Crystal chemistry, thermal expansion and dielectric properties of (Bi1.5Zn0.5)(Sb1.5Zn0.5)O7 pyrochlore

Materials Research Bulletin, Vol. 32, No. 2, pp. 175-189, 1997 Copyright 0 1997 E1setie.r Science Ltd Printed in the USA. All rights reserved 0025-5408/97 $17.00 +,OO

Pergamon

PI1 SOO25-5408(96)00186-9

CRYSTAL CHEMISTRY, THERMAL EXPANSION AND DIELECTRIC PROPERTIES OF (Bil.sZn&(Sbl.sZno.s)O, PYROCHLORE

A. Mergen and W.E. Lee

University of Sheffteld, Department of Engineering Materials, Sheffield, S 1 3JD, UK (Refereed) (Received August 26, 1996; Accepted October 10, 1996)

ABSTRACT

Cd and Mg replacement for Zn confirms that the Zn cation distributes between A and B sites in the BZS pyrochlore crystal structure. Thermal expansion of BZS pyrochlore was determined to be 7.92 x IO6 K-’ between 20650°C while its dielectric properties showed a weak temperature dependence with dielectric constant of around 32 and dielectric loss of 0.005 at room temperature. copytight 0 1997 Ek~er Science Ltd KEYWORDS: A. ceramics. C. X-ray diffraction D. dielectric properties, D. thermal expansion. INTRODUCTION

Pyrochlore compounds have a wide range of possible applications. Pyrochlores can be used in solid state devices such as high permittivity ceramics (CdzNbzO,, Ln2Ti20r), thermistors (BizRuz07 pyrochlore suitably modified by solid solution with CdzNbzOT, BhCrNb07, and B&CrTaO,), thick film resistors and materials for screen printing (many Pb and Bi containing precious metal pyrochlores) and switching elements (CdzOszO7, CazOsz07, and TlzRuz07) (1). In addition, some nonferroelectric pyrochlores may serve as technologically useful dielectrics in applications such as temperature-stable and temperature-compensating dielectrics or microwave dielectrics, e.g., Pb(Cd)BiMwSb07 (where M”’ = Ti, Zr, Sn) (2) pyrochlore compounds in the BizOs-ZnO-Nb205 system (3,4) and lead-based nonferroelectric pyrochlores (PbI.e,Mg0.29’Nb,.7,06.39 and PbzFeWOa,s). The name pyrochlore originally referred to the mineral (Ca, Na, U)&Nb, Ta)zOG(OH, F) (JCPDS Card No 13-254) which can be generalized as A2B2X7 or AzBzXbZ. The simple pyrochlore structure is face centered cubic with space group Fd3m (No. 227). There are eight molecules per unit cell (Z = 8) and for a stoichiometric pyrochlore structure (A&X&Z) 175

