The crystal structure of the mixed-layer Aurivillius phase Bi5Ti1.5W1.5O15

Solid State Sciences 7 (2005) 1025–1034 www.elsevier.com/locate/ssscie

The crystal structure of the mixed-layer Aurivillius phase Bi5Ti1.5W1.5O15 J. Tellier, Ph. Boullay ∗ , N. Créon, D. Mercurio Laboratoire de Sciences des Procédés Céramiques et Traitements de Surface (CNRS UMR 6638), 123 Av. Albert Thomas, Université de Limoges, F-87060 Limoges Cedex, France Received 7 December 2004; received in revised form 19 January 2005; accepted 19 January 2005 Available online 16 June 2005

Abstract The crystal structure of the 1 + 2 mixed-layer Aurivillius phase Bi5 Ti1.5 W1.5 O15 (SG I2cm no 46: −cba, Z = 4, a = 5.4092(3) Å, b = 5.3843(3) Å and c = 41.529(3) Å) consisting of the ordered intergrowth of one and two octahedra thick perovskite-type blocks separated by [Bi2 O2 ]2+ slabs is reported. Supported by an electron diffraction investigation and, using the Rietveld analysis, it is shown that this compound should be described using a I-centering lattice in agreement with the generalised structural model of the Aurivillius type compounds recently presented by the authors. The structure of this Bi5 Ti1.5 W1.5 O15 phase is analyzed in comparison with the related simple members (Bi2 WO6 and Bi3 Ti1.5 W0.5 O9 ). The crystal structure of Bi3 Ti1.5 W0.5 O9 is also reported.  2005 Elsevier SAS. All rights reserved. Keywords: Bismuth layered oxides; Intergrowth compounds; Ferroelectric phase transition

1. Introduction The Aurivillius family of bismuth layered oxides [1] includes a large number of ferroelectric materials that can be described as resulting from the regular stacking of [M2 O2 ] slabs and perovskite-like [Ap−1 Bp O3p+1 ]2− blocks. While the perovskite-like blocks offer large possibilities in terms of compositional flexibility, the cation sites in the interleave [M2 O2 ] slabs are almost exclusively occupied by Bi3+ cations forming [Bi2 O2 ]2+ slabs and leading, for simple members, to the chemical composition Bi2 Ap−1 Bp O3p+3 where p corresponds to the number of sheets of corner sharing BO6 octahedra forming the perovskite-like blocks. Beside these simple members, which have been reported for p values up to 7 (see [2] and references therein), various ordered intergrowths of Aurivillius phases have been prepared as macroscopic pure phases [2–6] and can be described using the general formula Bi2m An−m Bn O3(n+m) . While investigations using Transmission Electron Microscopy (TEM) reveal the formation, at a local scale, of com* Corresponding author. Tel.: +33 5 55 45 72 48; Fax: +33 5 55 45 72 70.

E-mail address: [email protected] (Ph. Boullay). 1293-2558/$ – see front matter  2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.01.025

plex intergrowth (see [4,6,7], for instance), the most stable intergrowths contain usually one perovskite block of size p and one of size p + 1 such as Bi7 Ti4 NbO21 [3,4] (m = 2, n = 5: intergrowth “2 + 3”, i.e., Bi3 TiNbO9 + Bi4 Ti3 O12 ) or MII Bi8 Ti7 O27 [5,6] (m = 2, n = 7: intergrowth “3 + 4”, i.e., Bi4 Ti3 O12 + MII Bi4 Ti4 O15 ). The first report on the existence of 1 + 2 mixed-layer Aurivillius-type intergrowths (m = 2, n = 3) was made by Kikuchi et al. [3] with the compounds Bi5 TiNbWO15 and Na0.5 Bi4.5 Nb2 WO15 . Kikuchi [8] was also the first to report the formation of a new Aurivillius compound (Bi3 Ti1.5 W0.5 O9 , m = 1, n = 2) in the pseudo-binary Bi4 Ti3 O12 –Bi2 WO6 . This system was further investigated by Shebanov et al. [9] where the first mention of the compound under consideration in the present work, Bi5 Ti1.5 W1.5 O15 (m = 2, n = 3), was made. Voronkova et al. [10] then properly identifies this compound as an Aurivillius-type intergrowth having √ an orthorhombic unit cell with parameters a ∼ b ∼ ap 2 and c ∼ 41.6 Å. Since these earlier studies, no crystal structure refinement of such a 1 + 2 mixed-layer Aurivillius compounds was reported and, generally speaking, even the assignment of the proper space group and cell parameters for these mixed-layer Aurivillius still poses problems as illustrated in some recent

