Cation disorder in Bi2Ln2Ti3O12 Aurivillius phases (Ln = La, Pr, Nd and Sm)

Materials Research Bulletin 38 (2003) 837–846

Cation disorder in Bi2Ln2Ti3O12 Aurivillius phases (Ln ¼ La, Pr, Nd and Sm) Neil C. Hyatta,b,*, Joseph A. Hriljacc, Tim P. Comynd a

Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b London & Scandinavian Metallurgical Co Limited, Fullerton Road, Rotherham S60 1DL, UK c School of Chemical Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK d Ecertec Limited, 175 Woodhouse Lane, Leeds LS2 9JT, UK Received 9 August 2002; received in revised form 18 January 2003; accepted 27 January 2003

Abstract The synthesis and structure of triple layered Bi2Ln2Ti3O12 Aurivillius phases (Ln ¼ La, Pr, Nd and Sm), prepared from K2Ln2Ti3O10 Ruddlesden-Popper precursors, has been investigated. These materials adopt a body ˚ and c  33 A ˚ ) comprising centred tetragonal structure (space group I4/mmm, with unit cell parameters a  3:8 A 2þ 2 a regular intergrowth of [Bi2O2] fluorite-type and [Ln2Ti3O10] perovskite-type layers. A significant degree of cation disorder is present in the Bi2Ln2Ti3O12 system, involving the cross-substitution of Ln/Bi cations onto the Bi/Ln sites in the fluorite- and perovskite-type layers, respectively. As the size of the lanthanide cation is reduced, Bi/Ln disorder is significantly suppressed due to the effect of bond length mismatch in the perovskite-type layer in the crystal structure of Bi2Ln2Ti3O12. This offers a potential strategy for the chemical control of cation disorder in the Bi2Ln2Ti3O12 system. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Crystal structure; D. Ferroelectricity

1. Introduction Ferroelectric perovskite-type oxides are currently the focus of attention in the development of nonvolatile Ferroelectric Random Access Memory (FRAM) devices, where digital information is stored in the switchable spontaneous polarisation state of the ferroelectric component [1]. Promising candidate materials for such applications are the Aurivillius phases of general formula Bi2An1BnO3nþ3 [2–4], where A is a large 12-co-ordinate cation and B is a small 6-co-ordinate cation with a d0 electron *

Corresponding author. Tel.: þ44-114-222-5973; fax: þ44-114-222-5943. E-mail address: [email protected] (N.C. Hyatt).

0025-5408/03/$ – see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00032-1

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configuration [5]. In structural terms, these materials may be considered as being composed of a regular intergrowth of [Bi2O2]2þ fluorite-type and [An1BnO3nþ1]2 perovskite-type layers. Of particular interest is the triple layered (n ¼ 3) Aurivillius phase Bi4xLaxTi3O12: thin films of this material, with the composition Bi3.25La0.75Ti3O12, have been demonstrated to be excellent candidate materials for use in FRAM applications [3,4]. Importantly, structural studies of the Bi4xLaxTi3O12 system have demonstrated the presence of significant cation disorder in members of this solid solution [6]. Thus, the composition Bi2La2Ti3O12 is perhaps more properly formulated (Bi0.79La0.21)2(La0.79Bi0.21)2Ti3O12, in order to reflect the crosssubstitution of 21% of the La/Bi cations onto the Bi/La sites in the fluorite- and perovskite-type layers, respectively [6]. The extent of such cation disorder in the ferroelectric component of FRAM devices is believed to influence device fatigue, that is, the reduction of polarisation or stored charge with repeated switching (corresponding to read/write operations) [3,7]. Thus, the control of cation disorder in Aurivillius phases is a potentially important strategy for improving the inherent fatigue characteristics of FRAM devices fabricated from such materials. In the double layered (n ¼ 2) Aurivillius phases, ABi2Nb2O9 (A ¼ Pb, Sr, Ba and Ca), the extent of Bi/A cation disorder between the fluorite- and perovskite-type layers has been found to be dependent both on the size of the A2þ cation and thermal annealing conditions [8–10]. We have, therefore, explored the possibility that cation disorder in the Bi2Ln2Ti3O12 system may be suppressed through careful control of lanthanide ion size and synthetic conditions. The investigation presented here describes the synthesis and structure of Bi2Ln2Ti3O12 Aurivillius phases (Ln ¼ La, Pr, Nd and Sm) prepared via a low temperature metathesis reaction, recently described by Gopalakrishnan et al. [11]. This metathesis reaction involves the prior preparation of a triple layered Ruddlesden-Popper phase, K2Ln2Ti3O10, and the subsequent reaction of this material with a stoichiometric quantity of BiOCl to form the target phase, Bi2Ln2Ti3O12, as shown schematically in Fig. 1. Whereas the synthesis of Bi2Ln2Ti3O12 via the solid-state reaction of stoichiometric quantities of metal oxides requires temperatures of 1000–1150 8C [6,12,13], the reaction summarised by Fig. 1 proceeds at a significantly lower temperature of 800 8C, offering a potential strategy for the synthetic

