Cation disorder in the ferroelectric Aurivillius phase PbBi2Nb2O9: an anamolous dispersion X-ray diffraction study

Solid State Ionics 112 (1998) 281–289

Cation disorder in the ferroelectric Aurivillius phase PbBi 2 Nb 2 O 9 : an anamolous dispersion X-ray diffraction study a b a, Ismunandar , Brett A. Hunter , Brendan J. Kennedy * a

b

School of Chemistry, The University of Sydney, Sydney NSW 2006, Australia Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai NSW 2234, Australia Received 9 June 1998; accepted 18 July 1998

Abstract The effect of high temperature annealing on cation disorder in PbBi 2 Nb 2 O 9 has been studied using a combination of powder neutron and anomalous dispersion X-ray diffraction methods. High resolution diffraction data shows that the orthorhombic cell volume is greater in rapidly cooled samples compared to slowly cooled samples. Rietveld analysis of the diffraction data gave the following cell parameters, space group A 21 am, a 5 5.4909(1) and 5.4879(1), b 5 5.4998(1) and ˚ for quenched and slowly cooled samples respectively. Anomalous dispersion 5.4989(1), c 5 25.5313(2) and 25.5390(2) A X-ray diffraction methods have shown this is a result of disorder of the Pb 21 and Bi 31 cations. In the rapidly cooled sample a near statistical distribution of Pb 21 and Bi 31 in the 4a perovskite type sites was observed, the occupancies being 33 and 67% of Pb 21 and Bi 31 respectively. This occurs since both Pb 21 and Bi 31 have lone pair electrons and at high temperatures these cations have a similar preference for the 4a and 8b (Bi 2 O 2 ) type sites. In the slowly cooled sample the 4a sites are occupied by 53 Pb 21 and 47% Bi 31 suggesting Bi has a preference for the 8b sites.  1998 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectric; Metal oxide; Cation disorder powder diffraction; Crystal structure

1. Introduction In 1949, Aurivillius first reported the formation of a series of layered bismuth oxides of the general formula Bi 2 A m21 B m O 3m13 (m 5 1, 2, 3, 4). These ‘Aurivillius type oxides’ consist of a-PbO-type [Bi 2 O 2 ] 21 layers interwoven with (m 2 1) perovskite-type layers having the composition [A m21 Bm O 3m21 ] 22 [1]. Shortly thereafter, Smolenski and Subbarao [2–4] identified these materials as *Corresponding author. Tel.: 1 61-2-93512742; Fax: 1 61-293513329. E-mail: [email protected]

promising ferroelectrics, prompting numerous studies during the 1960’s and early 1970’s on the preparation and electronic properties of these types of oxides. A large number of these early studies were the subject of the review by Subbarao [5] in 1973. Aurivillius type oxides exhibit a great variability in the metal cation stoichiometry, thus presenting the potential for systematic control of their physical and electronic properties. The A-site cations include Ca, Sr, Ba, Pb, Bi, Na, rare-earth ions, or mixtures of these, while the octahedral B-site invariably contains small highly charged cations such as Ti 41 , Nb 51 , Ta 51 , W 61 or Mo 61 .