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articles ([11] for 1 + 2 and [12] for 3 + 4). Following the above arguments, it was clear for us that the crystal structure refinement of Bi5 Ti1.5 W1.5 O15 could be of interest since, except for Bi7 Ti4 NbO21 [13], no other valid crystal structure determination of a mixed-layer Aurivillius compound was reported (see bibliographic updates in Section 5).

2. Experimental Powder material corresponding to the composition Bi3 Ti1.5 W0.5 O9 and Bi5 Ti1.5 W1.5 O15 were prepared by a solid state reaction of the precursor oxides Bi2 O3 , TiO2 and WO3 mixed in a stoichiometric amount and pressed into pellets. For Bi3 Ti1.5 W0.5 O9 , they were heated at 900 ◦ C for 20 hours. For Bi5 Ti1.5 W1.5 O15 , they were heated at 1000 ◦ C for 10 hours and this thermal treatment was repeated three times after intermediate grindings. The XRPD pattern used for the structural determination was recorded in the 2θ range of 12◦ to 112◦ and 15◦ to 112◦ for Bi3 Ti1.5 W0.5 O9 and Bi5 Ti1.5 W1.5 O15 , respectively, with a 0.02◦ step using a Siemens D5000 diffractometer (CuKα1 /Kα2 —Graphite monochromator). The electron microscopy study was carried out with a JEOL 2010 microscope working at 200 kV and fitted with a double-tilt (±30◦ ) rotating sample holder. The powder was crushed in an agate mortar to obtain small fragments that were put in a suspension in alcohol. A drop of the suspension was then deposited and dried on a copper grid previously coated with a thin film of amorphous carbon.

3. Results and discussion From the investigation of the reciprocal space performed by SAED, the observed patterns (observed reflection conditions are hkl: h + k + l = 2n and h0l: h, l = 2n) can be indexed properly considering the space group I2cm (SG no 46: −cba) with the cell parameters a ∼ 5.4 Å, b ∼ 5.4 Å and c ∼ 42 Å (see Fig. 1). From these results, it is noticeable that the space group of this 1 + 2 intergrowth is the same as the one of the 2 + 3 Bi7 Ti4 NbO21 intergrowth [13]. The HREM image represented in Fig. 2 shows that a regular intergrowth between perovskite-type blocks of size p1 = 1 and p2 = 2 can be observed over a large area. Regarding the crystal structure determination, one way to obtain starting atomic positions for the Rietveld refinement of the structure of Bi5 Ti1.5 W1.5 O15 is to estimate them by building an idealized 1 + 2 intergrowth of one p = 1 and one p = 2 perovskite blocks separated by one [Bi2 O2 ]2+ slab. The Rietveld refinement of this starting structural model was then performed using the JANA2000 software [14]. Considering the large difference in the scattering factors for Ti4+ and W6+ cations, the Ti/W occupancy of the two B-sites was refined and the necessity to incorporate Ti cations in