Fig. 1. Scheme showing the formation of the Aurivillius phase, Bi2Ln2Ti3O12, from a Ruddlesden-Popper precursor, K2Ln2Ti3O10.

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control of cation disorder in the Bi2Ln2Ti3O12 system. Alternatively, cation disorder in the Bi2Ln2Ti3O12 system may be influenced by the size of the lanthanide cation providing a means of chemical control of cation disorder in these materials.

2. Experimental The synthesis of polycrystalline samples of Bi2Ln2Ti3O12 (Ln ¼ La, Pr, Nd and Sm) was achieved by the reaction of K2Ln2Ti3O10 with a stoichiometric quantity of BiOCl at 800 8C in air for 6 h, as described by Gopalakrishnan et al. [11]. The precursor, K2Ln2Ti3O10, was prepared by reaction of stoichiometric quantities of TiO2 and Ln2O3 (Ln ¼ La, Nd and Sm) or Pr6O11 with a 50 mol% excess of K2CO3 (to compensate for loss due to volatilisation), at 1050 8C for 12 h, in air. The product was washed with distilled water, to remove excess K2CO3, and dried at 200 8C, in air. Since K2Ln2Ti3O10 phases undergo rapid hydration on exposure to the atmosphere [14], these materials were stored under vacuum and dried at 400 8C, for 1 h in air, prior to the reaction with BiOCl. Following this reaction, the product material was washed with distilled water, to remove the KCl by-product and subsequently dried at 110 8C in air. X-ray powder diffraction data were acquired at room temperature using a Siemens D5000 diffractometer, equipped with a primary beam curved single-crystal Ge-(2 2 0) monochromator, affording Cu Ka1 radiation. The instrument operates in transmission mode and is fitted with a Position Sensitive Detector (PSD). During data collection, specimens (consisting of a thin layer of sample dispersed on Mylar film) were rotated in order to alleviate preferred orientation effects. For the purpose of structure refinement studies, the diffraction data were corrected for the effect of sample absorption using the method of Klug and Alexander [15]. Crystal structure refinements by the Rietveld Method, were undertaken with the GSAS suite of programs [16], as discussed in Section 3.2.

3. Results and discussion 3.1. Synthesis of Bi2Ln2Ti3O12 (Ln ¼ La, Pr, Nd and Sm) The formation of essentially single-phase Bi2Ln2Ti3O12 materials (Ln ¼ La, Pr, Nd and Sm) via the metathesis reaction described in Section 2 was confirmed by X-ray powder diffraction studies. A preliminary examination of the diffraction patterns revealed reflection conditions consistent with the ˚ and c  33 A ˚ . This body centred tetragonal space group I4/mmm, with unit cell constants of a  3:8 A cell is in agreement with that described by Hervoches and Lightfoot for Bi2La2Ti3O12 materials prepared by the solid-state reaction method [6]. Gopalakrishnan et al. [11] reported an orthorhombic ˚ and c ¼ 32:944 A ˚ for Bi2La2Ti3O12 prepared by the metathesis reaction unit cell, with a and b  5:4 A between K2La2Ti3O10 and BiOCl. However, careful inspection of the X-ray powder diffraction data for samples prepared in this investigation provided no evidence for an orthorhombic distortion of the body centred tetragonal cell described above. A subtle variation in the colour of the polycrystalline Bi2Ln2Ti3O12 materials was apparent; whereas, Bi2La2Ti3O12 is pale cream, Bi2Pr2Ti3O12 is pale green, Bi2Nd2Ti3O12 is pale lilac and Bi2Sm2Ti3O12 is pale yellow. These colours are similar to those observed in the hydrated salts of Pr3þ, Nd3þ and Sm3þ ions; the weak intensity being due to the small crystal field effects associated with such Ln3þ cations [17].