176

A. MERGENet al.

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there are 88 atoms in a unit cell: 16 A cations in position (c), 16 B cations in position (d), 8 Z anions in position (a), and the remaining 48 X anions in position (f). The coordinations of ions in a normal pyrochlore are AXsZz, BX6, XA&, and ZA4 and LIB4 (0 is a vacancy). The larger A cations are eight coordinated and located within scalenohedra (distorted cubes) that contain six equally spaced anions (the X anions) and two additional axial anions (the Z anions) at a slightly shorter distance from the central cations. The smaller B cations are six coordinated and located within trigonal antiprisms (distorted octahedra) with all six anions equidistant from the central cations. The atomic arrangement in the pyrochlore structure is completely specified except for the x coordinate of the 48f positions, which is found to range from 0.375 to 0.4375 (with the A ion chosen as origin) depending on the ionic radii, since 48f anions are somewhat shifted from the center X6Z stoichiometry. By removing combinations of A and Z ions, a variety of defect structures can be produced with the general formula AIdBZX&+~ (such as A~BzX~,ABzX6). The rules regarding the allowed fundamental reflexions in the pyrochlore Fd3m space group are available in the International Tables for X-ray Crystallography. However, superlattice or ordering reflections (when supposing the pyrochlore as a fluorite derivative) are (11 I), (331), and (531) and arise from both cations and anions. The presence of these reflections with h, k, 1 all odd is indicative of A, B cations and X, Z anions ordering on the 16c, 16d, 48f, and 8a sites respectively in space group Fd3m. In addition, vacant sites will be ordered on 8b sites. However, when pyrochlores have anion or cation defect structures the ordering on their individual sites will change although the general ordering (16c, 16d, 48f, 8a) is the same. When the cations in the pyrochlore structure disorder (in this case the pyrochlore crystal structure approaches the defect fluorite structure (5)), these ordering reflections will diminish in intensity (6). Since the ideal formula of pyrochlore type compounds is A2BzX7, it is interesting to predict the cation distribution in the Bil.SZnSbl.S07 (BZS) pyrochlore. If the composition of the BZS pyrochlore corresponds to the initial mixture (i.e., if there is no material lost by volatilization, no unreacted material and no remaining amorphous phases) and taking into account the ionic radii of the cations (from Shannon (7)) the most probable formula of the BZS pyrochlore is (Bil,sZnos)(Sbl.sZno.s)Or. To determine whether Zn goes to the A and/or B sites in the pyrochlore structure various cations (Cd, Sr, and Ca known to fit only on the A site and Mg only the B site) were doped into the BZS pyrochlore structure to replace the Zn content. The phase purity after doping and lattice distortion due to the cation incorporation were investigated by XRD. In addition, the thermal expansion and dielectric properties of BZS pyrochlore were also investigated as a function of temperature. EXPERIMENTAL Powder Processing. (Bi& x = o.~,o.sZno.s-x)(SbI.5Zno.~)07 and (Bii.5Zno.s)(Sbi.5BY= o._,o.~ Zno.+)07 hypothetical pyrochlores (A = Cd, Sr, and Ca; B = Mg) were produced from mixed oxides of B&O3 (99.9%), ZnO (99.9%), SbzOx (99%), Cd0 (99.5%), CaO (99.9%), MgO (99%), and SrCO3 (99%). All chemicals were supplied from Aldrich Chemical Company Ltd., The Old Brickyard, New Road, Gillingham, Dorset, SP8 4JL, England. Powders were wet mixed in ethanol for 4 h using zirconia balls and dried at lOO-130°C for 24 h. Dried powders were calcined at 700°C for 4 h in a closed alumina crucible and ground in a mechanical agate mortar and pestle. The resulting powders were uniaxially pressed into

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pellets at approximately 150 MPa and sintered in a closed alumina crucible to prevent evaporation of volatile oxides for 4 h between 1100 and 1195°C with heating and cooling rates of 2-5 K/mm For dielectric measurements, BZS samples were produced using mixed oxides and coprecipitation of metal salts produced as described in ref. 8. X-ray Diffraction (XRD). XRD with a Philips difhactometer using Cu Ka radiation from 10-85” 28 at a speed of lo/mitt was used to reveal the phase purity and crystal structure of doped samples and to check the lattice distortion caused by cation substitution. Observed and calculated (using the program Crystallographica) interplanar spacings and intensities of the pure (Bil.sZno.s)(Sbl.sZno.~)07compound are given in Table 1. The difference in the intensities for some of the strong reflexions is likely due to a combination of texture effects and because peak heights and not integrated intensities were used (8). Lattice parameters of the substituted pyrochlores were determined between 95” and 130” using the d-spacings of approximately 1.199, 1.169, 1.068, and 1.007 A, which correspond, respectively, to the (l&l) values of (662), (840), (844), and (10,22). Slow scan rates of 0.25’/min and an internal gold standard was used and lattice parameters calculated using a least-squares method. It should be noted that during this study although XRD was used for checking for impurity phases in the pyrochlore it is limited to detecting l-5 ~01% of second phases (9,lO). Thermal Expansion Measurement. The linear thermal expansion coefficients (01) were determined for BZS pyrochlore using 2.6 cm long x 5 mm dia rod specimens which were TABLE 1 Observed and Calculated Powder XRD of Pure (Bil.sZno.s)(Sbl,sZnO.~)O, Pyrochlore bkl