the p = 1 perovskite-type block was evidenced from our experimental data. In the last stage of the refinement, O–O soft distances restraints were added in order to avoid exaggerated deformation of the oxygen octahedral and this without significant change in the reliability factors of the refinement. The measured, calculated and difference diagrams resulting from the Rietveld refinement of the XRPD patterns are shown in Fig. 3. The atomic positions, the thermal factors and the occupancy rates obtained for this refinement are summarized in Table 1 together with the reliability factors. The main inter-atomic distances in the coordination sphere of the cationic sites are given in Table 2. A representation of the crystal structure projected onto the (y0z) and (x0y) planes is presented in Fig. 4. Despite the fact that the compound Bi3 Ti1.5 W0.5 O9 (p = 2) has been evidenced early [8] and that a recent article [15] claimed its crystal structure analysis by the Rietveld method, no crystal structure data is available for this compound (only pattern matching profile analysis is presented in [15]—see bibliographic updates in Section 5). Thus, in order to compare the structural distortions of each perovskite blocks present in the Bi5 Ti1.5 W1.5 O15 intergrowth compound with respect to the corresponding simple members, we have refined the crystal structure of Bi3 Ti1.5 W0.5 O9 using the Rietveld method. This compound crystallizes in the A21 am space group as most of the others p = 2 Aurivillius compounds reported to date. Main structural parameters for Bi3 Ti1.5 W0.5 O9 are summarized in Table 3 and, in Fig. 5, a representation of the crystal structure is displayed together with the XRPD patterns. The Bi5 Ti1.5 W1.5 O15 intergrowth compound exhibits the characteristics common to all the Aurivillius phases in terms of layer stacking along the c-axis with the regular alternation of [Bi2 O2 ] slabs and perovskite blocks (with two different size here). Also characteristic, one observes an expansion of the cationic sublattice combined with a compression of the anionic sublattice along the c-axis [7]. The cationic layers of the perovskite blocks are displaced along the c-axis towards the oxygen layers belonging to the [Bi2 O2 ] slabs. The displacement is more pronounced while going closer to these [Bi2 O2 ] slabs, which leads to the existence of one short apical B–O distance (see Ti/W(1)–O(7) in Table 2). Notice that these displacements along the c-axis do not contribute here to the observed spontaneous polarization (Tc ≈ 710 ◦ C [9,10]) but is only a consequence of the B-site deficiency existing within the [Bi2 O2 ] slabs. As depicted in the early crystal structure analyses of Aurivillius phases, the structural distortions associated with the ferroelectric phase transition can be described by considering distortions to an archetypal HT paraelectric phase.1 Similarly to other Aurivillius ferroelectric phases (see footnote #3), the structure observed at RT presents three distortions from the archetypal structure: 1 Notice that for mixed-layer Aurivillius intergrowths, the archetypal phase has a tetragonal lattice with aRT = at + bt , bRT = bt − at , cRT = 2ct and a P4/mmm space group.

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Fig. 1. SAED patterns observed for the Bi5 Ti1.5 W1.5 O15 compound: (a) [001] zone axis patterns (ZAP), (b) [010] ZAP, (c) [−110] ZAP and (d) [100] ZAP.

– a tilting of the octahedra around the a-axis (Fig. 4); – a rotation of the octahedra around the c-axis (Fig. 4); – atomic displacements along the polar a-axis. Regarding the level of structural distortions, it was reported on a broad range of Aurivillius phases [16] that the structural distortion increases as the perovskite tolerance √ factor t = (RA + RO )/ 2(RB + RO ) decreases. Nonetheless, in the present case, RTi4+(VI) and RW6+(VI) being almost identical (RTi4+(VI) = 0.605 Å [17], RW6+(VI) = 0.600 Å [17]), one expects that the level of structural distortions in the intergrowth and the related simple members (p = 1 Bi2 WO6 [18]2 and p = 2 Bi3 Ti1.5 W0.5 O9 this study 2 In this reference the crystal structure is refined using the space group Pca21 (no 29) with the unit cell parameters: a = 5.4373(2) Å,

and [20]) would not be different even if the cationic distribution onto the B sites differs. At first, considering our experimental results, it is clear that Bi5 Ti1.5 W1.5 O15 cannot be regarded as a “true intergrowth” of the simple members p = 1 and p = 2 with, notably, the presence of Ti cations in the p = 1 perovskite-type block. More precisely, one notices: – for p = 1: the disappearance of the rotation of the B(2)O6 octahedra around the c-axis with conservation of the atomic displacement along the polar a-axis within the [Bi2 O2 ] slabs (Fig. 6(a)); b = 16.4302(5) Å and c = 5.4584(2) Å. In the following, we will use the space group P21 ab (no 29: cab) to ensure a coherency with the other presented structures, i.e., the polar axis is a and the stacking direction is along c.

