- Email: [email protected]
The structure of a PWO:La3+ crystal
Journal of Alloys and Compounds 307 (2000) 245–248
L
www.elsevier.com / locate / jallcom
The structure of a PWO:La 31 crystal a, a b Qisheng Lin *, Xiqi Feng , Jiutong Chen a
Laboratory of Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 25 January 2000; accepted 25 February 2000
Abstract A lead tungstate crystal, with 1% mol La 31 doping, was grown by the Czochralski method and its structure determined by ˚ c 5 12.038(1) single-crystal X-ray diffraction techniques. It retains the scheelite-type structure with cell constants a 5 b 5 5.4587(6) A, ˚A, V 5 358.7(1) A ˚ 3 , Z 5 4. The trivalent La 31 ions are located at the 4(b) positions with occupancy 0.01 and share this site, statistically, with Pb 21 ions. The structural discrepancy between the La 31 doped and the undoped PWO comes from the positions of the oxygen atoms, which results that the PLO 8 polyhedron and WO 4 tetrahedron rotate anticlockwise by about 57.88 along their S4 axis compared with those of the undoped PWO crystal. The reasons for this are discussed. 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical materials; X-ray diffraction; Crystal structure
1. Introduction To improve the optical transmittance, decay time and radiation hardness of PbWO 4 (PWO) scintillating crystals, much research, including doping and annealing treatments, has been carried out [1–4]. Several studies on the structural characterization of PWO by X-ray and neutron diffraction techniques have been performed to illustrate the presence of defects in PWO crystals [5–7]. As a matter of fact, the most intensively studied dopants are mainly trivalent ions, because they are considered as being able to replace the Pb 21 ions and to compensate the Pb 21 deficiency taking account of their radius and electronegativity. For example, Kobayashi et al. reported that the scintillating properties could be significantly improved by doping with trivalent ions, such as La 31 , Y 31 , Gd 31 and Lu 31 . They considered that these ions could compensate the Pb 21 deficiency, thus reduce the densities of the Pb 31 / O 2 related defects [8]. Han et al. initially measured the dielectric relaxation spectra of La:PWO and concluded that the observed polarization was most probably due to ? the creation of [2(La 31 Pb ) –V 99 Pb ] dipole complexes [9]. However, no results on the structure or microstructure of *Corresponding author. E-mail address: [email protected] (Q. Lin)
the doped PWO have been reported. As part of our research, we have tried to study the relationship between the structure and properties of PWO:La 31 crystals. Here, we report the single-crystal X-ray results of 1% mol La 31 doped PWO.
2. Experimental The investigated crystal was grown from an initial mixture of stoichiometric PbO and WO 3 (.99.999%) with 1% mol La 31 ion doped in a platinum crucible using the Czochralski method in air. For X-ray analysis, a single crystal with approximate dimensions 0.2 mm30.4 mm30.2 mm was physically separated and selected for X-ray diffraction analyses at room temperature. Photographs show that the crystal retains the same tetragonal lattice as that of undoped PWO.
3. Single-crystal X-ray structure analysis A total of 481 reflections were collected at 298 K on an AFC7R Rigaku diffractometer with graphite monochro˚ using the v / 2u mated Mo Ka radiation ( l 5 0.71069 A)
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00823-9
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Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248
scan mode in the range 08 , 2u , 69.88, of which 415 were unique and 132 with I $ 3s (I) were used for structure determination. The cell parameters determined from 25 reflections in the range 9.88 , u , 18.38 are a 5 ˚ c 5 12.038(1) A, ˚ V 5 358.7(1) A ˚ 3 , Z 5 4. 5.4587(6) A, From the systematic absence of hkl, h 1 k 1 l 5 2n; hk0, h 5 2n; 00l, l 5 4n and from the subsequent least-squares refinement, the space group was determined to be I41 /a (No. 88). Data reduction and corrections for empirical absorption based on c-scan techniques were applied. Lorentz and polarization corrections were also applied to the data. After structure refinement with isotropic thermal parameters, an empirical absorption correction was made using the program DIFABS. A secondary extinction correction was applied. Intensities of equivalent reflections were averaged with an agreement factor R int 5 0.035. The PL (0.99Pb10.01La) and W atoms were determined directly from an E-map and the O atom by difference Fourier syntheses. The structure was refined by a full-matrix least-squares method for 10 variables affording the final R 5 0.0288, R w 5 0.0356, w 5 1 / [s 2 (F ) 1 0.020F 1 1.0], S 5 1.33, D /s 5 0.00. The residual peaks (Dr ) max and (Dr ) min in the final difference Fourier map are ˚ 3 , respectively. The scattering factors 2.53 and 22.53 e / A 21 31 for the Pb , La , W 61 and O 2 ions were taken from International Tables for X-ray Crystallography (1974, Vol.