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3.2. Structure refinement of Bi2Ln2Ti3O12 (Ln ¼ La, Pr, Nd and Sm) Structure refinement of Bi2Ln2Ti3O12 materials was carried out according to the following stepwise strategy. In the initial step, the structural model proposed by Hervoches and Lightfoot [6] for Bi2La2Ti3O12 was modified, such that the Bi and Ln cations were fully ordered on their respective sites in the fluorite- and perovskite-type layers, and subsequently refined. In the second step, the Bi and Ln cations were allowed to disorder over the two available cation sites, subject to the following constraints: the occupation of each of the mixed Bi/Ln sites was constrained to sum to unity; the Bi:Ln stoichiometry was constrained to remain constant at 1:1; the z co-ordinates and isotropic thermal parameters of the Bi and Ln ions occupying the two distinct cation sites were constrained to be equal. In the third and final step, the O1 oxygen atom was allowed to displace from the ideal 4e position, (0, 1/2, 0), to a new 8j position, (x, 1/2, 0), corresponding to a rotation of the Ti1 octahedra about the <0 0 1> axis. A significant improvement in the model was obtained at each step according to Hamilton R-factor Ratio tests [18]. The improvement in the model obtained in the second step was highly significant at a confidence level of a ¼ 0:005; i.e. the probability that the Bi/Ln cations are perfectly ordered was less than 0.5%. The improvement in the model obtained in the second step was only significant at a level of a ¼ 0:25, reflecting the poor precision of X-ray powder diffraction in locating weakly scattering elements such as oxygen in the presence of much stronger scatterers such as bismuth and the lanthanide elements. However, the displacement of the O1 oxygen atom was effective in affording a more sensible co-ordination environment about the Ti1 cation and reducing the isotropic thermal parameter of the O1 atom to a more acceptable value, as discussed below. For these reasons, we have accepted the displacement of the O1 oxygen atom as significant. The refined atomic parameters and selected bond lengths for Bi2Ln2Ti3O12 materials, with Ln ¼ La, Pr, Nd and Sm, are given in Tables 1 and 2, respectively. The final profile fit to the X-ray powder diffraction data for Bi2La2Ti3O12 is shown in Fig. 2. The unit cell constants of the Bi2Ln2Ti3O12 structure (Ln ¼ La, Pr, Nd and Sm) decrease with the size of the lanthanide ion, as indicated in Table 1. This contraction of the unit cell is reflected in a small decrease in the key metal–oxide bond lengths, as summarised in Table 2. 3.3. Discussion The refined crystal structure of Bi2Ln2Ti3O12 (Ln ¼ La, Pr, Nd and Sm), shown schematically in Fig. 3, exhibits several interesting features. The co-ordination environment of the Ti2 cation in the outer TiO2 plane of the perovskite-type layer is considerably distorted from ideal octahedral geometry, as shown in Fig. 4. This distortion, in which the Ti2 cation is displaced toward the O4 oxygen, results in significant buckling of the outer TiO2 planes. The co-ordination environment of the Ti1 cation also appears to be distorted, although the nature of this distortion, involving two longer apical Ti–O2 bonds and four shorter equatorial Ti–O1 bonds is symmetric, in contrast with the asymmetric distortion of the Ti2 octahedron. Interestingly, the Ti1 octahedron appears to become more symmetric, as the size of the Ln3þ ion decreases. The asymmetric co-ordination environment of the titanium cations in Bi2Ln2Ti3O12 is a common feature of transition metal oxides in which the transition metal possesses a d0 electron configuration [19] and is considered to arise from a second order Jahn-Teller effect [20]. Bond valence sum calculations [21], summarised in Table 3, indicate that the Ti cations are somewhat over-bonded in the crystal structure of Bi2Ln2Ti3O12. Co-operative rotation of the Ti1 octahedra by 108 about the <0 0 1> axis is effective in increasing the Ti1–O1 bond length by