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Fig. 2. HREM images of Bi5 Ti1.5 W1.5 O15 . A regular intergrowth between perovskite-type blocks of size p1 = 1 and p2 = 2 can be observed over a large area.

Table 1 Refined structural parameters for Bi5 Ti1.5 W1.5 O15 (Z = 4) Atom

Wyckoff

x

y

z

Uiso (Å2 )

Occupancy

Bi(1) Bi(2) Bi(3) Ti/W(1) Ti/W(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8)

4b 8c 8c 8c 4a 8c 8c 8c 8c 8c 8c 8c 4b

0.0233(18) 0.0096(14) 0.0338(15) −0.017(2) 0b −0.033(7) 0.217(6) 0.268(8) 0.260(14) 0.260(14) 0.161(6) −0.024(6) −0.073(8)

−0.0087(13) −0.0135(10) 0.4790(8) 0.500(2) 0 −0.056(6) 0.252(3) 0.300(4) 0.251(14) 0.251(14) 0.194(4) 0.466(6) 0.467(13)

0.25 0.62944(4) 0.56701(5) 0.30280(8) 0 0.0456(4) 0.0056(6) 0.2042(6) 0.4040(7) 0.6001(7) 0.8027(7) 0.1556(4) 0.75

0.0352(11) 0.0081(7) 0.0102(7) 0.0014(2)a 0.0014(2)a 0.010b 0.010b 0.010b 0.010b 0.010b 0.010b 0.010b 0.010b

1 1 1 0.673(3)/0.327(3) 0.154(6)/0.846(6) 1 1 1 1 1 1 1 1

Space group I2cm (no 46: −cba) with a = 5.4092(3) Å, b = 5.3843(3) Å and c = 41.529(3) Å. Reliability factors: Robs = 1.83%, Rwp = 8.07% and χ 2 = 2.79. a Constrained to the same refined value;

b Fixed parameters.

– for p = 2: the conservation, but with a lower magnitude, of the octahedral rotations around both a-axis and c-axis (Fig. 6(b)); – an increase of the average B(2)–O distance which goes from 1.87 to 1.93 Å. This can actually be decomposed (Fig. 7(a)) into, first, a regularization of the B(2)–O distances within the (a, b) plane and, second, an increase of the B(2)–O distances along the c-axis (1.87 to 1.92 Å);

– almost no change in the average B(1)–O distance (1.98 vs. 1.97 Å) but a reduction of the B(1)–O(7) distance, i.e., the “bridging” oxygen between the perovskite block and the [Bi2 O2 ] slabs (Fig. 7(b)). These modifications of the B–O distances along the caxis are compensated by an evolution of the Bi–O distances between the Bi atoms of the [Bi2 O2 ] slabs and the “bridging”

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Fig. 3. Final observed, calculated and difference plots for the XRPD Rietveld refinement of Bi5 Ti1.5 W1.5 O15 . The set of tick marks represents the reflection associated to Bi5 Ti1.5 W1.5 O15 . Table 2 Main interatomic distances in the coordination sphere (< 3 Å) of Bi and Ti/W atomic positions obtained from the Bi5 Ti1.5 W1.5 O15 structure refinement Ti/W(1) –O(3) –O(3) –O(6) –O(6) –O(7) –O(8)

2.01(3) Å 1.90(4) Å 2.03(3) Å 1.91(3) Å 1.74(2) Å 2.22(1) Å

×1 Ti/W(2) –O(1) 1.924(19) Å ×1 –O(2) 1.81(3) Å ×1 –O(2) 2.05(3) Å ×1 ×1 ×1

×2 ×2 ×2

Bi(1)

–O(3) –O(3) –O(6) –O(8) –O(8) –O(8)

