© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0031-8965/03/19603-00R7 $ 17.50+.50/0

phys. stat. sol. (a) 196, No. 2, R7–R9 (2003) / DOI 10.1002/pssa.200306595

ph

ysic

a st

atu

s so

lidi ⋅ R

apid

Res

earc

h N

ote

Rapid Research Note

Dielectric response of F–-doped PbWO4 single crystal

Hongwei Huang1, Xiqi Feng*, 1, Tong B. Tang2, Ming Dong3, and Zuo-Guang Ye3 1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China 2 Physics Department, H.K. Baptist University, Waterloo Road, Kowloon, Hong Kong SAR, China 3 Department of Chemistry, Simon Fraser university, Burnaby, BC, V5A 1S6, Canada

Received 19 February 2003, revised 14 March 2003, accepted 14 March 2003 Published online 17 March 2003

PACS 77.22.Gm

PbF2 doping is known to affect significantly the scintillating properties of PbWO4 single crystal, yet the nature of the extrinsic defects so induced remains unclear. In this paper, complex impedance spectroscopy was used to study the dielectric relaxation of PbF2-doped PWO crystals and provided information on its dipole relaxation and evidence for its defect complexes [2(FO

2–)•–VPb″].

Doping with aliovalent ions has an important influence on the scintillating properties of PbWO4 (PWO hereafter for short) single crystals [1], however, the dopants are largely concentrated on the metallic oxides, such as La2O3, Gd2O3, Y2O3, and Nb2O5 etc. PbF2 doping is known to affect significantly the properties of PWO single crystal [2], yet the nature of the extrinsic defects so induced remains unclear. Over the past few years, impedance spectroscopy has gradually been recognized as a convenient and useful method for probing the defect structure of PWO, but such studies are confined to cationic dopings [3–5]. As anionic dopants may lead to very different situations, we have, in this work, applied this tech-nique to fluoride-doped PWO crystals. Our samples were cut from an ingot grown with the Czochralski method, that was transparent, colour-less and without visible defects with proper doping concentration in the melt. Plates of 10 × 10 × 1 mm3 in dimensions, with their large faces normal to the crystal c-axis, were examined either as-grown or after annealing at 850 °C for 3 h in air. Platinum thin films were then sputtered on their large faces to serve as electrodes. The measurements were proceeded in a Solartron complex impedance analytical system, consisting of a 1260 Impedance Analyzer and a 1296 Dielectric Interface that scanned between 10 Hz and 1 MHz, with temperatures increased in steps of 20 °C from 200 to 320 °C. Each frequency sweep commenced after the set temperature point had stabilized for 20 min. According to the classical theory of dielectric relaxation [6], the dielectric response at angular fre-quency ω obeys the equation

0 0* ( )/(1 ) ,iε ε ε ε ωτ∞ ∞

− = − + (1)

* Corresponding author: e-mail: xqfeng@sunm.shcnc.ac.cn

R8 Hongwei Huang et al.: Dielectric response of F–-doped PbWO4 single crystal

ph

ysic

a st

atu

s so

lidi ⋅ R

apid

Res

earc

h N

ote

where ε* is complex dielectric constant, and ε0 and ε

∞ denote re-

spectively the “static” and “infinite frequency” dielectric constants, their difference being attributable to dielectric polarization, of which τ0 is the relaxation time. For the more general case of dispersive relaxations Cole and Cole have [7] proposed the following modified relation:

10 0* ( )/[1 ( ) ]i α

ε ε ε ε ωτ−

∞ ∞− = − + .

Here α lying between 0 and 1 can be regarded as a dispersion parameter for the width of the distribution in relaxation times. As shown below, this relation adequately describes our experimental results for PbF2-doped PWO. Figure 1 presents a typical Cole–Cole plot for an as-grown crystal at 300 °C. It may be seen that the locus of the dielectric constant on the complex plane falls on a circular arc. Similar plots for pristine as well as annealed samples at different temperatures are all well fitted by Eq. (2). In the former cases, the fitted values of ε

∞, ε0 and α are in the respective ranges of 5.93–6.22, 23.40–23.62 and 0.06–0.07; for annealed crystals, the ranges are 4.27–4.50, 23.95–24.35 and 0.04–0.07. These values of doped PWO do not deviate far from the published data on the pure material, for which ε∞ = 3.67 and ε0 = 21.55 when E || c, at 300 K, whereas the indices of refraction in the visible regions predict ε0 = 5.19 when E || c, and ε′(E || c) = 31.0 at 1.59 kHz at 24.5 °C [8]. Figure 2 depicts the dielectric loss tangent as a function of frequency, at various temperatures T (specified in K by the data point symbols) for as-grown crystal. Its maximum fmax shifted to higher fre-quency with increasing T. This quantity, the inverse of the characteristic time of the relaxation process, is found to satisfy an Arrhenius type of thermal dependence

fmax = ν exp (–E/κT), (3)

where E stands for the activation energy of the relaxation process, and ν its frequency factor, κ being of course the Boltzmann constant. The values so calculated are 1.94 ± 0.02 eV and 2.3 × 10–22 s–1. The an-

Fig. 1 Cole–Cole plot of as-grown PWO : F– at 300 °C; solid line repre-sents fitted circular arc.