J-WA) 111 220 311 222 400 331 422 511 440 531 620 533 622 444 71 l/551 73 l/553 800 733 822 7511555 662 840

Calculated

Observed I&

6.042 3.699 3.154 3.02 2.616 2.401 2.134 2.015 1.851 1.769 1.655

5 1 2 100 24 4 1 2 26 1 1

1.578 1.511 1.466 1.363 1.309

22 8 1 1 2

1.234

1

1.201 1.175

6 0

d(A) 6.0287 3.6918 3.1484 3.0143 2.6105 2.3956 2.1315 2.0096 1.8459 1.765 1.651 1.5924 1.5742 1.5072 1.4622 1.3594 1.3053 1.2757 1.2306 1.2057 1.1978 1.1675

Ifio = ii.3 2 100 30 4 0.2 2 50 2 0 0.3 41 9 1 0.6 7 0.7 0 0.4 16 11

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prepared by uniaxial pressing at 250 MPa. The samples were around 95% of theoretical density after firing 4 h at 1200°C. Parallel ends, which were at right angle to the axes of the rods, were obtained by grinding with 600 grade Sic powder and polishing with 6 pm diamond paste. The rod expansion was measured relative to that of silica. The temperature of the furnace was raised at 5 K/min and the thermal expansion coefficient is given by:

where L is original specimen length, AL is the increase in length interval, and AT is the temperature range of the experiment. Using this formula, the average coefficient a over the temperature range 20 to 650°C was calculated by a least-squares method. Dielectric Measurements. To perform dielectric measurements sintered samples (-1 mm thick and -10 mm in diameter) were ground parallel, electroded using silver paint (DuPont 7095) and fired at 550°C for 30 min. Relative permittivity and dissipation factors were measured using a HP4284A precision LCR meter at 100 kHz at temperatures of between 20-120°C at the Department Metallurgy and Materials Science, University of Manchester. The relative permittivities reported have not been corrected for porosity levels in the pellets which ranged from 90-98% theoretical density. RESULTS AND DISCUSSION Characterization of Doped-BZS Pyrochlores. Although there is a definite correlation between ionic radii of A and B cations and the stabilization of the pyrochlore structure, other factors such as electronegativity of cations, charge neutrality, and the thermodynamic stability of competitive phases are also important (11). However, in general the ionic sizes should be comparable to those of the A and B site cations and the combination must yield the same average charge as the A and B cations to maintain charge neutrality. In addition, as an indication of formation of the pyrochlore crystal structure the radius ratio of the cations (rA/rB)is also important (1). The substituted cations have suitable ionic radii for the pyrochlore structure according to the upper and lower radius limits given by Subramanian et al. (1). These values are given as 0.87 < rA < 1.17, 0.58 < rB < 0.775 a and 0.96 < rA < 1.29, 0.54 < rB < 0.76 = A, respectively, for A i + Bi + 07 and A i + Bz + 07 pyrochlores. Moreover, Subramanian et al. (1) gave the radius ratio limits of pyrochlores as 1.46 < rA/rB< 1.80 and 1.4 < r&B < 2.2, respectively, for 3+, 4+, and 2+, 5+ pyrochlores. The radius ratio of BZS pyrochlore is around 1.74 within the limits given by Subramanian et al. (1). While examples of Sr and Ca containing pyrochlores are rare, there are some pyrochlore compounds containing these ions such as SrLaSnNbOT, CaLaSnNbO, (12), and SrLiTazOeF (13). In addition, SrzOsz06.4, and Ca20s207 pyrochlores have been prepared (1). A-Site Substitution. XRD of Cd-doped BZS pyrochlore (Bil sZno.~_~Cd,Sb,.sZno.s07) sintered for 4 h at 115O’C showed that until x = 0.2 only pyrochlore peaks occur, but when x > 0.2, a trace of p-B&O, (JCPDS 29-236) is additionally found (Fig 1). However, these extra