2.85(3) Å 2.60(3) Å 2.52(3) Å 2.19(4) Å 2.52(7) Å 2.96(7) Å

×2 Bi(2) ×2 ×2 ×1 ×1 ×1

×1 ×1 ×1 ×1 ×1 ×1

–O(1) –O(1) –O(4) –O(4) –O(5) –O(5)

2.51(4) Å 2.47(3) Å 2.25(7) Å 2.41(7) Å 2.21(6) Å 2.37(7) Å

×1 ×1 ×1 ×1 ×1 ×1

Bi(3)

–O(4) –O(4) –O(5) –O(5) –O(7) –O(7)

2.32(6) Å 2.31(6) Å 2.31(7) Å 2.30(7) Å 2.75(3) Å 2.68(3) Å

oxygens of the perovskite blocks: 2.51 to 2.49 Å for Bi(3)– O(1) and 2.60 to 2.71 Å for Bi(2)–O(7) (Tables 2 and 3).

This results in a regularization of the BiO6 [4 + 2] polyhedrons within the [Bi2 O2 ] slabs through an increase of the shortest Bi–O distances, i.e., Bi(3)–O(5) (2.18 Å in Bi2 WO6 vs. 2.21 Å—Figs. 6(a) and 7(a)) and Bi(2)–O(4) (2.25 Å in Bi3 Ti1.5 W0.5 O9 vs. 2.32 Å—Figs. 6(b) and 7(b)). This also corresponds to an opposite displacement of the couples of atoms O(1)/Bi(2) and O(7)/Bi(3) in the directions [110] and [−110] towards their ideal positions in the prototype structure.

4. Conclusion In this paper, the crystal structure of one 1 + 2 mixedlayer Aurivillius phase (m = 2, n = 3: Bi5 Ti1.5 W1.5 O15 ) is reported based on a X-ray Powder Diffraction (XRPD) study and supported by an Electron Diffraction investigation (ED). The crystal structure of Bi3 Ti1.5 W0.5 O9 is also reported. It is noticeable that the space group of this 1 + 2 Bi5 Ti1.5 W1.5 O15 intergrowth is the same as for the 2 + 3 Bi7 Ti4 NbO21 intergrowth [13]. The structural characteristics are likewise closely related, which strongly supports our recent proposition for an unified approach to the crystal-

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Fig. 4. Crystal structure of Bi5 Ti1.5 W1.5 O15 projected onto the (y0z) plane. Parts of the structure are also represented projected onto the (x0y) plane in order to visualize the c-axis rotation of the octahedral network. The size of the spheres used to represent the atomic positions correspond to the isotropic thermal displacement parameters.

chemistry of the Aurivillius phases [7]. It seems to us of importance to state here that the existence within such intergrowth structures of two perovskite-type blocks with an odd and even number of octahedral layers (“mixed-layer”) combined with the specific rotation of the octahedra within these layers imposes to consider two times the p and p + 1 blocks to obtain the periodicity along the c-axis. Also, it can be assumed that the a and b parameters should be

√ close to a ∼ b ∼ ap 2. As a direct consequence, it follows that these ferroelectric mixed-layer Aurivillius phases possess necessarily a I-centering lattice. Moreover, as indicated in [7], the number of possible space groups for such mixed-layer compounds is strongly limited and I2cm (SG no 46: −cba) appears, at least for Bi7 Ti4 NbO21 and Bi5 Ti1.5 W1.5 O15 , the favored one. In most of the recent publications reporting the ferroelectric properties of mixed-

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Fig. 5. Top: Final observed, calculated and difference plots for the XRPD Rietveld refinement of Bi3 Ti1.5 W0.5 O9 . The set of tick marks represents the reflection associated to Bi3 Ti1.5 W0.5 O9 . Bottom: Crystal structure of Bi3 Ti1.5 W0.5 O9 projected onto the (y0z) plane. Parts of the structure are also represented projected onto the (x0y) plane in order to visualize the c-axis rotation of the octahedral network. The size of the spheres used to represent the atomic positions correspond to the isotropic thermal displacement parameters.

