
Table 1 Empirically derived potential parameters used in PWO The maximum short-potential cutoff is 15.0 A˚ Interactions
r (A˚)
A (eV)
(a) Short-range potential parameters. 9547.96 O2–O2 Pb2+–Pb2+ 18912.114 Pb2+–O2 8086.8038 767.43 W6+–O2 K+–O2 3587.570 Special (b) Shell parameters Pb2+ W6+ O2
0.2192 0.313781 0.264866 0.4386 0.300
Y (e)
10.09 5.89 2.04
C (eV A˚6)
Reference
32.0 2.6 3.5636 0.0 0.0
[8,19,20] [8,19,20] [8,19,20] [8,19,20] [21]
K (eV A˚2)
Reference
21006.539 7.690 6.30
[8,19,20] [8,19,20] [8,19,20]
Table 2 Energies of isolated point defects and cluster defect (a) Lattice energy (eV/formula) PbWO4 WO3
247.67 213.38
PbO K2O
37.89 23.18
(b) Isolated point defects Defect
Energy (eV)
Defect
Energy (eV)
V2 Pb K Pb
25.55 18.160
V2þ O þ ½O3 2
18.72 11.03
(c) Defect cluster pair Configuration
Cluster energy (eV) per defect
2þ K Pb 2VO pair þ KPb 2½O3 2 pair
17.778 13.265
PWO is a typical non-stoichiometric crystal containing intrinsic lead vacancies V2 Pb and oxygen vacancies V2þ in the crystal. The amount of V2 Pb is O 2þ larger than that of VO : Most of the V2 Pb are primarily bonded by V2þ forming defect pairs O 2þ 2 V2 2V : The excess V ; which should trap Pb Pb O holes to be a compensator, forms an isolated intrinsic defect in the as-grown crystal [12,22]. However, there are eight nearest neighbors O2
surrounding V2 Pb that are divided into two groups, each having four equal O2 symmetrically distributed around V2 Pb : No one can be prior to the others to trap holes. Compared to the formation of hole-centers in alkali halides [23], once a negative alkali ion vacancy is formed, the compensator is a hole shared by two halide ions nearest to the alkali ion vacancy forming a halide diatomic molecular ion rather than being trapped by one halide ion
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nearest to the alkali vacancy forming a halide atom. It can be reasonably believed that a similar result will occur in an oxide compound crystal. Therefore, once a hole is created in the crystal, the hole would be shared by the two nearest oxygen ions forming an oxygen diatomic molecular ion 2 O3 2 ; rather than trapped by one oxygen ion O forming oxygen ion O . The reaction formula can be written as 2O2 þ h ¼ O3 2 : In the crystal growing process of PWO, once a V2 Pb is formed, the lattice relaxation decreases the electrical negativity on V2 Pb site, and increases the electrical positivity on O2 site that shifts outwards. Therefore, once a V2 Pb is formed in PWO crystal, its two compensating holes will turn to be distributed as one hole shared by two oxygen ions nearest to V2 Pb and the other hole shared by two oxygen ions near V2 Pb forming a totally neutral color center pair or an aggregate color center. The previous one is a diatomic oxygen molecular ion perturbed by V2 Pb possessing mononegative, and the subsequent one is a pure diatomic oxygen ion possessing monopositive. According to the definition of color centers in alkali halides, the previous hole-centre is defined by Vþ F and the subsequent hole-center is defined by Vþ K and the 350 nm þ absorption band is attributed to Vþ K and VF aggregate color center. When PWO crystal is doped K2O, the K ion will occupy the Pb site in the crystal as the defect chemistry Eqs. (3.1) and (3.2) show. Any other defect chemistry possibilities have been excluded in other works [8]. Further calculations give the solution energy 6.577 eV for Eq. (3.1) and 4.148 eV for Eq. (3.2). 1 2K2 O
3 þ þ PWO ! K Pb þ ½O2 þ PbO;
2þ K2 O þ PWO ! 2K Pb þ VO þ 2PbO:
(3.1) (3.2)
The simulation results show that the formation 3 þ energy of the K Pb 2½O2 cluster is less than that 2þ of KPb 2VO cluster. From the calculated result, in the low-doped K+:PWO crystal, Kþ ion will occupy the Pb vacancy-related 350 nm absorption þ band, and an oxygen molecular ion ½O3 will 2 exist near KPb as the charge compensation. This process is shown in the defect solution Eqs. (3.1), and Fig. 1 is the conformation in this situation. On
c
−
K
VK+ (2O2− + h → O23 )
Oxygen ion K
Potassium ion
Fig. 1. Micro-structural models of Vþ K and KPb color centers in the as-grown PWO crystal.