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10

20

30

40

50

179

60

70

60

60

70

80

60

70

80

80

70

80

Ddgreel Z-Theta

10

20

30

40

50

Degrees Z-Theta

10

20

30

40

50

Degrees Z-Theta

10

20

20

40

50

Degrees Z-Theta

FIG. 1 XRD of cadmium-doped BZS pyrochlore, Bil.~Z~.~_,Cd,Sb~,~Z~.~O,, sintered for 4 h at 115OOCwith x = 0.1 (a), 0.2 (b), 0.3 (c), and 0.5 (d)* P-BhOj (*) (JCPDS 29-236) starts to form when x > 0.2. All peaks pyrochlore unless indicated otherwise.

A. MERGENet al.

180

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peaks were not seen when the same doped pyrochlore compositions were sintered at lower temperature (11 OO’C) (Fig. 2). Only single phase pyrochlore occurs after sintering at 1150°C for x = 0.1 and 0.2 and at 1100°C for all x values (0.1 I x I 0.5). Approximately 2.6% and 3.2% weight losses were observed for x = 0.4 and 0.5 after sintering at 115O”C, suggesting that Bi203 evaporation occurs at this temperature. However, these same pyrochlore compositions, x > 0.2, showed no weight loss when sintered at lower temperatures (1lOO“C) and no bismuth oxide was observed in the XRD. Nevertheless, when x = 0.5 line splitting was observed (Figs. Id and 2c) indicating a distortion from cubic symmetry. In a cubic crystal structure, any distortion of the unit cell which decreases its symmetry increases the number of lines in the pattern (9). XRD of Sr-doped BZS pyrochlore, (Bi I.sZno.s_xSr,Sbl.sZn~.~O,), suggesting that the solid solution member of x = 0.1 could be prepared as single phase material (Fig. 3a). However, when x > 0.1, a strontium antimony compound (SrSbzOb, JCPDS 1l-45) started to form and increased in amount with x when sintered at 1195°C (Figs. 3b and c). The formation of this compound was also detected for the same compositions sintered at lower temperatures. Figure 4 shows the relative peak intensities of the SrSbz06 reflection corresponding to 3.46 A d-spacing (crystal structure and hkl values are not known), and the BZS pyrochlore (222) reflection, which corresponds to the d-spacing of -3.001 A, as a function of Sr concentration, x. This is a semiquantitative measure of strontium antimonate to pyrochlore ratio and indicates that as the Sr incorporation increases the SrSbz06 percentage also

10

20

30

so

40 Lkgrcas

60

70

80

90

Z-Theta

FIG. 2 XRD of Cd-doped BZS pyrochlore sintered for 4 h at 1100°C with x = 0.1 (a), 0.3 (b), and 0.5 (c). All peaks correspond to pyrochlore.

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xi

Xl

40

50

60

181

70

80

93

Degrees 2-Theta

FIG. 3 XRD of strontium-doped BZS pyrochlore, Bi,.sZno.s_xSr,SblsZrb.507, sintered 4 h at 1195°C for x = 0.1 (a), 0.3 (b), and 0.5 (c). When x > 0.1, strontium antimony oxide compound, SrSbz06, (*) started to form. P = pyrochlore. 1.00

0.70 g f

0.50 -

z 2

0.50 -

5 6 I L

0.40 0.30 --

FIG. 4 Relative peak intensity ratio of SrSbzOb (d = 3.46 A) to BZS pyrochlore (d = 3.001 A) as a function of Sr concentration, x, in Bil,sZno.s-xSr,Sb,.sZno507.