Table 1 Coordinates and thermal parameters for PbWO 4 :La 31 Atom
x
y
z
˚ 2) Beq (A
PL a W O
0.000 0.000 0.142(2)
0.250 0.250 0.015(1)
0.625 0.125 0.2071(7)
0.0082(1) 0.0047(1) 0.005(7)b
a PL indicates the 4(b) site occupied by Pb 21 and La 31 in the ratio 0.99:0.01. b Oxygen atom was refined isotropically.
IV, Table 2.2B). All calculations were performed by the MOLEN program package [10].
4. Discussion A drawing of the unit cell projected along the c direction is shown in Fig. 1. It is characterized by identical columns with PL and W atoms regularly stacked along the c direction and with the surrounding oxygen atoms forming a screw. The atomic coordinates and the thermal parameters are listed in Table 1, in which PL represents a statistical distribution of ions with 0.99Pb 21 10.01La 31 . The selected bond distances and bond angles are given in Tables 2 and 3, respectively. The PL atoms are eightcoordinated by O atoms and the W atoms four-coordinated,
Fig. 1. Arrangement of PL atoms and WO 4 tetrahedron in PWO:La 31 in the projection along the c-axis. Note the positions of oxygen atoms surrounding the columns made up of PL and W atoms alternating along the c direction.
Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248 Table 2 Selected bond distances for PbWO 4 :La 31 Bond
Distance ˚ (A)
Bond
Distance ˚ (A)
PL–O(a) PL–O(b) PL–O(c) PL–O(d) PL–O(e) PL–O(f)
2.627(9) 2.605(9) 2.627(9) 2.605(9) 2.605(8) 2.627(8)
PL–O(g) PL–O(h) W–O W–O(i) W–O(j) W–O(k)
2.605(8) 2.627(8) 1.793(8) 1.793(8) 1.793(9) 1.793(9)
Table 3 Selected bond angles for PbWO 4 :La 31
247
with both coordination polyhedra in S4 symmetry. As a matter of fact, the PLO 8 polyhedra can be divided into two tetrahedra, of which the one with four longer bonds slightly extends along the c direction and the other with shorter bonds is slightly squeezed in the same direction. Due to some of the Pb 21 ions being replaced by the smaller radius La 31 ions, the average PL–O bond length ˚ is smaller than that of undoped PWO (2.62 A), ˚ (2.616 A)
a
Angles
(deg)
Angles
(deg)
Angles
(deg)
O(a)–PL–O(b) O(a)–PL–O(c) O(a)–PL–O(d) O(a)–PL–O(e) O(a)–PL–O(f) O(a)–PL–O(g) O(a)–PL–O(h) O(b)–PL–O(c) O(b)–PL–O(d) O(b)–PL–O(e) O(b)–PL–O(f) O(b)–PL–O(g) O(b)–PL–O(h)
150.4(3) 135.8(3) 73.4(3) 67.8(3) 98.1(3) 78.1(3) 98.1(3) 73.4(3) 78.2(3) 127.0(3) 78.1(3) 127.0(3) 67.8(3)
O(c)–PL–O(d) O(c)–PL–O(e) O(c)–PL–O(f) O(c)–PL–O(g) O(c)–PL–O(h) O(d)–PL–O(e) O(d)–PL–O(f) O(d)–PL–O(g) O(d)–PL–O(h) O(e)–PL–O(f) O(e)–PL–O(g) O(e)–PL–O(h) O(f)–PL–O(g)
150.4(3) 78.1(3) 98.1(3) 67.8(3) 98.1(3) 127.0(3) 67.8(3) 127.0(3) 78.1(3) 150.4(3) 78.2(3) 73.4(3) 73.4(3)
O(f)–PL–O(h) O(g)–PL–O(h) O–W–O(i) O–W–O(j) O–W–O(k) O(i)–W–O(j) O(i)–W–O(k) O(j)–W–O(k) PL(l)–O–PL(m) PL(l)–O–W PL(m)–O–W
135.8(3) 150.4(3) 113.1(4) 107.7(4) 107.7(4) 107.7(4) 107.7(4) 113.1(4) 101.9(3) 120.5(4) 134.2(4)
a
Symmetry code: a (1 / 2 2 x, 2 y, 1 / 2 1 z); b ( 2 1 / 4 2 y, 1 / 4 1 x, 1 / 4 1 z); c ( 2 1 / 2 1 x, 1 / 2 1 y, 1 / 2 1 z); d (1 / 4 1 y, 1 / 4 2 x, 1 / 4 1 z); e ( 2 x, 2 y, 1 2 z); f (1 / 4 1 y, 3 / 4 2 x, 3 / 4 2 z); g (x, 1 / 2 1 y, 1 2 z); h ( 2 1 / 4 2 y, 2 1 / 4 2 x, 3 / 4 2 z); i ( 2 x, 1 / 2 2 y, z); j (1 / 4 2 y, 1 / 4 1 x, 1 / 4 2 z); k ( 2 1 / 4 1 y, 1 / 4 2 x, 1 / 4 2 z); l (1 / 2 2 x, 2 y, 2 1 / 2 1 z); m ( 2 1 / 4 1 y, 2 1 / 4 2 x, 2 1 / 4 1 z).