0167-2738 / 98 / $ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00222-7

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Despite the importance of these materials, relatively few detailed high resolution structural studies of Aurivillius type oxides have been described, an obvious exception being the recent work of Rae, Thompson and Withers on Bi 4 Ti 3 O 12 , Bi 3 TiNbO 9 and Bi 2 SrTa 2 O 9 [6–8]. These high resolution studies have shown that the earlier work of Aurivillius [1] did not resolve small structural distortions that now appear to be important in determining the ferroelectric properties of these types of oxides. Most of the m 5 2 oxides, with the general formula ABi 2 B 2 O 9 , show small distortions resulting in orthorhombic symmetry, although some adopt the archetypal tetragonal structure [7]. The orthorhombic structures are usually described by space group A2 1 am, with a and ˚ and c ¯ 25 A. ˚ There are four formula units b¯5 A per unit cell and the atoms are distributed in eight crystallographic positions. Atom A and one of the oxygen atom occupy the special 4a positions, whilst the remaining oxygen and B type atoms occupy the general 8b positions. In comparison with the ease of substitution into the perovskite layers it has long been thought that it is not possible to substitute other cations into the Bi 2 O 2 layers without destroying the structure [3,9]. The Bi 2 O 2 layers are comprised of a square planar net of oxygen anions with the Bi 31 cations alternatively above and below the plane and can be described as forming caps of the BiO 4 square pyramids. The asymmetrical coordination environment of the Bi cations is due to the stereochemical activity of the 6s 2 lone pair electrons. It is this distorted environment that is thought to limit cation substitution into the Bi 2 O 2 layers. Recently it has been established that other cations with sterochemically active lone pair electrons such as Sn 21 , Sb 31 , Pb 21 or Te 41 can be introduced, at least in part, into the Bi 2 O 2 layers [10–15]. PbBi 2 Nb 2 O 9 was one of the Aurivillius type oxides originally studied by Subbarao [3] who demonstrated that its Curie temperature was sensitive to the heat treatment of the sample. It was postulated that this behaviour was due to thermally induced disorder in the distribution of the Pb 21 and Bi 31 ions over the 4a and 8b sites [2,3]. The first direct experimental evidence for this disorder was provided by Srikanth, Subbarao and co-workers [16] using powder neutron diffraction methods. But as noted by these authors the small

difference in the neutron scattering lengths of Pb and Bi limits the precision of their determination [16]. Recent structural studies on ABi 2 Nb 2 O 9 , A 5 Sr, Ba gave contradictory results [17,18]. In accordance with the argument that the presence of lone pair electrons is essential for the substitution into the [Bi 2 O 2 ] 21 layers Ismunandar et al. found that there was no cation disorder in these oxides [17]. Conversely, Blake and co-workers reported a slight disorder in these systems [18], and they postulated that the differences observed between the two studies may have resulted from the differing annealing conditions used. Obviously conventional X-ray methods cannot be used to study disorder between isoelectronic ions such as Pb 21 and Bi 31 , however anomalous dispersion diffraction methods are capable of distinguishing between these two elements. In order to verify and quantify any cation disorder in PbBi 2 Nb 2 O 9 , we have carried out a detailed structural study of this material using a combination of powder neutron and anomalous dispersion synchrotron X-ray diffraction methods. This allows for an unambiguous determination of site disorder in this complex oxide.

2. Experimental A polycrystalline sample of PbBi 2 Nb 2 O 9 was prepared by the solid state reaction of stoichiometric quantities of PbO (Aldrich, 99.99%), Bi 2 O 3 (Aldrich, 99.995%), and Nb 2 O 5 (Aldrich, 99.999%). The intimately mixed materials were heated in air at 7008C for 12 h, 8008C for 24 h, 9008C for 24 h and finally at 10008C for 24 h. The formation of a single phase oxide was confirmed by powder XRD measurements at room temperature on a Siemens D5000 diffractometer. The sample was then pressed into 13 mm dia. pellets. These pellets were annealed at 11008C for 4 h, at which time half of the sample was removed from the furnace and quenched to room temperature, whilst the remaining portion of the sample was slowly cooled (5 K / min) to room temperature The powder neutron diffraction patterns were recorded in 0.058 steps in the range 08 , 2u , 1568 at room temperature using neutrons of wavelength ˚ on the high-resolution powder diffractome1.8849 A