182

A. MERGEN et al.

10

20

30

40

50

Dagrns

10

20

30

40

Vol. 32, No. 2

60

70

80

60

70

80

60

70

Bo

60

70

80

Z-Theta

so

Degrees Z-Theta

10

M

30

40

50

Degrees Z-Theta

10

20

30

40

50

Degrees I-Theta

FIG. 5 XRD of Ca-doped BZS pyrochlore, Bi, sZno,s-xCa,SbI,~Zno.s07, sintered 4 h at 1180°C for x = 0.1 (a), 0.2 (b), 0.3 (c), and 0.5 (d). (* = P-Bi203, BC= Bis.llCao.g90s.sa,and P = pyrochlore).

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TABLE 2 Lattice Constants of Cd, Sr, Ca, and Mg Doped BZS Pyrochlore as a Function of Composition Phase (Bh sZno.~xCd,)(Sb 1.5Znd%

(%.&no.~xSrx)W I.sZ~O.SP~ (Bidno dax)@b 1.~Zno.5)07

(Bil.sZmd(Sb

I.5Zno.~,Mgx)07

X

0.1 0.2

Lattice Constant (A)

0.3

10.4515(8) 10.4558(9) 10.4579(10)

0.4

10.4610(9)

0.5

10.4566(8)

0.1

10.4596(9)

0.1

10.4546(9)

0.2

10.4586(8)

0.3

10.4548(10)

0.1

10.4412(8)

0.5

10.4378flOj

increases. This reveals that increasing Sr addition increased the amount of SrSbz06 rather than causing substitutional solid solution into the pyrochlore when x > 0.1. XRD of Ca-doped BZS pyrochlore revealed only single phase pyrochlore when x = 0.1 (Fig. 5a). However, when x > 0.1, a trace of P-BizOJ was invariably found after 4 h at 118O’C as for Cd-doped BZS pyrochlore (Fig. 1). The amount of P-Bi203 increases with the concentration of Ca. In addition, when x = 0.5 a bismuth calcium compound, Bi311Caa.s90556 (JCPDS 40-317), forms as well as pyrochlore and P-Bi20, (Fig. 5d). Consequently, as observed for Sr doping, Ca incorporation did not result in single phase pyrochlore formation and thus cannot be used as evidence for Zn replacement on the A site of pyrochlore. B-Site and Mixed Site Substitutions. To determine the possible Zn cation distribution on the B site of the pyrochlore crystal structure Mg, which has a suitable radius for the B site, was also doped into the BZS pyrochlore (Bil sZno.~Sb1.5Zno.5_xMg,07, x = 0.1 and 0.5). Mgdoped BZS pyrochlore sintered at 1150°C was single phase for x values of 0.1 and 0.5, indicating that half of the Zn (Zno.s) can easily be accommodated on the B-site of pyrochlore since no impurity phase was observed in XRD after Mg replacement for Zn. In addition, XRD of samples doped with both Cd and Mg instead of Zn, (Bi&do,5)(Sbl,5Mg,,5)07, showed only single phase pyrochlore suggesting that Zn cation may go to both the A and B-sites in the BZS pyrochlore. Lattice Parameter in Doped Pyrochlores The lattice parameters of doped pyrochlores were found by XRD only for the samples that gave single phase pyrochlore since it is thought that impurities or minor second phases like BizOs, Bb.~,Cao.~~0~.5~ and SrSbzOh may change the pyrochlore stoichiometry and complicate the relation between pyrochlore lattice constant and concentration of cation dopant. Lattice parameters of the doped samples are given in Table 2. Figure 6 shows lattice constant, h, versus x for the Cd-doped BZS pyrochlores sintered for 4 h at 1100°C. The curve is linear from x = 0.1 to 0.4 indicating that a well-behaved solid solution exists within this range according to Vegard’s law which