Fig. 2. Atomic environment of W atoms. Bold lines represent the WO 4 tetrahedron in PWO:La 31 , and double lines represent the WO 4 tetrahedron in undoped PbWO 4 . Arrows indicate the sense of rotation from the positions in PbWO 4 [5]. Note that the W atom is set as the origin.
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Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248
which was calculated according to the data given in Ref. [5]. Although the coordination environments of PL and W do not vary compared with those of undoped PWO, the decrease of the PL–O bond length induces the PbO 8 and WO 4 polyhedra to rotate anticlockwise by about 57.88 along their S4 axis. To clarify the significant variations, the environment of W is shown in Fig. 2. The arrows indicate the sense of rotation in the scheelite PWO. The introduction of La 31 ions requires structural distortions. In ferroelectric ceramic materials, structural distortions always occur in the following two ways. First, cations shift some distance along a special direction such as Ba 21 ions in BaTiO 3 [11]. Second, the cation polyhedra rotate along their symmetric axis, which always take place, for example, in Bi 2 WO 6 [12] and Bi 3 TiNbO 9 compounds [13]. Because it requires a lower potential energy, the second way is easier than the first. This can explain the above situation in the PWO:La 31 crystal.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No. 59732040).
References [1] R.Y. Zhu, D.A. Ma, H.B. Newman, C.L. Woody, J.A. Kierstead, S.P. Stoll, P.W. Levy, Nucl. Instr. Meth. A 376 (1996) 319.
[2] A. Annenkov, E. Auffray, M.V. Korzhik, P. Lecoq, J.-P. Peigneux, Phys. Status Solidi (a) 170 (1998) 47. [3] M.V. Korzhik, in: Proceedings of an International Conference on Inorganic Scintillators and Their Applications, SCINT95, Delft University Press, Delft, 1996, p. 241. [4] B. Han, X. Feng, G. Hu, Y. Zhang, Z. Yin, J. Appl. Phys. 86 (1999) 3571. [5] J.M. Moreau, Ph. Galez, J.P. Peigneux, M.V. Korzhik, J. Alloys Comp. 46 (1996) 238. [6] J.M. Moreau, R.E. Gladyshevskii, Ph. Galez, J.P. Peigneux, M.V. Korzhik, J. Alloys Comp. 284 (1999) 104. [7] A. Cousson, W. Paulus, R. Chipaux, CMS Conference Report, 1999 / 022, August 16, 1999. [8] M. Kobayashi, Y. Usuki, M. Ishii, N. Senguttuvan, K. Tanji, M. Chiba, K. Hara, H. Takano, M. Nikl, P. Bohacek, S. Baccaro, A. Cecilia, M. Diemoz, Nucl. Instr. Meth. A 434 (1999) 412. [9] B. Han, X. Feng, G. Hu, P. Wang, Z. Yin, J. Appl. Phys. 84 (1998) 2831. [10] C.K. Fair, MolEN. An Interactive Intelligent System for Crystal Structure Analysis, Enraf-Nonius, Delft, 1990. [11] K. Itoh, L.Z. Zeng, E. Nakamura, N. Mishima, Ferroelectrics 63 (1985) 29. [12] J. Harada, T. Pedersen, Z. Barnea, Acta. Crystallogr. A 26 (1970) 336. [13] R.E. Newnham, R.W. Wolfe, J.F. Dorrian, Mater. Res. Bull. 6 (1971) 1029.