Ismunandar et al. / Solid State Ionics 112 (1998) 281 – 289

ter (HRPD) [19] at the HIFAR reactor operated by the Australian Nuclear Science and Technology Organisation (ANSTO). The lightly ground sample was contained in a thin-walled 12 mm-diameter vanadium can that was slowly rotated during the measurements to minimise the effects of preferred orientation. The synchrotron diffraction patterns were recorded in 0.0058 steps at room temperature using X-rays of wavelength 0.70036, 0.92438 or ˚ on the high resolution powder diffrac0.95167 A tometer, X7A, at the National Synchrotron Light Source, Brookhaven National Laboratory [20]. For these measurements the diffractometer was operated in u 2 2u mode and the flat plate sample was rocked throughout the measurements. The Rietveld refinement [21] of the structure of PbBi 2 Nb 2 O 9 using neutron and X-ray data was undertaken with the PC version of the computer program LHPM [22]. For the neutron diffraction data the background was described by a fourth-order polynomial in 2u and was refined simultaneously with the peak profile parameters. A Voigt function was chosen to generate the line shape of the diffraction peaks. The Gaussian widths were given by the function: (FWHM)2 5 U tan 2u 1V tan u 1 W where U, V, and W are refinable parameters and the width of Lorentzian component was varied as h sec u to model particle size effect. For the X-ray diffraction data the same background function was employed and a pseudo-Voigt function was chosen to generate the peak profiles. The values of f ´and f 0, calculated using the program FPRIME [23], and of the neutron scattering lengths [24] used in the refinements are listed in Table 1.

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3. Structure refinement Examination of the powder X-ray diffraction patterns, collected using a laboratory diffractometer, confirmed that the samples were orthorhombic and all the peaks could be indexed in space group A2 1 am. The cell volumes for the sample quenched from 11008C and that slowly cooled to room temperature were significantly different, 771.00(2) and ˚ 3 respectively. These results are similar 770.70(2) A to those reported by Srikanth and co-workers who found quenching of the sample from higher temperatures resulted in larger cell volumes [16]. Neutron powder diffraction data were collected on both samples. The structures of the two samples were initially refined in space group A2 1 am using the model reported by Srikanth et al. [16]. Initially it was assumed that the cations were fully ordered, that is the Pb exclusively occupies the 4a site. The background, a scale factor, lattice parameters, profile parameters, atomic positions and anisotropic thermal displacement parameters, for all atoms but Nb, were refined. It was found that when anisotropic atomic displacement parameters of the Nb atoms were refined, unrealistic ellipsoids were obtained. Consequently isotropic thermal parameters were used for the Nb atoms. Once convergence had been reached the possibility of disorder of the bismuth and lead atoms was considered. The occupancies of the 4a and 8b sites were constrained so that both sites were fully occupied and the Pb:Bi stoichiometry was maintained. It was assumed that the Pb and Bi occupied identical positions in the lattice and that the temperature factors of the atoms in these sites were

Table 1 Values of f 9 and f 0 (eV) and of the neutron scattering lengths, b, used in the structural refinements

Pb Bi Nb O

f9 f0 f9 f0 f0 f0 f9 f0

0.70036

˚ l X-ray (A) 0.92438

0.95167

2 3.903 9.904 2 4.563 10.357 2 2.344 0.605 0.008 0.006

2 9.819 9.656 2 17.806 3.957 2 1.092 1.002 0.016 0.011

2 17.087 3.943 2 9.961 4.148 2 1.027 1.056 0.018 0.011

b (fm) 0.853 0.9402 0.7054 0.5803

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equal. Unexpectedly these refinements suggest there is a slight increase in the extent of disorder upon slow cooling of the sample. Intuitively it is expected that rapid cooling of the sample should increase the extent of disorder by freezing in a random high temperature distribution. The estimated standard deviations of the refined occupancies are sufficiently high in the neutron refinement to preclude discussion of this observation (Table 2) other than to note that they are similar to that previously reported by Srikanth and co-workers [16]. The bulk structure of the materials were well described by the neutron refinements at this stage and the results are in good agreement with those described in the earlier study by Srikanth et al. [16]. The refined positional and atomic displacement parameters together with selected bond distances are listed in Tables 3–5 respectively. A typical fit of the neutron patterns is shown in Fig. 1. To resolve the question of disorder, we turned to the X-ray diffraction data that had been collected at ˚ three wavelengths, 0.70036, 0.92438 and 0.95167 A. 31 21 ˚ At 0.70036 A, the scattering power of Bi and Pb are similar and as consequence structural refinements using only this data invariably failed to give meaningful occupancies for the Pb and Bi. Fig. 2 shows the calculated change in u fBi 2 fPb u 2 as a function of X-ray wavelength. The two maxima correspond to the Pb L and Bi L edges and the diffraction patterns were collected ca. 20 eV below these maxima (indicated by the dotted lines) to maximise the contrast between Pb and Bi. As illustrated in Fig. 3

