P E R G A M O N

Progress in

Crystal Growth

and Characterization Progress in Crystal Growth and Characterization of Materials

of Materials (2000) 177-182 http://www.elsevier.com/locate/pcrysgrow

S P E C T R O S C O P I C E V I D E N C E F O R

D E F E C T S T R U C T U R E I N I R R A D I A T E D

PbWO4 S I N G L E C R Y S T A L

Wensheng LI and Tong B. TANG

Department of Physics, H.K. Baptist University, Waterloo Road, Kowloon, Hong Kong, China

Abstract

PbWO 4 has recently attracted much attention because of its scintillation

applications. We have measured its dielectric response subsequent to Ar + and UV irradiation,

respectively. Dielectric relaxation was observed, which we attribute to a complex color

center in the WO ~2 tetrahedron. The complex color center in turn led to the distortion of the

WO22 tetrahedron, which was verified by FTIR spectra showing that the v 3 and v 4

degeneracies were removed.

INTRODUCTION

Radiation damage in PbWO 4 (or PWO for short) crystals is coming under close scrutiny due to

the use of the material in scintillation detectors[l-41. According to Nikl[5], the process of the production

of color centers in PWO can be generally viewed as, (i) creation of hot electrons and holes by the

interaction with high energy photons; (ii) their separation and diffusion; and (iii) localization at specific

lattice sites, resulting in color centers. However the exact nature of such lattice sites and color centers

remain unclear. In the present work, a model of the complex color centers dominant in PWO produced

by different radiation is proposed. The AC impedance spectra of PWO after Ar + and UV irradiation was

observed; it is known that the effects of UV light are similar to those of the 7-rays[4]. IR spectra

revealed that the origin Of the dielectric relaxation could be attributed to the formation of a complex

color center in the WO 2 ~ tetrahedron.

E X P E R I M E N T

PWO ingots were grown from 5N-purity powder using an improved Bridgman method in the

Shanghai Institute of Ceramics. Samples were cut into 8x8xl mm 3 shapes, their large faces being

0960-8974/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. PII: $0960-8974(00)00005-X

78 Wensheng LI and Tong B. Tang / Prog. Crystal Growth and Charact. 40 (2000) 177-182

perpendicular to the crystal c-axis. Some samples were subjected to UV radiation from a 1 kW high-

pressure Hg lamp, and some to Ar ÷ ion irradiation using the ion gun in a Quantum 2000 Scanning ESCA

microprobe, with the energy set at 1 keV, each for the same 5 rain period. XPS spectra were taken in the

so said ESCA microprobe, using C~ line at 284.6 eV for energy calibration. FTIR spectra taken on the

Nicolet FTIR-550. The dielectric responses of PWO were recorded with an HP 4284a meter, Oxygen

annealing, when desired, proceeded at 1000°C for 12 hr.

RESULTS AND DISCUSSION

Fig.1 presents the dissipation factors measured at room temperature in a typical as-grown PWO

crystal and in the same sample after oxygen annealing. The only difference between the two curves lies

at the low frequency end, where annealing has resulted in a lower tan& Fig.2 shows the dielectric

responses of two samples irradiated with At+and UV radiation at various temperatures.

1.6

2 . 0

1 .2

1.0

~" 0.8

0.6 d o

0.4

c~ 0.2

0.0

0.8

"~ 0.4

.M

N

i~-6 o.8 I 0 .6

. . . . . . . . , . . . . . . . . , . . . . . . . . , . . . . . . . . , . . . . . . . . , 0 . 4

10 ~ 10 2 10 3 10 4 10 ~ 10 ~

Frequency (Hz) 0 . 2

Figure I . Die lectr ic losses of a crysta l

(a) a s - g r o w n a n d (b) a f ter a n n e a l i n g 0.O . . . . . . . . ~ . . . . . . . . , . . . . . . . . , . . . . . . . . ~ . . . . . . . . ~' 101 10 2 1 0 s 1 0 4 1 0 5 10

F r e q u e n c y ( H z )

Figure 2. Die lectr ic losses a f ter (a) A r + or (b) U V

i r rad i a t ion m e a s u r e d at v a r i o u s t e m p e r a t u r e ,

(A) 290, (B) 310, (C) 335, (D) 360 and (E) 385 K.