Fig. 2 Dielectric loss relaxation spectra of an as-grown sample, for various temperatures in °C (indicated by different symbols for data points).

phys. stat. sol. (a) 196, No. 2 (2003) R9

ph

ysic

a st

atu

s so

lidi ⋅ R

apid

Res

earc

h N

ote

Table 1 Dipole relaxations in PWO doped with a trivalent metal ion or with F–.

Dopant (concentration) Dipole complex Activation energy (eV) Reference

La (500 ppm) (150 ppm)

[2(LaPb3+)• – VPb″] 0.55

0.49 [3]

Y (400 ppm) (150 ppm)

[2(YPb3+)• – VPb″] 0.46

0.43 [4]

Gd [2(GdPb3+)• – VPb″] 0.51 [5]

F [2(FO2–)• – VPb″] 1.94 this work

nealed sample exhibited similar characteristics (not shown), and its E and ν come out as 1.49 ± 0.02 eV and 3.9 × 10–18 s–1, respectively. (The errors in E have been estimated from least-squares fits of the Ar-rhenius plots.) We now discuss how these results may be interpreted within the probable defects structure model. Table 1 compares materials doped with some trivalent rare-earth cation to ours doped with F–, all un-annealed. In the former cases, dielectric relaxations have always been observed and ascribed to [2(REPb

3+)•–VPb″] dipole complexes. For our samples, when the monovalent impurity anion substitutes at a divalent oxide site, the same single positive charge center forms, an analogous compensation mecha-nism is likely to operate, hence the defect complex should be [2(FO

2–)•–VPb″], also an impurity–vacancy dipole. The comparison in the table is therefore instructive, but it indicates that our material exhibits an exceptionally high E. This activation energy measures the potential barrier blocking vacancy jumps. In the PWO lattice, the Pb–Pb interatomic distance is considerably larger than the distance from Pb to a nearest O ion. Consequently, the barrier for VPb to jump in [2(FO

2–)•–VPb″] is much higher, as found ex-perimentally. Dielectric loss in fluoride doped PWO is found to be much smaller than those in RE3+ : PWO. This may be explained by the fact that F is closer to O in both electronegativity and electronic configuration than the RE are to Pb, and so partly of WO4 can be replaced by WO3F, thanks to significant hybridization among the 2p states of F and those of O. Annealing treatment decreases the activation energy from 1.94 eV to 1.49 eV. Obviously, during this process, volatile fluorine, which had occupied oxygen sites, would be re-substituted by oxygen from air. The subsequent dissociation of free VPb from the extrinsic defect complexes leads to relaxation in the bound VPb. The reduction in F renders the crystal more homogeneous, a trend that is reflected also by the observed decrease in the dispersion parameter α.

Acknowledgement This work was supported by the NSFC of China (Grant No. 50172054), the Baptist university of Hong Kong S.A.R. (Grant No. FRG/00-01/II-35) and the Natural Sciences and Engineering Council of Canada (NSERC). The authors are grateful to Dr. A. A. Bokev for helpful discussion.

References

[1] M. Nikl, phys. stat. sol. (a) 178, 595 (2000). [2] X. Liu, G. Hu, X. Feng, Y. Huang, and Y. Zhang, phys. stat. sol. (a) 190, R1 (2002). [3] B. Han, X. Feng, G. Hu, Y. Zhang, and Z. Yin, J. Appl. Phys. 84, 2831 (1998). [4] H. Huang, W. Li, X. Feng, and P. Wang, phys. stat. sol. (a) 187, 563 (2001). [5] H. Huang, Z. Ye, M. Dong, W. Zhu, and X. Feng, Jpn. J. Appl. Phys. 41, L713 (2002). [6] P. Debye, Polar Molecules (New York, 1929). [7] K. S. Cole and R. H. Cole, J. Chem. Phys. 9, 341 (1941). [8] J. M. Stencel, E. Silberman, and J. Springer, Phys. Rev. B 14, 5435 (1976).