the patterns collected at these three wavelengths show a number of differences and these provide the necessary sensitivity to the Pb and Bi occupancies. The structure was then refined from the X-ray diffraction data, using the neutron diffraction results as a starting model. In these refinements the positional parameters of the lighter oxygen atoms were fixed at the values obtained from the neutron diffraction study. The refinement strategy initially used a fully ordered structure and firstly the background and profile parameters were optimised, followed by the positional parameters of the heavy atoms and the isotropic displacement parameters. Once these refinements had converged, the occupancy factors were varied. For the slowly cooled sample the refinements were straightforward, in each case the heavy atoms did not move from the position obtained from neutron refinements and by refining the occupancy factors the R values all decreased. For the quenched sample very strong preferred orientation was observed and this significantly affected the intensity of the (001) peaks, in particular the (004), (115) and (0010) peaks. The preferred orientation was seen on at all three wavelengths. Refinements using the March-Dollase preferred orientation model were partially successful, but better fits were obtained by excluding the three largest effected peaks. The occupations of the A and Bi sites were not affected by excluding or not excluding these peaks. The final occupancy factors are listed in Table 2, and an example of the fit to a X-ray powder diffraction pattern is shown in Fig. 4.

Table 2 Percentage occupancies in the Pb / Bi sites obtained from structural refinements using either neutron diffraction data or anomalous X-ray diffraction data

% Occup.

Quenched Cooled

R factors

Quenched

Cooled

4(a) Pb / Bi 8(b) Bi / Pb 4(a) Pb / Bi 8(b) Bi / Pb R p (%) R wp (%) GOF R Bragg (%) R p (%) R wp (%) GOF R Bragg (%)

Neutron

Anomalous

92(17) / 8(17) 96(17) / 4(17) 88(17) / 12(17) 94(17) / 6(17) 8.39 9.59 6.5 5.24 7.01 8.19 3.0 3.46

33(2) / 67(2) 67(2) / 33(2) 53(1) / 47(1) 76(1) / 24(1) 19.55 25.41 4.07 18.90 / 16.45 10.92 15.29 2.15 7.10 / 7.20

Ismunandar et al. / Solid State Ionics 112 (1998) 281 – 289 Table 3 Cell, positional, and isotropic thermal parameters of quenched and cooled PbBi 2 Nb 2 O 9 from neutron diffraction data.

Pb

4a

Bi

8b

Nb

8b

O1

O2

O3

O4

O5

4a

8b

8b

8b

8b

Cell Parameters ˚ (A) ˚ 3) V (A R p (%) R wp (%) GOF R Bragg (%)

x y z U x y z U x y z U x y z U x y z U x y z U x y z U x y z U a b c

˚ 2) (A

˚ 2) (A

˚ 2) (A

˚ 2) (A

˚ 2) (A

˚ 2) (A

˚ 2) (A

˚ 2) (A

Quenched

Cooled

0.25 0.267(3) 0.5 0.033(2) 0.247(4) 0.743(1) 0.2013(1) 0.018(1) 0.262(4) 0.751(1) 0.4113(1) 0.03(1) 0.286(6) 0.202(4) 0 0.026(3) 0.277(5) 0.267(2) 0.1593(2) 0.029(2) 0.006(4) 2 0.003(3) 0.2505(4) 0.012(1) 0.505(4) 0.008(3) 0.5739(4) 0.024(2) 0.567(4) 0.530(2) 0.5806(4) 0.020(2) 5.4909(1) 5.4998(1) 25.5313(2) 771.00(2) 7.13 8.44 5.2 3.83