The relaxation phenomena in these two cases show close resemblance. Each follows a Debye dispersion

relation with a single relaxation time "c, which varies with temperature T in an exponential manner,

Wensheng LI and Tong B. Tang /Ping. Crystal Growth and Charact. 40 (2000) 177-182 179

i - E ic =f,,,,,, = v(,exp (%--~-") (1) l " K I

where, f,,~ is the frequency of loss peak, and both v~, and E are materials constants. Following gq(1), we

determine the activation energies E in the range of 0.30_+0.01 to 0.32_+0.02 eV using a frequency factor

v 0 = 107 for both Ar + and UV irradiated samples. The XPS spectra indicate that, before irradiation, the

elements were in a single valence state in the crystals (Fig.3). The situation appeared unchanged after

UV irradiation. However, Ar + ion bombardment led to the emergence of low B.E. subpeaks at the side of

the main peaks (Fig.4), indicating the presence of Pb ~) and W 4+ as deduced from their respective chemical

shifts.

z > ,

i , i , i , f , i , i

135 137 139 141 143 145 W or ig in W 4:/7,, 2 ~2

3 2 3 4 3 6 3 8 4 0

B i n d i n g E n e r g y ( e V )

Figure 3. XPS spectra, before irradiation

> .

6)

P b

i , i , i , i , i , i , i

134 136 138 140 142 144 146

2 ; ' 3'0 ' 3'2 ' 3~4 ' 3'6 ' 3 ; ' 410

Binding Energy (eV)

Figure 4. XPS spectra, after Ar ÷ bombardment

To make clear the origin of the dielectric spectra observed, IR spectra were taken. It was

confirmed that in the WO 2- tetrahedron only the v~ and v 4 modes are IR active[6], and the v 3 and v 4 mode

are in the wave number range of 400 - 900 cm ~, with a very broad band[6,7] as show in Fig5(a). The

broad band is owing to the strong absorption of the crystal and the degeneracy of the v 3 and v~ modes,

each of them is a triply degeneracy vibration. After irradiation, the IR spectra are evidently changed

(Fig5(b) and Fig5(c)). Sharp peaks appeared in the said band revealing that the vibration mode

degeneracy was somehow removed owing to the distortion of the WO ]- tetrabedron by the emergence of

complex color centers.

80 Wensheng LI and Tong B. Tang / Prog. Crystal Growth and Charact. 40 (2000) 177-182

:(c) I

: (b)

t - O

0 t./)

J L i

:(a)

1 4 0 0 ' 1 2 ; 0 ' 1 0 ' 0 0 ' 8 ; 0 400 6(?0 Wave Number (cm -1)

Figure 5. IR Spectra of (a) as-grown, (b) UV i r r a d i a t e d a n d (c) Ar* i o n i r r a d i a t e d samples

It was confirmed using a fine XRD diffraction experiment[8] that there always exist some level

of lead vacancies Vpb in the crystal owing to the evaporation of PbO during crystal growth, thus in the

,' 1 II as-grown samples, the charge compensation for balance can generally be described as [ Vpb ] = ~lVo ]

+ [Pb3*]. In a crystal having a large forbidden gap such as PWO, the dc conduction only comes from the

charge hopping, mainly the hoping of the mobile point defects. Since in the annealed sample the

concentration of oxygen vacancies is greatly diminished as a result its dc conduction becomes much

smaller leading to a dielectric loss at low frequency lower than in the as-grown one (Fig.l).