the other hand, when the density of Kþ ion ð½K þ Þ 2þ 2 is so great that ½Kþ 4½V2 Pb ½VO ; all the VPb þ related 350 nm band will be occupied by K : The remaining Kþ will take the V2 Pb related 420 nm absorption band, and the V2þ will therefore O become the charge compensation, as the defect solution Eqs. (3.2) show. In the purity as-grown PWO crystal, as previously illustrated, a part of V2 Pb will exist in the 2þ form V2 2V pair, and the remaining V2 Pb Pb will O 3 þ þ trap two holes ð½O2 Þ near it to form VK and Vþ F centers [12,19,22]. In the low doping situation, since K Pb is monovalent, which will trap only one hole near it, the Vþ K center existing in the as-grown crystal will disappear and the Vþ F center disturbed þ by V2 will become the new V center as shown in Pb K Eq. (3.3). Since the old Vþ center disappears, the K 350 nm absorption band, which is a compound band of 330 and 360 nm, will be restrained, and the 360 nm band will be annihilated. This point well agrees with the experimental result [7]. 3 þ 2 þ þ Vþ F þ K ! ½VPb þ O2 þ K ! KPb þ þ þ O3 2 ! KPb þ VK
ð3:3Þ
þ þ The amount of V2 Pb trapping VK and VF center 2 is finite, and more VPb exists in the form of the 2þ V2 Pb 2VO pair, which will open when the crystal is radiated even by ultraviolet irradiation and the 420 nm absorption band will be stimulated greatly. With the increase of the density of K+ ion, when 2þ 2 ½Kþ 4½V2 Pb ½VO ; the VPb neighboring with 2þ VO will be taken by the spare K+ ion. Thus, the
ARTICLE IN PRESS F. Zhang et al. / Physica B 355 (2005) 427–431
420 nm-induced absorption band will be restricted, which will improve the scintillation property and the radiation hardness of PWO. As the previous experiment results show, the segregation coefficient of K+ ion in PWO is about 0.04 only [7,9], and the grown PWO-doped K+ ion will exhibit great asymmetry of the K+ ion distribution concentration. The K+ ion concentration doping in PWO will increase from the top to the bottom of the crystal. Therefore, there will be low concentration doping K+ ion in the top of the crystal and heavy doping K+ ion in the bottom of the same crystal. As argued in this paper, the crystal-doped K+ ion will have different optical absorption behaviors because of the different K+ ion doping concentration in the different position form the top to the bottom of the PWO crystal. The computer simulation in this work presents theoretical evidence that even in the same K+:PWO crystal there will be a block cutt from the bottom of the crystal, which achieves so high dopant concentration that could suppress the 420 nm absorption band. Further experiments should be carried out to confirm the calculated results.
4. Conclusions A computer simulation technique has been used to study the mechanisms of the substitution reaction. The present study demonstrates the local structure features of different density K+ ion doped PWO, which are difficult to probe by conventional experimental techniques. Our results and discussion have drawn attention to two conclusions as follows: 1. light-doped K+:PWO, the energetically favorable solution is when the monovalent ion enters the Pb-lattice, simultaneously producing an þ oxygen molecular ion ½O3 which is also 2 þ called VF center to compensate for the excess negative charge. This will partly restrain the 350 nm absorption band. 2. When the doped K+ density is greater than + 2þ ½V2 occupies the Pb-lattice which Pb 2½VO ; K neighbors an oxygen vacancy in the as-grown
431
crystal, and the ½2Kþ 2V2þ O dimers will increase. This will be helpful to improve the radiation hardness and increase the light yield. The calculated results well coincide with the experimental evidence.