L
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The structure of a PWO:La 31 crystal a, a b Qisheng Lin *, Xiqi Feng , Jiutong Chen a
Laboratory of Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 25 January 2000; accepted 25 February 2000
Abstract A lead tungstate crystal, with 1% mol La 31 doping, was grown by the Czochralski method and its structure determined by ˚ c 5 12.038(1) single-crystal X-ray diffraction techniques. It retains the scheelite-type structure with cell constants a 5 b 5 5.4587(6) A, ˚A, V 5 358.7(1) A ˚ 3 , Z 5 4. The trivalent La 31 ions are located at the 4(b) positions with occupancy 0.01 and share this site, statistically, with Pb 21 ions. The structural discrepancy between the La 31 doped and the undoped PWO comes from the positions of the oxygen atoms, which results that the PLO 8 polyhedron and WO 4 tetrahedron rotate anticlockwise by about 57.88 along their S4 axis compared with those of the undoped PWO crystal. The reasons for this are discussed. 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical materials; X-ray diffraction; Crystal structure
1. Introduction To improve the optical transmittance, decay time and radiation hardness of PbWO 4 (PWO) scintillating crystals, much research, including doping and annealing treatments, has been carried out [1–4]. Several studies on the structural characterization of PWO by X-ray and neutron diffraction techniques have been performed to illustrate the presence of defects in PWO crystals [5–7]. As a matter of fact, the most intensively studied dopants are mainly trivalent ions, because they are considered as being able to replace the Pb 21 ions and to compensate the Pb 21 deficiency taking account of their radius and electronegativity. For example, Kobayashi et al. reported that the scintillating properties could be significantly improved by doping with trivalent ions, such as La 31 , Y 31 , Gd 31 and Lu 31 . They considered that these ions could compensate the Pb 21 deficiency, thus reduce the densities of the Pb 31 / O 2 related defects [8]. Han et al. initially measured the dielectric relaxation spectra of La:PWO and concluded that the observed polarization was most probably due to ? the creation of [2(La 31 Pb ) –V 99 Pb ] dipole complexes [9]. However, no results on the structure or microstructure of *Corresponding author. E-mail address: [email protected] (Q. Lin)
the doped PWO have been reported. As part of our research, we have tried to study the relationship between the structure and properties of PWO:La 31 crystals. Here, we report the single-crystal X-ray results of 1% mol La 31 doped PWO.
2. Experimental The investigated crystal was grown from an initial mixture of stoichiometric PbO and WO 3 (.99.999%) with 1% mol La 31 ion doped in a platinum crucible using the Czochralski method in air. For X-ray analysis, a single crystal with approximate dimensions 0.2 mm30.4 mm30.2 mm was physically separated and selected for X-ray diffraction analyses at room temperature. Photographs show that the crystal retains the same tetragonal lattice as that of undoped PWO.
3. Single-crystal X-ray structure analysis A total of 481 reflections were collected at 298 K on an AFC7R Rigaku diffractometer with graphite monochro˚ using the v / 2u mated Mo Ka radiation ( l 5 0.71069 A)
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00823-9
246
Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248
scan mode in the range 08 , 2u , 69.88, of which 415 were unique and 132 with I $ 3s (I) were used for structure determination. The cell parameters determined from 25 reflections in the range 9.88 , u , 18.38 are a 5 ˚ c 5 12.038(1) A, ˚ V 5 358.7(1) A ˚ 3 , Z 5 4. 5.4587(6) A, From the systematic absence of hkl, h 1 k 1 l 5 2n; hk0, h 5 2n; 00l, l 5 4n and from the subsequent least-squares refinement, the space group was determined to be I41 /a (No. 88). Data reduction and corrections for empirical absorption based on c-scan techniques were applied. Lorentz and polarization corrections were also applied to the data. After structure refinement with isotropic thermal parameters, an empirical absorption correction was made using the program DIFABS. A secondary extinction correction was applied. Intensities of equivalent reflections were averaged with an agreement factor R int 5 0.035. The PL (0.99Pb10.01La) and W atoms were determined directly from an E-map and the O atom by difference Fourier syntheses. The structure was refined by a full-matrix least-squares method for 10 variables affording the final R 5 0.0288, R w 5 0.0356, w 5 1 / [s 2 (F ) 1 0.020F 1 1.0], S 5 1.33, D /s 5 0.00. The residual peaks (Dr ) max and (Dr ) min in the final difference Fourier map are ˚ 3 , respectively. The scattering factors 2.53 and 22.53 e / A 21 31 for the Pb , La , W 61 and O 2 ions were taken from International Tables for X-ray Crystallography (1974, Vol.