. In the Ar + ion bombardment process, Pb ~ was created, therefore the concentration of [V"pb I in

crystal increased which in turn leads to the increase of IV;'I and [Pb! +] in order to keep the charge

balance. As a result [(W4')"-V;'] were formed in the WO]- tetrahedra. In the UV radiated case, it was

suggested[3] that the following processes occurred,

Vo + 2e --> F (2)

02, + h --e O (3)

Wensheng L[ and Tong ]3. Tang / Prog. Crystal Growth and Charact, 40 (2000) 177-182 18

Therefore, In the WO 4- tetrahedron~ the trapping sites lead to the formation of a complex color center

[(F)"-(20)"]. Similar behavior involving dielectric relaxation lie in the fact that the complex color

centers in both irradiation cases are relate to WO ~-tetrahedra.

CONCLUSION

It is confirmed that the dielectric responses of PWO after Ar ÷ ion and UVirradiation are resemble

to each other. Based on impedance, IR spectroscopic data and XPS, we propose that both cause complex

color centers located at the WOZ-tetrahedra in the crystal lattice. High-temperature annealing is

observed to reduce the concentration of oxygen vacancies, which are the mobile defects.

ACKNOWLEDEGMENT

We thank Prof. X.Q.Feng (Shanghai Institute of Ceramics) for helpful discussions. This work

was partly supported by The National Nature Science Foundation of China (Grant No.59732040).

REFERENCE:

1. E. Auffray, I. Dafinei, F. Gautheron, O. Lafont-Puyet, P. Lecoq and M. Schneegans, in

Inorganic Scintillators and TheirAppIications (Delft University Press, Delft, 1996) 282.

2. M. Nikl, P. BobS.ek, K. Nitsch, E. Mihokovfi, M. Martini, A. Vedda, S. Croci, G. P. Pazzi, P. Fabeni,

S. Baccaro, B. Borgia, I. Dafinei, M. Diemoz, G. Organtini, E. Auffray, P. Lecoq, M. Kobayashi, M.

Ishii and Y. Usuki, Appl. Phys, Lett. 71, 3755 (1997)

3. Baoguo Han, Xiqi Feng, Guanqin Hu, Yanxing Zhang and Zhiwen Yin, J. Appl. Phys. 86, 3497

(1999).

4. I. Dafinei, B. Borgia, F.Cavallari, M. Diemoz, E. Longo, S. Baccaro, A. Cecilia, M. Montecchi, G.

Organtini, S. Salvatori and M. Nikl, In Proc. Int. Co~f on Inorganic Scintillators and their

applications, ed. Z.W. Yin et al.,(Shanghai China,1997) 219.

5. M, Nikl, K, Nitsch, S. Baccaro, A. Cecilia, M. Montecchi, B. Borgia, I. Dafinei, M. Diemoz,

M. Martini, E. Rosetta, G. Spinolo, A. Vedda, M. Kobayashi, M. Ishii, Y. Usuki, O. Jarolimek

and P. Reiche ,., J. Appl. Phys. 82, 5758 (1997).

6. J.A. Gadsden, " h~[?ared Spectra of Minerals and Related Inorganic Compounds",

The Butterworth Group, 1975.

7. J.M.Stencel, E.Silberman and J.Springer, Phys. Rev. B ,14, 5435 (1976).

8. P. Galez and J.M, Moreau, J. Alloys and Compounds, 46, 238(1996).

82 Wensheng LI and Tong B. Tang/Prog. Crystal Growth and Charact. 40 (2000) 177-J82

Brief biography of the authors

w.s . L_i

W.S.Li joined the Shanghai Institute of Ceramic, Chinese Academy of Sciences and worked on electrical and optical properties of crystals and ceramics after he gained B.Sc. from Tong-Ji University. He is now working for his Ph.D. degree in Hong Kong Baptist University.

T.B. Tang

T.B. Tang obtained his Ph.D from University of Cambridge and then worked on organic superconductors in Japan as a Royal Society - JSPS Fellow, before joining the Hong Kong Baptist University, where he is a Professor of Physics.