Acknowledgment We are grateful to Dr. J.D. Gale for providing the GULP code.
References [1] F.A. Kro¨ger, Some Aspects of the Luminescence of Solids, Elsevier, Amsterdam, 1948. [2] ECAL CMS, Technical Design Rep. Electromagnetic Calorimeter, CERN/LHCC, December 14, 1997, pp. 97–33. [3] P. Lecoq, et al., Nucl. Instrum. Methods A 365 (1995) 291. [4] M. Kobayashi, et al., Nucl. Instrum. Methods Phys. Res. A 399 (1997) 261. [5] M. Nikl, et al., Appl. Phys. Lett. 71 (1997) 3755. [6] Y. Huang, et al., Solid State Commun. 127 (2003) 1. [7] M. Nikl, et al., Phys. Stat. Sol. B 196 (1996) K7. [8] Qisheng Lin, Xiqi Feng, J. Phys.: Condens. Matter 15 (2003) 1963. [9] M. Nikl, et al., J. Appl. Phys. 82 (1997) 5758. [10] J.M. Moreau, et al., J. Alloys Compd. 238 (1996) 46. [11] A. N. Annenkov, et al., CMS Note (1998) 41. [12] Qisheng Zhang, et al., PRB 68 (2003) 064108. [13] J.D. Gale, J. Chem. Soc. Faraday Trans. 93 (1997) 629. [14] N.F. Mott, M.J. Littlelton, Trans. Faraday Soc. 34 (1938) 485. [15] J.D. Gale, General Utility Lattice Program, Imperial College, London, 1996. [16] J.D. Gale, Philos. Mag. B 73 (1996) 3. [17] C.R.A. Catlow, J. Chem. Soc. Faraday Trans. 2 (85) (1989) 335. [18] A.B. Liduard, J. Chem. Soc. Faraday Trans. 2 (85) (1989) 341. [19] Zhang Feiwu, et al., J. University of Shanghai for Science and Technology 26 (6) (2004) 9. [20] Qisheng Lin, et al., PRB 63 (2001) 134105. [21] T.S. Bush, J.D. Gale, C.R.A. Catlow, et al., J. Mater. Chem. 4 (1994) 831. [22] Zhang Qiren, et al., Chin. Phys. Lett. 21 (6) (2004) 1131. [23] S.G. Fang, Q.R. Zhang, Physics of Colour Centres in Crystals, Shanghai Jiao-tong University press, 1989 (in Chinese).
Physica B 355 (2005) 427–431 www.elsevier.com/locate/physb
Influence in the absorption spectrum of PbWO4 crystal by K+ doping Feiwu Zhang, Qiren Zhang, Yuanyuan Sun, Kun Tao College of Science, University of Shanghai for Science and Technology, Box 251, 516 Jungong Road, Shanghai 200093, PR China Received 26 September 2004; received in revised form 7 November 2004; accepted 11 November 2004
Abstract The positions of the impurity K ion in the crystal are simulated by computer technology. The various kinds of defects corresponding in the K:PbWO4 are calculated. The defect chemistry and the defect reactions with the different impurity concentration doping PWO crystal have been studied. The origin of the decrease of the 350 nm absorption band of K:PbWO4 under low concentration is discussed, and the previous experimental results recur by computer simulation. The calculated results show that the 420 nm absorption band will be eliminated and the property of the light-induced refractive will be enhanced under the condition of heavily doped K:PbWO4. r 2004 Elsevier B.V. All rights reserved. PACS: 74.62.Dh; 61.72.Ji Keywords: PWO; Doping; Computer simulation; GULP
1. Introduction Although the luminescence of lead tungstate crystal, PbWO4 (PWO), has been reported [1] for a half-century since 1948, it has not been intensively investigated until the design and construction of a Large Hadron Collider (LHC) at CERN, which requires scintillators to make an Electromagnetic Calorimeter (ECAL) of CMS experiment [2,3]. Corresponding author. Tel.: +86 21 65694943;
fax: +86 21 65684057. E-mail address: [email protected] (F. Zhang).