184

A. MERGENet al.

10.482 -

Vol. 32, No. 2

10.4780

10.461 -

-m P

Cdca

vZn

Sr

10.460 10.450 -I z

10.458 -

i

10.457

‘d

5 ; 0

10.450

;10.4580 P

z

5

Et E

:: 10.455 3 z

-

3

10.454 -~

10.4400 -

10.453 10.452 10.4380 -1' 10.451 ~~ 0.0

0.1

0.2

0.3

0.4

0.5

0.70

0.80

0.90

1.00

1.10

1.20

1.30

Csllon Radius ("A)

X

FIG. 6 Lattice parameter of Cd-doped BZS pyrochlore, Bil.~Zno.s_xCd,Sb~.~Zn~.~O~, sintered at 1100°C for 4 h as a function of Cd incorporation, x.

FIG. 7 Lattice parameter of Cd, Sr, Ca and Mg doped BZS pyrochlore for x = 0.1 as a function of dopant cation radii.

states that the lattice parameter of the solid solution is directly proportional to the atomic percent solute present (9). As more Cd is incorporated into the BZS pyrochlore structure, the lattice parameter of pyrochlore also increases, since the Cd ion has a larger radius than Zn. Attempts to prepare solid solutions for x = 0.5, (Bi&do.s)(Sbl.sZnO,~)O,, produced line splitting in the XRD (Fig. 2), indicating that there was a distortion from cubic symmetry so that the lattice parameter of this composition was not compared to the other compositions (Table 2). In Table 2 the lattice parameters of &doped samples are given for x = 0.1, 0.2, and 0.3, although they are inconsistent. This is thought to be due to the B&O, formed along with pyrochlore which altered the pyrochlore stoichiometry. The lattice parameters of the Mg-doped BZS (Table 2) indicate that as Mg incorporation increases the lattice constant of pyrochlore decreases since Mg is slightly smaller than Zn. Figure 7 illustrates the straight-line relationship observed between the lattice constant of BZS pyrochlore with different cation dopings and the radii of the dopant cations. Only low dopant concentrations, x = 0.1, were compared since single phase pyrochlore was only obtained for all cations at this level. As can be seen from Figure 7, the lattice parameter of the pyrochlore increases with the cation radius. Doping into the A site of pyrochlore showed that the Zn cation may go to the A site in the BZS pyrochlore although the results for Sr and Ca doping were not definitive since second phase impurities started to form for x > 0.1. However, Cd doping showed that half of the Zn cations in the BZS pyrochlore may easily be accommodated on the A site of pyrochlore since incorporation of Cd between 0.1 < x I 0.5 gave only single phase and no impurity phases were detected by XRD in samples sintered at 1100°C (assuming that there is no unreacted material and no remaining amorphous material). In addition, in the x = 0.5 composition sintered at 1IOO‘C line splitting occurred (Fig. 2), but no second phases were