0.25 0.268(3) 0.5 0.026(1) 0.248(2) 0.749(1) 0.2012(1) 0.014(1) 0.265(3) 0.750(1) 0.4114(1) 0.002(1) 0.298(3) 0.213(5) 0 0.030(3) 0.285(3) 0.286(2) 0.1583(2) 0.025(2) 0.026(3) 0.000(3) 0.2512(4) 0.010(1) 0.515(3) 0.006(3) 0.5732(3) 0.020(2) 0.579(3) 0.533(3) 0.5814(4) 0.016(2) 5.4879(1) 5.4989(1) 25.5390(2) 770.70(2) 6.17 7.26 2.4 2.23

4. Discussion The refined occupancies obtained from the anomalous X-ray diffraction measurements demonstrate that the Pb and Bi cations are disordered over the 4a and 8b sites. The extent of this disorder is greatest in the rapidly cooled sample, where the Pb 21 cations occupy 33(2)% of the perovskite type 4a sites compared to 53(1)% in the slowly cooled

285

sample. This is consistent with the view that at high temperature Pb and Bi will have a similar preference for the perovskite layer 4a and Bi 2 O 2 layer 8b sites resulting in a statistical distribution over these two sites. This cation distribution is maintained upon quenching the sample to room temperature. Failure to observe the cation disorder in either the present, or in previous, neutron diffraction studies [16] is probably a result of the similar neutron scattering lengths of Pb and Bi. The ability of Pb to replace all of the Bi cations in the Bi 2 O 2 layer has been observed in Bi 22x Pb x Sr 12x Nd x Nb 2 O 9 [11], although in the Pb rich oxides a distortion to a monoclinic cell was observed. No such lowering of symmetry was observed in the present work. The increased cation order in the slowly cooled sample, relative to that in quenched sample, demonstrates that Bi has a thermodynamic preference for the Bi 2 O 2 layers. Since both Bi 31 and Pb 21 have 6s 2 lone pair electrons, the preference of Bi 31 for the Bi 2 O 2 layer can not be attributed to the presence of the lone pair electrons but possibly is the result of the size and / or charge difference between Pb 21 and Bi 31 . The cell volume of the quenched sample is larger than that of the cooled sample. This is a result of compressive strain and a marginally larger orthorhombic distortion is observed in the slowly cooled sample. The relative expansion of the cell in quenched sample is anisotropic, both the a and b axes are slightly larger than in the slowly cooled sample while the c axis is smaller (Table 3). We believe this reflects the expansion of Bi 2 O 2 layers in the quenched sample due to increased substitution of the larger Pb 21 cations for the smaller Bi 31 cations, and compression of the perovskite layer as a result of the increased amount of the smaller Bi 31 cations present in these layers. The average bond lengths in the two samples are very similar and are generally unremarkable. The ˚ is similar to that average Nb–O distance of 2.00 A observed in other octahedrally coordinated Nb 51 oxides such as RhNbO 4 [25], or ABi 2 Nb 2 O 9 (A 5 Sr, Ba) [17]. The Bi or Pb atoms in the 4a sites are coordinated to 12 oxygen atoms with an average ˚ The Bi and Pb atoms, M–O distance of 2.78 A. situated at the 8b sites are coordinated to eight oxygen atoms forming a distorted square antiprism ˚ and four with four short bonds to O(3) at ca. 2.30 A

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Table 4 3 ˚ 2 ) of quenched and cooled PbBi 2 Nb 2 O 9 obtained by refinement from the powder neutron Anisotropic thermal parameters ( 3 10 A diffraction data U11 Pb Bi Nb O(1) O(2) O(3) O(4) O(5)

U12

U13

47(9) 9(3)

13(3) 23(2)

2 43(8) 0(3)

0 2 21(5)

0 7(3)

74(15) 59(8) 20(6) 29(5) 46(10)

95(18) 21(6) 17(6) 14(3) 27(8)

3(3) 15(3) 19(3) 36(3) 26(3)

47(12) 32(9) 2 39(4) 2 1(3) 17(4)

0 12(7) 11(5) 23(4) 20(6)

0 4(6) 2 35(5) 2 13(4) 2 4(6)

U11 73(8) 6(3)

U22 44(8) 6(3)

Cooled U33 10(3) 15(2)

U12 2 37(8) 2 8(3)

U13 0 8(3)

U23 0 4(3)

67(15) 22(4) 12(5) 15(3) 53(8)

13(3) 19(3) 13(2) 33(3) 13(3)

2 20(7) 18(8) 2 31(4) 0(3) 26(6)

0 2 2(7) 6(4) 18(3) 14(5)

0 2(6) 2 12(6) 2 12(4) 2 5(5)

87(9) 14(3)

U23

a

Pb Bi Nb O(1) O(2) O(3) O(4) O(5) a

Quenched U33

U22

a

20(9) 27(6) 12(5) 24(3) 47(11)

Only isotropic thermal parameter refined.