PWO crystal has been chosen for the scintillation detector due to its high density, short radiation length, and fast decay time [3]. Apart from most investigations mainly focused on the factors that affect spectroscopy and scintillation properties of PWO, the nature and role of various defects in PWO crystal became a subject of debate. Extensive researches have been performed by doping or codoping in the PWO with the impurity ions for effectively improving the luminescence and scintillation characteristics [4–6]. In 1996, Nikl et al. [7] doped K+ ion in PWO, and observed some improvements in the 350 nm absorption band.
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.072
ARTICLE IN PRESS 428
F. Zhang et al. / Physica B 355 (2005) 427–431
Unfortunately, the research of K+:PWO was not developed in depth. Even in the PWO crystal computer simulation [8], the situation of heavily doped K+:PbWO4 was not considered. As some theory models were put out [9–12], we could explain the microcosmic mechanism of different density doping in the crystal and foretell some experimental results. The most recent defect theory model [12] refers the 350 nm absorption band to þ the Vþ K and VF color centers and the 420 nm band 2þ to the lead and oxygen vacancies pair V2 Pb 2VO : All of the proposed defect microstructure is unfortunately based on hypotheses, none of which was directly confirmed by experiments [8,9–12]. Due to the limited theory knowledge and experimental evidence about the complex defects in PWO, the computer simulation techniques that have seldom been used in the PWO study will effectively compensate the insufficiency of the present experiment results and explore the defects in PWO crystal. In this paper, we used computer simulation technology to investigate the defect chemistry in the different density doping crystal K+:PWO and discussed their microcosmic mechanism.
2. Methods of simulation The lattice simulations were performed using the freeware general utility lattice program (GULP) program [13] that is based upon the Mott–Littleton methodology for accurate modeling of defective lattices. The program GULP [13–16] optimizes the structure with respect to the asymmetric unit fractional coordinates and cell strains, using analytical symmetry-adapted first and second derivatives within a Newton–Raphson procedure starting from the exact Hessian matrix. An important feature of these calculations is the modeling of defects. The simplification of the Mott–Littleton method is to divide the crystal lattice that surrounds the defect into three regions known as 1, 2a, and 2b [14,17,18]. In the inner region, all interactions are treated at an atomistic level and the ions are explicitly allowed to relax in response to the defect, while the remainder of the crystal, where the defect forces are relatively weak,
is treated by more approximate quasi-continuum methods. In this way, local relaxation is effectively modeled and the crystal is not considered simply as a rigid lattice through which ion species diffuse. In this study, the inner defect region was set at 7.5 A˚, which was found to be adequate for the convergence of the computed energies, and the result almost approximates the one when the inner defect region was set at 15.0 A˚. Both perfect- and defect-lattice calculations are formulated within the framework of the Born-like model [21]. In this approximation, the potentials describing the interatomic interactions between two ions, with distance r, are presented as follows: Z i Z j e2 r C U ij ðrÞ ¼ þ A exp 6; r r r where the first part is the long-range Coulombic term and the latter are the short-range term described by the two-body Buckingham form [17], which is composed of the short-range Pauli repulsion and the leading term of the dispersion energy. Therefore, Z i is the formal charge of atom i, and A, r; and C are the adjustable potential parameters, respectively. Because charged defects will polarize other ions in the lattice, ionic polarizability (a) is also incorporated into the potential model. A shellmodel [22] treatment of such effects is described in terms of a shell with charge Y connected via an isotropic harmonic spring of force constant k to a massive core of charge ZY, namely, a¼
Y2 : k
In the calculations, all ions are treated as polarizable. Table 1 gives the short-range potential parameters and shell-model parameters, where the maximum short-range cutoff is 15.0 A˚.
3. Simulation results and discussion Table 2 presents the calculated energies of isolated point defects and cluster defect in the PWO crystal. Some of the computed results coincide with previous work [8,20].