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0.004

0.003

L

f

#

O*Oo2 0.001

0.000

0

loo

200

500

400

100

000

7w

T@mpmturm*C

FIG. 8

Thermal expansion of BisnZnSb3nOTpyrochlore sintered 4 h at 1200%

found. This indicates that Cd incorporation at this level also led to only single phase pyrochlore but distorted the cubic lattice possibly due to its higher ionic radius. Matsuo (14) also produced a pyrochlore phase, (Pb,.sZn&Nb~Or, with Zn on the A site. Another conclusion which may be drawn from this study is that although Cd and Ca have similar ionic radii (1 .lO and 1.12 A, respectively) Ca doping does not result in single phase pyrochlore except with x = 0.1. Ion size is clearly not the only significant variable affecting the stability of the pyrochlore structure. Replacement of Zn with Mg indicated that the Zn cation can also go to the B-site in the BZS pyrochlore structure. In addition, mixed cation doping (Cd and Mg simultaneously) replacing all the Zn in the BZS confirmed that the Zn cation can go to the A and B sites in the BZS pyrochlore. Jeanne et al. (15) determined that the Zn cation distribution between the A and B sites. They performed structure factor calculations for different possible distributions of cations and found the lowest R factor (R = C 1 I, - IJ / C I,, where I, is observed X-ray intensity and I, is calculated X-ray intensity) for (Bi~.sZno.~)(Tal.sZno.s)O,, where Zn is distributed between A and B sites. The present doping study also showed that the Zn cation is distributed between A and B sites in the BZS pyrochlore structure giving the formula of (Bil,sZno,s)(Sb,.sZ~.5)07. The thermal expansion coefficient of Thermal Expansion of BZS Pyrochlore. Bi,,zZnSb,/zOr pyrochlore polycrystalline ceramic sintered 4 h at 1200°C was determined to be 7.92 x lo6 K-’ between room temperature and 650°C. Two different samples of the same size were used for measurements and the difference between them was within the error limit of k0.64 x lo+. Figure 8 indicates that the expansion rate is a linear function of temperature in the range 20-65O”C.

A. MERGEN et al.

186

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TABLE 3 Thermal Expansion Coefficients of RZTi207and R&Ifz07 Pyrochlores (R = Rare-Earth Elements) at Temperatures over the Range 20-800°C and 2&9OO”C, Respectively R,Hf*O, (17) R La Pr Nd Sm ELI Gd

a x 106, K-’ (20-900 “0 7.85 9.13 9.27 10.60 10.82

a x IO”, K-’ (20-800 “C) 9.46

; HO

Er Tm Yb Lu Y

9.80 10.16 10.70 10.29 10.35 10.31 10.25 10.66 10.41 10.9 9.87 10.46

8.50 9.75 9.75 9.65 9.62 10.40 11.80 8.72

32.5

=" $ E

r t

32.4 32.3

s

0.005 -

2=

0.004 -:;:-

I-

p' 32.2

0.006 -

.I

P 5

32.1

0.003-

32.0

0.002 -

oz

’.

0.001

0.000 20 30 40 50 SO 70 50 00 100110120130 Tsmprralurr/*C

1r,

I

(

I,

I

j

/,

,

. . .._.. ,

,

,

,

(

I,

I,

I

20 so 40 50 SO 70 80 80 100 110120130 Temperdur~/~C

FIG. 9 The temperature dependence of a)dielectric constant and b)dielectric loss of BZS pyrochlore produced from mixed oxide (SS) and coprecipitated (Pr) powders and sintered 4 h at 1200°C.

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BISMUTH ZINC ANTIMONATE

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TABLE 4

Dielectric Constant (K) and Dielectric Loss (tan@ Values of Doped Pyrochlores at Low Concentrations of Cd, Ca and Sr Composition (Bi~,sZno.&ddSb 1.5Zw)% (Bi~.sZn&b)W 1.sZno.5)07 (Bi~.~ZwCadSb I.sZ~O.S)O 7 (Bi I.&&dW I.Jno.dO 7

Sintering Temperature (“C)