Table 1 Coordinates and thermal parameters for PbWO 4 :La 31 Atom
x
y
z
˚ 2) Beq (A
PL a W O
0.000 0.000 0.142(2)
0.250 0.250 0.015(1)
0.625 0.125 0.2071(7)
0.0082(1) 0.0047(1) 0.005(7)b
a PL indicates the 4(b) site occupied by Pb 21 and La 31 in the ratio 0.99:0.01. b Oxygen atom was refined isotropically.
IV, Table 2.2B). All calculations were performed by the MOLEN program package [10].
4. Discussion A drawing of the unit cell projected along the c direction is shown in Fig. 1. It is characterized by identical columns with PL and W atoms regularly stacked along the c direction and with the surrounding oxygen atoms forming a screw. The atomic coordinates and the thermal parameters are listed in Table 1, in which PL represents a statistical distribution of ions with 0.99Pb 21 10.01La 31 . The selected bond distances and bond angles are given in Tables 2 and 3, respectively. The PL atoms are eightcoordinated by O atoms and the W atoms four-coordinated,
Fig. 1. Arrangement of PL atoms and WO 4 tetrahedron in PWO:La 31 in the projection along the c-axis. Note the positions of oxygen atoms surrounding the columns made up of PL and W atoms alternating along the c direction.
Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248 Table 2 Selected bond distances for PbWO 4 :La 31 Bond
Distance ˚ (A)
Bond
Distance ˚ (A)
PL–O(a) PL–O(b) PL–O(c) PL–O(d) PL–O(e) PL–O(f)
2.627(9) 2.605(9) 2.627(9) 2.605(9) 2.605(8) 2.627(8)
PL–O(g) PL–O(h) W–O W–O(i) W–O(j) W–O(k)
2.605(8) 2.627(8) 1.793(8) 1.793(8) 1.793(9) 1.793(9)
Table 3 Selected bond angles for PbWO 4 :La 31
247
with both coordination polyhedra in S4 symmetry. As a matter of fact, the PLO 8 polyhedra can be divided into two tetrahedra, of which the one with four longer bonds slightly extends along the c direction and the other with shorter bonds is slightly squeezed in the same direction. Due to some of the Pb 21 ions being replaced by the smaller radius La 31 ions, the average PL–O bond length ˚ is smaller than that of undoped PWO (2.62 A), ˚ (2.616 A)
a
Angles
(deg)
Angles
(deg)
Angles
(deg)
O(a)–PL–O(b) O(a)–PL–O(c) O(a)–PL–O(d) O(a)–PL–O(e) O(a)–PL–O(f) O(a)–PL–O(g) O(a)–PL–O(h) O(b)–PL–O(c) O(b)–PL–O(d) O(b)–PL–O(e) O(b)–PL–O(f) O(b)–PL–O(g) O(b)–PL–O(h)
150.4(3) 135.8(3) 73.4(3) 67.8(3) 98.1(3) 78.1(3) 98.1(3) 73.4(3) 78.2(3) 127.0(3) 78.1(3) 127.0(3) 67.8(3)
O(c)–PL–O(d) O(c)–PL–O(e) O(c)–PL–O(f) O(c)–PL–O(g) O(c)–PL–O(h) O(d)–PL–O(e) O(d)–PL–O(f) O(d)–PL–O(g) O(d)–PL–O(h) O(e)–PL–O(f) O(e)–PL–O(g) O(e)–PL–O(h) O(f)–PL–O(g)
150.4(3) 78.1(3) 98.1(3) 67.8(3) 98.1(3) 127.0(3) 67.8(3) 127.0(3) 78.1(3) 150.4(3) 78.2(3) 73.4(3) 73.4(3)
O(f)–PL–O(h) O(g)–PL–O(h) O–W–O(i) O–W–O(j) O–W–O(k) O(i)–W–O(j) O(i)–W–O(k) O(j)–W–O(k) PL(l)–O–PL(m) PL(l)–O–W PL(m)–O–W
135.8(3) 150.4(3) 113.1(4) 107.7(4) 107.7(4) 107.7(4) 107.7(4) 113.1(4) 101.9(3) 120.5(4) 134.2(4)
a
Symmetry code: a (1 / 2 2 x, 2 y, 1 / 2 1 z); b ( 2 1 / 4 2 y, 1 / 4 1 x, 1 / 4 1 z); c ( 2 1 / 2 1 x, 1 / 2 1 y, 1 / 2 1 z); d (1 / 4 1 y, 1 / 4 2 x, 1 / 4 1 z); e ( 2 x, 2 y, 1 2 z); f (1 / 4 1 y, 3 / 4 2 x, 3 / 4 2 z); g (x, 1 / 2 1 y, 1 2 z); h ( 2 1 / 4 2 y, 2 1 / 4 2 x, 3 / 4 2 z); i ( 2 x, 1 / 2 2 y, z); j (1 / 4 2 y, 1 / 4 1 x, 1 / 4 2 z); k ( 2 1 / 4 1 y, 1 / 4 2 x, 1 / 4 2 z); l (1 / 2 2 x, 2 y, 2 1 / 2 1 z); m ( 2 1 / 4 1 y, 2 1 / 4 2 x, 2 1 / 4 1 z).