ARTICLE IN PRESS F. Zhang et al. / Physica B 355 (2005) 427–431
429
Table 1 Empirically derived potential parameters used in PWO The maximum short-potential cutoff is 15.0 A˚ Interactions
r (A˚)
A (eV)
(a) Short-range potential parameters. 9547.96 O2–O2 Pb2+–Pb2+ 18912.114 Pb2+–O2 8086.8038 767.43 W6+–O2 K+–O2 3587.570 Special (b) Shell parameters Pb2+ W6+ O2
0.2192 0.313781 0.264866 0.4386 0.300
Y (e)
10.09 5.89 2.04
C (eV A˚6)
Reference
32.0 2.6 3.5636 0.0 0.0
[8,19,20] [8,19,20] [8,19,20] [8,19,20] [21]
K (eV A˚2)
Reference
21006.539 7.690 6.30
[8,19,20] [8,19,20] [8,19,20]
Table 2 Energies of isolated point defects and cluster defect (a) Lattice energy (eV/formula) PbWO4 WO3
247.67 213.38
PbO K2O
37.89 23.18
(b) Isolated point defects Defect
Energy (eV)
Defect
Energy (eV)
V2 Pb K Pb
25.55 18.160
V2þ O þ ½O3 2
18.72 11.03
(c) Defect cluster pair Configuration
Cluster energy (eV) per defect
2þ K Pb 2VO pair þ KPb 2½O3 2 pair
17.778 13.265
PWO is a typical non-stoichiometric crystal containing intrinsic lead vacancies V2 Pb and oxygen vacancies V2þ in the crystal. The amount of V2 Pb is O 2þ larger than that of VO : Most of the V2 Pb are primarily bonded by V2þ forming defect pairs O 2þ 2 V2 2V : The excess V ; which should trap Pb Pb O holes to be a compensator, forms an isolated intrinsic defect in the as-grown crystal [12,22]. However, there are eight nearest neighbors O2
surrounding V2 Pb that are divided into two groups, each having four equal O2 symmetrically distributed around V2 Pb : No one can be prior to the others to trap holes. Compared to the formation of hole-centers in alkali halides [23], once a negative alkali ion vacancy is formed, the compensator is a hole shared by two halide ions nearest to the alkali ion vacancy forming a halide diatomic molecular ion rather than being trapped by one halide ion
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nearest to the alkali vacancy forming a halide atom. It can be reasonably believed that a similar result will occur in an oxide compound crystal. Therefore, once a hole is created in the crystal, the hole would be shared by the two nearest oxygen ions forming an oxygen diatomic molecular ion 2 O3 2 ; rather than trapped by one oxygen ion O forming oxygen ion O . The reaction formula can be written as 2O2 þ h ¼ O3 2 : In the crystal growing process of PWO, once a V2 Pb is formed, the lattice relaxation decreases the electrical negativity on V2 Pb site, and increases the electrical positivity on O2 site that shifts outwards. Therefore, once a V2 Pb is formed in PWO crystal, its two compensating holes will turn to be distributed as one hole shared by two oxygen ions nearest to V2 Pb and the other hole shared by two oxygen ions near V2 Pb forming a totally neutral color center pair or an aggregate color center. The previous one is a diatomic oxygen molecular ion perturbed by V2 Pb possessing mononegative, and the subsequent one is a pure diatomic oxygen ion possessing monopositive. According to the definition of color centers in alkali halides, the previous hole-centre is defined by Vþ F and the subsequent hole-center is defined by Vþ K and the 350 nm þ absorption band is attributed to Vþ K and VF aggregate color center. When PWO crystal is doped K2O, the K ion will occupy the Pb site in the crystal as the defect chemistry Eqs. (3.1) and (3.2) show. Any other defect chemistry possibilities have been excluded in other works [8]. Further calculations give the solution energy 6.577 eV for Eq. (3.1) and 4.148 eV for Eq. (3.2). 1 2K2 O
3 þ þ PWO ! K Pb þ ½O2 þ PbO;
2þ K2 O þ PWO ! 2K Pb þ VO þ 2PbO:
(3.1) (3.2)
The simulation results show that the formation 3 þ energy of the K Pb 2½O2 cluster is less than that 2þ of KPb 2VO cluster. From the calculated result, in the low-doped K+:PWO crystal, Kþ ion will occupy the Pb vacancy-related 350 nm absorption þ band, and an oxygen molecular ion ½O3 will 2 exist near KPb as the charge compensation. This process is shown in the defect solution Eqs. (3.1), and Fig. 1 is the conformation in this situation. On
c
−
K
VK+ (2O2− + h → O23 )
Oxygen ion K
Potassium ion
Fig. 1. Micro-structural models of Vþ K and KPb color centers in the as-grown PWO crystal.