K

tan6

1150 1150 1180 1195

33.4 34.8 31.9 33.9

0.002 0.0025 0.0046 0.001

For comparison the linear thermal expansion values of other pyrochlore compounds are given in Table 3. In addition, Bayer (18) found the thermal expansion coefficients of YzTizOr and La&iz07 pyrochlores as 10.9 x 10” and 8.0 x 10” R’ in the temperature range 20-520°C and 11.O x 10” and 8.3 x lo6 K-’ in the temperature range 20-1020°C. Martin (19) investigated the thermal expansion coefficient of the defect pyrochlore PbzTizOc at temperatures of between 25 and 33O’C and found it to be 9.6 x IO” K-l. Dielectric Properties of BZS Pyrochlore. Figures 9a and 9b show the relative permittivity and dielectric loss of BZS produced from both coprecipitated and mixed oxide powders (8) sintered 4 h at 12OO”C,as a function of temperature at 100 kHz. Pellet densities were more than 90% of theoretical (8). Figure 9a revealed that there is only a slight difference between the dielectric constants of pellets produced from both methods, 32.5 for coprecipitation and 32.4 for mixed oxide at room temperature. As the temperature was raised to 12O”C, the magnitude of the dielectric constants of both pellets decreased slightly, from 32.5 to 32.2 for coprecipitated powder pellets and from 32.4 to 3 1.9 for mixed oxide pellets. The dielectric constants of both pellets showed a rather weak temperature dependence. In addition, the dissipation factor (loss tangent, tar&) (Fig 9b) behaves in the same manner as dielectric constant when measured against temperature and the room temperature dielectric loss was around 0.005 for both pellets. The dielectric properties of doped single-phase pyrochlores were also measured at room temperature and are given in Table 4 which shows that Cd incorporation replacing Zn led to higher dielectric constant and lower dielectric loss values. In addition, the sintering temperature of Cd-doped pyrochlore was also lower than BZS pyrochlore. The dielectric constant of BZS pyrochlore observed in this study was typical of other pyrochlores. When CdzNbzO-Ipyrochlore was found to be ferroelectric (20), a great deal of interest was generated in the pyrochlores. However, subsequent investigations of other pyrochlore systems did not reveal any ferroelectric behavior in these structures and most pyrochlore compounds have a small dielectric constant. In Table 5 dielectric properties of some of the pyrochlore compounds are given for comparison. The dielectric constants of pyrochlores range from 20 to 170 and are generally associated with a high loss value. Nevertheless, if dielectric loss values of pyrochlores can be decreased to low levels, they may serve as technologically useful dielectrics in some applications.

188

A. MERGEN et al.

Vol. 32, No. 2

TABLE 5 Room Temperature Dielectric Constant (K) and Dielectric Loss (tan@ of Some Compounds with Pyrochlore Structure K

Comuosition Pb2FeW06.5 Pb(Cd)BiM’“SbOr

-65 (Ml”= Ti, Zr, Sn)

BirZnU3Nb4/304+3~r 1 < x 5 1.83 Bi,Znwj,Nb4,30wti, 1.33 I x < 1.83 BixZnr_2~Nb2_~307r

(Bit.SZno.s)(Zl~.sNb1.5)07

flo,sBit,~Pb207 CdLaTiNbOr

-0.008 10-30x -

21 10-3

~3)@6.8~0.2)

-170

-

-152 67

-

78

5 x 10”

22 23

32x 1O-3

20

12

PbBiTiTaO r

140 95

-

Pb(Zn,Nb I-&.s-I.~~

130


PbLaZrNbO 7

2 4

1 < x I 1.5

(Bir.5Nio.rZn&(Zno.sTio.zNb Bi2Pb20r

30-60 80-150

Reference

tan6

24

CONCLUSIONS 1.

2. 3. 4.

Doping either Cd or Mg and mixing cation doping (both Cd and Mg simultaneously) into BZS pyrochlore is consistent with the Zn being distributed between A and B sites giving the formula of (Bi1,5Zno.s)(Sbl,5Zno.~)07. Sr and Ca incorporation into BZS pyrochlore resulted in formation of other phases and therefore cannot be used as evidence for Zn replacement on the A site of BZS. Thermal expansion of BZS pyrochlore was determined to be 7.92 x 10” K-’ between 20-65O’C. BZS pyrochlore has a dielectric constant of -32 with dielectric loss of -0.005 at room temperature with a weak temperature dependence between 20-12O’C. ACKNOWLEDGMENTS

The authors are grateful to the Turkish Ministry of Education for a scholarship for A.M. and to Prof. J.H. Sharp, Department of Engineering Materials, University of Sheffield, for helpful discussions. REFERENCES 1. 2. 3. 4. 5.

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6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

BISMUTH ZINC ANTIMONATE

189

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