Table 5 ˚ for the quenched and slowly cooled Selected bond distances (A) samples of PbBi 2 Nb 2 O 9

[(Bi,Pb) 2 O 2 ] layer

21

[(Pb,Bi)Nb 2 O 7 ] 22 perovskite layer

(Bi,Pb)–O(2) (Bi,Pb)–O(2) (Bi,Pb)–O(2) (Bi,Pb)–O(2) (Bi,Pb)–O(3) (Bi,Pb)–O(3) (Bi,Pb)–O(3) (Bi,Pb)–O(3) (Pb,Bi)–O(1) (Pb,Bi)–O(1) (Pb,Bi)–O(1) (Pb,Bi)–O(1) (Pb,Bi)–O(4) (Pb,Bi)–O(4) (Pb,Bi)–O(5) (Pb,Bi)–O(5) Nb–O(1) Nb–O(2) Nb–O(4) Nb–O(4) Nb–O(5) Nb–O(5)

31 31 31 31 31 31 31 31 31 31 31 31 32 32 32 32 31 31 31 31 31 31

Quenched

Cooled

3.07(1) 2.82(1) 3.10(3) 2.80(3) 2.30(2) 2.30(2) 2.38(2) 2.25(2) 2.39(2) 3.11(2) 2.95(2) 2.56(2) 2.75(1) 2.76(1) 3.05(1) 2.55(1) 2.284(4) 1.805(6) 1.97(2) 1.98(2) 1.89(2) 2.08(2)

3.14(1) 2.77(1) 3.15(1) 2.77(2) 2.24(2) 2.39(2) 2.43(2) 2.21(2) 2.46(2) 3.06(2) 3.02(2) 2.49(2) 2.77(1) 2.73(1) 3.11(1) 2.53(1) 2.279(4) 1.793(6) 1.96(2) 2.01(2) 1.87(2) 2.11(2)

Fig. 1. Observed, calculated, and difference neutron powder diffraction pattern for slowly cooled PbBi 2 Nb 2 O 9 . The observed data are indicated by crosses and the calculated profiles by the solid line. The short vertical lines below the profiles mark the position of all possible Bragg reflections.

˚ If only the four longer contacts to O(2) at ca. 3.00 A. short A–O(3) bonds are considered, the coordination of the A type cations can be described as an AO 4 square pyramid similar to that found in a-PbO [26]. Here the four strongly bonded oxygen atoms can be

Ismunandar et al. / Solid State Ionics 112 (1998) 281 – 289

287

Fig. 2. The variation in u fBi 2 fPb u 2 as a function of X-ray energy in the vicinity of the Pb and Bi L III absorption edges.

viewed as forming the base and the A type cation the apex of a pyramid. The 6s 2 lone pair electrons (E) are directed towards the O(2) anions and can be considered to constitute the fifth vertex of a AO 4 E square pyramid. Thus the lone pair electrons result in the square O(2) 4 face being much larger than the opposite O(3) 4 square face; the O(2)–O(2) distance ˚ whereas the O(3)–O(3) distance is 2.75 A, ˚ is 3.67 A Fig. 5. A similar stereochemical effect is also observed in nine coordinated Bi ions such as Bi 3 MSb 2 O 11 M 5 Al, Ga [27], and BiI 3 [28]. The atomic displacement parameters (ADP) of the atoms comprising the perovskite and Bi 2 O 2 layers are similar in both samples and, as expected, are highly anisotropic as a result of the stereochemistry of the individual atoms, Fig. 6. The thermal ellipsoid of O(1) which lies within perovskite-like layer is, for example, elongated perpendicular to Nb–O bonds since the closest contact to this atom is the two Nb ˚ whilst the Pb / Bi atoms that complete atoms at 2.2 A, the coordination spheres of O(1) are at much longer ˚ In contrast for O(2) the ellipsoid distance, ¯ 2.5 A. is elongated along Nb–O direction indicating dis˚ is placement towards the closest Nb atoms at 1.8 A strongly inhibited relative to displacement towards ˚ Pb / Bi atoms. More the more distant ( ¯ 2.8 A) interestingly we note that the magnitude of the atomic displacements in the quenched sample are noticeably larger than those for the slowly cooled sample atoms indicative of a larger amount of disorder, although the sense of the APD remains similar.