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Fig. 6. Rotation of the BO6 octahedra around the c-axis for (a) Bi2 WO6 (SG P21 ab no 29: cab) and Bi5 Ti1.5 W1.5 O15 (part p = 1) and for (b) Bi3 Ti1.5 W0.5 O9 and Bi5 Ti1.5 W1.5 O15 (part p = 2).

layer Aurivillius phases (1 + 2, 2 + 3 or 3 + 4), the proposed cell parameters and space group assignment are, most probably, erroneous simply by not taking the above comments into consideration. In a similar manner as the A- and B-lattice centering are admitted,3 respectively, for 3 It is noticeable that deviations from this general tendency are observed

for Aurivillius compounds having “relaxor” ferroelectric behaviour, as illustrated recently [19] in the comparative study of the two p = 4 related compounds CaBi4 Ti4 O15 (normal ferroelectric) and BaBi4 Ti4 O15 (relaxor-like ferroelectric).

even and odd-layer Aurivillius phases, we hope that the present structural determination will reinforce the idea of using a I-lattice centering for the mixed-layer Aurivillius phases.

5. Additional notes After acceptance, two articles in close relation with the present work were published [20,21]. Before the release of this article, the authors wanted to put them to the at-

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Fig. 7. Rotation of the BO6 octahedra around the a-axis for (a) Bi2 WO6 (SG P21 ab no 29: cab) and Bi5 Ti1.5 W1.5 O15 (part p = 1) and for (b) Bi3 Ti1.5 W0.5 O9 and Bi5 Ti1.5 W1.5 O15 (part p = 2). Table 3 Refined structural parameters for Bi3 Ti1.5 W0.5 O9 (Z = 4) Atom

Wyckoff

x

y

z

Uiso (Å2 )

Occupancy

Bi(1) Bi(2) Ti/W O(1) O(2) O(3) O(4) O(5)

4a 8b 8b 4a 8b 8b 8b 8b

−0.0500(11) −0.0392(11) 0a 0.031(5) 0.013(4) 0.217(9) 0.301(4) 0.223(7)

0.7603(10) 0.7337(5) 0.2465(18) 0.197(6) 0.292(4) 0.495(14) 0.036(4) 0.544(4)

0.5 0.69823(4) 0.58780(9) 0.5 0.6591(6) 0.2477(12) 0.5840(7) 0.5682(7)

0.0269(7) 0.0175(5) 0.0047(12) 0.010a 0.010a 0.010a 0.010a 0.010a

1 1 0.75/0.25a 1 1 1 1 1

Main interatomic distances in the coordination sphere (< 3 Å) of Bi and Ti/W atomic positions Ti/W Bi(1) Bi(2)

–O(1) –O(4) –O(1) –O(5) –O(2) –O(3)

2.212(5) Å 1.87(2) Å 2.39(3) Å 2.53(3) Å 2.58(2) Å 2.25(5) Å

×1 ×1 ×1 ×2 ×1 ×1

–O(2) –O(5) –O(1) –O(5) –O(2) –O(3)

1.796(14) Å 2.06(3) Å 2.28(3) Å 2.66(3) Å 2.61(2) Å 2.32(5) Å

×1 ×1 ×1 ×2 ×1 ×1

–O(4) –O(5) –O(4)

1.98(2) Å 1.94(3) Å 2.496(19) Å

×1 ×1 ×2

–O(3) –O(3)

2.33(6) Å 2.32(6) Å

×1 ×1

Space group A21 am (no 36: −cba) with a = 5.4017(2) Å, b = 5.3727(2) Å and c = 24.9406(6) Å. Reliability factors: Robs = 3.26%, Rwp = 9.24% and χ 2 = 4.07. a Fixed parameters.

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tention of the readers. They report the crystal structure analysis using neutron powder diffraction of, respectively, Bi3 Ti1.5 W0.5 O9 [20] and the 1 + 2 mixed-layer Aurivillius phase Bi5 TiNbWO15 [21]. Both articles are in agreement with the results presented here and give a strong support to this work.

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