Fig. 2. Atomic environment of W atoms. Bold lines represent the WO 4 tetrahedron in PWO:La 31 , and double lines represent the WO 4 tetrahedron in undoped PbWO 4 . Arrows indicate the sense of rotation from the positions in PbWO 4 [5]. Note that the W atom is set as the origin.
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Q. Lin et al. / Journal of Alloys and Compounds 307 (2000) 245 – 248
which was calculated according to the data given in Ref. [5]. Although the coordination environments of PL and W do not vary compared with those of undoped PWO, the decrease of the PL–O bond length induces the PbO 8 and WO 4 polyhedra to rotate anticlockwise by about 57.88 along their S4 axis. To clarify the significant variations, the environment of W is shown in Fig. 2. The arrows indicate the sense of rotation in the scheelite PWO. The introduction of La 31 ions requires structural distortions. In ferroelectric ceramic materials, structural distortions always occur in the following two ways. First, cations shift some distance along a special direction such as Ba 21 ions in BaTiO 3 [11]. Second, the cation polyhedra rotate along their symmetric axis, which always take place, for example, in Bi 2 WO 6 [12] and Bi 3 TiNbO 9 compounds [13]. Because it requires a lower potential energy, the second way is easier than the first. This can explain the above situation in the PWO:La 31 crystal.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No. 59732040).
References [1] R.Y. Zhu, D.A. Ma, H.B. Newman, C.L. Woody, J.A. Kierstead, S.P. Stoll, P.W. Levy, Nucl. Instr. Meth. A 376 (1996) 319.
[2] A. Annenkov, E. Auffray, M.V. Korzhik, P. Lecoq, J.-P. Peigneux, Phys. Status Solidi (a) 170 (1998) 47. [3] M.V. Korzhik, in: Proceedings of an International Conference on Inorganic Scintillators and Their Applications, SCINT95, Delft University Press, Delft, 1996, p. 241. [4] B. Han, X. Feng, G. Hu, Y. Zhang, Z. Yin, J. Appl. Phys. 86 (1999) 3571. [5] J.M. Moreau, Ph. Galez, J.P. Peigneux, M.V. Korzhik, J. Alloys Comp. 46 (1996) 238. [6] J.M. Moreau, R.E. Gladyshevskii, Ph. Galez, J.P. Peigneux, M.V. Korzhik, J. Alloys Comp. 284 (1999) 104. [7] A. Cousson, W. Paulus, R. Chipaux, CMS Conference Report, 1999 / 022, August 16, 1999. [8] M. Kobayashi, Y. Usuki, M. Ishii, N. Senguttuvan, K. Tanji, M. Chiba, K. Hara, H. Takano, M. Nikl, P. Bohacek, S. Baccaro, A. Cecilia, M. Diemoz, Nucl. Instr. Meth. A 434 (1999) 412. [9] B. Han, X. Feng, G. Hu, P. Wang, Z. Yin, J. Appl. Phys. 84 (1998) 2831. [10] C.K. Fair, MolEN. An Interactive Intelligent System for Crystal Structure Analysis, Enraf-Nonius, Delft, 1990. [11] K. Itoh, L.Z. Zeng, E. Nakamura, N. Mishima, Ferroelectrics 63 (1985) 29. [12] J. Harada, T. Pedersen, Z. Barnea, Acta. Crystallogr. A 26 (1970) 336. [13] R.E. Newnham, R.W. Wolfe, J.F. Dorrian, Mater. Res. Bull. 6 (1971) 1029.