Fig. 3. Details of X-ray powder diffraction patterns of slowly ˚ cooled PbBi 2 Nb 2 O 9 collected at 0.70036, 0.92438 and 0.95167 A. The ratio of the intensity of the (111) and (113) peaks at 0.70036 ˚ is I ( 111 ) : I ( 113 ) ¯ 2, and at 0.95167 is I ( 111 ) : I ( 113 ) (2.5, 0.92438 A ˚ it is I ( 111 ) : I ( 113 ) ¯ 1.25. A

288

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Fig. 4. Observed, calculated, and difference synchrotron X-ray powder diffraction pattern for slowly cooled PbBi 2 Nb 2 O 9 col˚ The format is the same as Fig. 1. lected using X-ray of 0.92438 A.

Fig. 5. View of the square antiprism of BiO 8 . Note the square pyramid formed by Bi and O(3).

As stated in the introduction, the Curie temperature of PbBi 2 Nb 2 O 9 is sensitive to the thermal treatment, in particular, rapidly cooling the sample increases T c [16]. The change in T c is similar to that Solodukha et al. introduced by substituting equimolar amounts of W 61 and Ti 41 for Nb 51 [29]. They attributed the increase in T c in the doped samples to an increase in the covalent character of the NbO 6 octahedra network. This mechanism does not explain the changes observed in undoped samples of PbBi 2 Nb 2 O 9 , where we have found the NbO 6 network is essentially unaltered by heat treatment. Our results suggest that the increase in T c is related to changes in the Bi 2 O 2 layer and / or the Pb / Bi–O bonds in the perovskite layer. Although the average

Fig. 6. Anisotropic displacement ellipsoids in (110) plane of the Bi 2 O 2 and perovskite layer of the quenched sample (a) and the slowly cooled sample (b). Note the similar feature of the two and the larger displacements in (a).

bond distances in the Bi 2 O 2 and perovskite layers are independent of heat treatment, the difference in the cation distribution alters the effective electron distribution in both layers. Valence bond sum calculations [30] indicates a slight increase in the underbonding behaviour of the cations in the Bi 2 O 2 layer and, possibly, in the perovskite layer in the quenched

Ismunandar et al. / Solid State Ionics 112 (1998) 281 – 289

sample relative to the slowly cooled sample. The VBS of the cation at the 8b site are 2.76 and 2.73 for cooled and quenched sample respectively, whilst those at the 4a site are 2.38 and 2.37. As discussed by Rae et al. [8] the large spontaneous polarisation in these oxides can be largely attributed to gross underbonding of the cation in the perovskite layer and only to lesser extent to the underbonding of the cation in the Bi 2 O 2 layer. Only when underbonding in the perovskite layer is approximately the same do changes in the Bi 2 O 2 layer become important.

5. Conclusions Using a combination of powder neutron and anomalous dispersion X-ray diffraction measurements we have established that in PbBi 2 Nb 2 O 9 there is considerable disorder between the Pb 21 and Bi 31 cations. The extent of this disorder is dependent on the thermal treatment of the sample. Rapid cooling of the sample increased the amount of disorder suggesting that the Pb and Bi may be statistically distributed over the two sites at elevated temperatures. It is well established that the Curie temperature in these oxides is sensitive to thermal treatment. It is now clear that this is the result of changes in the occupancies of the cations in the perovskite and Bi 2 O 3 layers.