the other hand, when the density of Kþ ion ð½K þ Þ 2þ 2 is so great that ½Kþ 4½V2 Pb ½VO ; all the VPb þ related 350 nm band will be occupied by K : The remaining Kþ will take the V2 Pb related 420 nm absorption band, and the V2þ will therefore O become the charge compensation, as the defect solution Eqs. (3.2) show. In the purity as-grown PWO crystal, as previously illustrated, a part of V2 Pb will exist in the 2þ form V2 2V pair, and the remaining V2 Pb Pb will O 3 þ þ trap two holes ð½O2 Þ near it to form VK and Vþ F centers [12,19,22]. In the low doping situation, since K Pb is monovalent, which will trap only one hole near it, the Vþ K center existing in the as-grown crystal will disappear and the Vþ F center disturbed þ by V2 will become the new V center as shown in Pb K Eq. (3.3). Since the old Vþ center disappears, the K 350 nm absorption band, which is a compound band of 330 and 360 nm, will be restrained, and the 360 nm band will be annihilated. This point well agrees with the experimental result [7]. 3 þ 2 þ þ Vþ F þ K ! ½VPb þ O2 þ K ! KPb þ þ þ O3 2 ! KPb þ VK
ð3:3Þ
þ þ The amount of V2 Pb trapping VK and VF center 2 is finite, and more VPb exists in the form of the 2þ V2 Pb 2VO pair, which will open when the crystal is radiated even by ultraviolet irradiation and the 420 nm absorption band will be stimulated greatly. With the increase of the density of K+ ion, when 2þ 2 ½Kþ 4½V2 Pb ½VO ; the VPb neighboring with 2þ VO will be taken by the spare K+ ion. Thus, the
ARTICLE IN PRESS F. Zhang et al. / Physica B 355 (2005) 427–431
420 nm-induced absorption band will be restricted, which will improve the scintillation property and the radiation hardness of PWO. As the previous experiment results show, the segregation coefficient of K+ ion in PWO is about 0.04 only [7,9], and the grown PWO-doped K+ ion will exhibit great asymmetry of the K+ ion distribution concentration. The K+ ion concentration doping in PWO will increase from the top to the bottom of the crystal. Therefore, there will be low concentration doping K+ ion in the top of the crystal and heavy doping K+ ion in the bottom of the same crystal. As argued in this paper, the crystal-doped K+ ion will have different optical absorption behaviors because of the different K+ ion doping concentration in the different position form the top to the bottom of the PWO crystal. The computer simulation in this work presents theoretical evidence that even in the same K+:PWO crystal there will be a block cutt from the bottom of the crystal, which achieves so high dopant concentration that could suppress the 420 nm absorption band. Further experiments should be carried out to confirm the calculated results.
4. Conclusions A computer simulation technique has been used to study the mechanisms of the substitution reaction. The present study demonstrates the local structure features of different density K+ ion doped PWO, which are difficult to probe by conventional experimental techniques. Our results and discussion have drawn attention to two conclusions as follows: 1. light-doped K+:PWO, the energetically favorable solution is when the monovalent ion enters the Pb-lattice, simultaneously producing an þ oxygen molecular ion ½O3 which is also 2 þ called VF center to compensate for the excess negative charge. This will partly restrain the 350 nm absorption band. 2. When the doped K+ density is greater than + 2þ ½V2 occupies the Pb-lattice which Pb 2½VO ; K neighbors an oxygen vacancy in the as-grown
431
crystal, and the ½2Kþ 2V2þ O dimers will increase. This will be helpful to improve the radiation hardness and increase the light yield. The calculated results well coincide with the experimental evidence.
Acknowledgment We are grateful to Dr. J.D. Gale for providing the GULP code.
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