Acknowledgements The assistance of Dr D. E. Cox at BNL is gratefully acknowledged. This work has been partially supported by the Australian Institute of Nuclear Science and Engineering and the Australian National Beamline Facility. Work at BNL is supported under contract DE-AC02-76CH00016, Division of Materials Science, US Department of Energy.

References [1] B. Aurivilllius, Arki. Kemi. 1 (1949) 463. [2] E.C. Subbarao, J. Phys. Chem. Solids 23 (1962) 665.

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[3] E.C. Subbarao, J. Am. Ceram. Soc. 45 (1962) 166. [4] G.A. Smolenski, V.A. Isupov, A.I. Agranovskaya, Sov. Phys. Solid State 3 (1959) 651. [5] E.C. Subbarao, Ferroelectrics 5 (1973) 267. [6] J.G. Thompson, A.D. Rae, R.L. Whithers, D.C. Craig, Acta Crystallogr. B47 (1991) 174. [7] A.D. Rae, J.G. Thompson, R.L. Whithers, A.C. Willis, Acta Crystallogr. B46 (1990) 474. [8] A.D. Rae, J.G. Thompson, R.L. Whithers, Acta Crystallogr. B48 (1992) 418. [9] R.E. Newnham, R.W. Wolfe, J.F. Dorrian, Mater. Res. Bull. 6 (1971) 1029. [10] T. Rentschler, Mater. Res. Bull. 32 (1997) 351. [11] T. Rentschler, M. Karus, A. Wellm, A. Reller, Solid State Ionics 90 (1996) 49. [12] P. Millan, A. Castro, J.B. Torrance, Mat. Res. Bul. 28 (1993) 117. [13] P. Millan, A. Ramirez, A. Castro, J. Mater. Sci. Lett. 14 (1995) 1657. [14] A. Castro, P. Millan, M.J. Martinez-Lope, J.B. Torrance, Solid State Ionics 63–65 (1993) 897. [15] A. Castro, P. Millan, R. Enjalbert, Mater. Res. Bull. 30 (1995) 871. [16] V. Srikanth, H. Idink, W.B. White, E.C. Subbarao, H. Rajagopal, A. Sequeira, Acta Crystallogr. B52 (1996) 432. [17] Ismunandar, B.J. Kennedy, Gunawan, Marsongkohadi, J. Solid State Chem. 126 (1996) 135. [18] S.M. Blake, M.J. Falconer, M. McCreedy, P. Lightfoot, J. Mater. Chem. 7 (1997) 1609. [19] C.J. Howard, C.J. Ball, R.L. Davis, M.M. Elcombe, Aust. J. Phys. 36 (1983) 507. [20] D.E. Cox, J.B. Hasting, L.P. Cardoso, L.W. Finger, Mater. Sci. Forum 9 (1986) 1. [21] H.M. Rietveld, J. App. Crystallogr. 2 (1969) 65. [22] R. J. Hill, C. J. Howard, Australian Atomic Energy Commision Report No.M112, AAEC(now ANSTO) Lucas Heights Research Laboratories 1986. [23] D.T. Cromer, D.A. Liberman, Acta Crystallogr. B47 (1981) 267. [24] V. F. Sears, Atomic Energy of Canada Limited Report AECL-8490, 1984. [25] B. Ismunandar, J. Kennedy, B.A. Hunter, Mater. Sci. Forum. 278–281 (1998) 714. [26] P. Garnier, J. Moreau, J.R. Gavari, Mat. Res. Bull. 25 (1990) 979. [27] Ismunandar, B.J. Kennedy, B. A. Hunter, J. Solid State Chem., 127 (1996) 178. [28] A.K. Cheetham, N. Norman, Acta Chem. Scand. A28 (1974) 55. [29] A.M. Solodukha, Z.A. Liberman, S.D. Milovidova, Inorg. Mater. 29 (1993) 1142. [30] N.E. Brese, M. O’Keefe, Acta Crystallogr. B47 (1991) 192.