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Table 1 Refined structural parameters of Bi2Ln2Ti3O12 materials, with Ln ¼ La, Pr, Nd and Sm Space group: I4/mmm

Bi1

4e (0, 0, z)

Ln1

4e (0, 0, z)

Ln2

4e (0, 0, z)

Bi2

4e (0, 0, z)

Ti1

2a (0, 0, 0)

Ti2

4e (0, 0, z)

O1

8j (x, 1/2, 0)

O2

4e (0, 0, z)

O3

8g (0, 1/2, z)

O4

4e (0, 0, z)

O5

4d (0, 1/2, 1/4)

Powder statistics

Lattice parameters

La

Pr

Nd

Sm

˚) a (A ˚) c (A n z ˚ 2) B (A n z ˚ 2) B (A n z ˚ 2) B (A n z ˚ 2) B (A n ˚ 2) B (A n z ˚ 2) B (A n x ˚ 2) B (A n z ˚ 2) B (A n z ˚ 2) B (A n z ˚ 2) B (A n ˚ 2) B (A

3.83166 (4) 33.0139 (5) 0.819 (3) 0.28836 (4) 1.68 (2) 0.181 (3) 0.28836 (4) 1.68 (2) 0.819 (3) 0.43366 (5) 1.68 (2) 0.181 (3) 0.43366 (5) 1.68 (2) 1 0.6 (2) 1 0.1291 (1) 1.8 (1) 1/2 0.063 (8) 2.3 (5) 1 0.0606 (4) 2.1 (3) 1 0.1167 (3) 1.1 (2) 1 0.1818 (4) 0.8 (3) 1 1.0 (4)

3.80953 (2) 32.8143 (3) 0.852 (3) 0.28863 (3) 1.41 (2) 0.148 (3) 0.28863 (3) 1.41 (2) 0.852 (3) 0.43386 (3) 1.41 (2) 0.148 (3) 0.43386(3) 1.41 (2) 1 0.7 (1) 1 0.1283 (1) 1.5 (1) 1/2 0.089 (5) 2.6 (4) 1 0.0594 (3) 2.3 (2) 1 0.1159 (2) 1.1 (2) 1 0.1825 (4) 0.9 (2) 1 0.9 (4)

3.80620 (2) 32.7647 (3) 0.860 (2) 0.28881 (3) 1.57 (2) 0.140 (3) 0.28882 (3) 1.57 (2) 0.860 (3) 0.43393 (3) 1.57 (2) 0.140 (3) 0.43393 (3) 1.57 (2) 1 0.9 (1) 1 0.1279 (1) 1.7 (1) 1/2 0.087 (5) 2.6 (4) 1 0.0594 (3) 2.3 (3) 1 0.1156 (2) 1.6 (2) 1 0.1821 (4) 1.6 (2) 1 0.5 (2)

3.79570 (3) 32.7099 (4) 0.879 (3) 0.28896 (4) 1.46 (2) 0.121 (3) 0.28896 (4) 1.46 (2) 0.879 (3) 0.43388 (5) 1.46 (2) 0.121 (3) 0.43388 (5) 1.46 (2) 1 0.4 (1) 1 0.1278 (1) 1.4 (1) 1/2 0.094 (9) 2.7 (5) 1 0.0585 (4) 2.4 (4) 1 0.1155 (3) 1.6 (2) 1 0.1822 (5) 1.6 (3) 1 0.8 (3)

Rwp (%) Rp (%) w2

3.20 2.66 4.31

3.60 2.68 4.29

3.12 2.35 4.32

2.90 2.18 3.70

ca. 1–2%, thus reducing the over-bonding of the Ti1 cation to a more acceptable level. Notably, the over-bonding of the Ti2 cation is essentially constant as a function of lanthanide ion size, whereas the over-bonding of the Ti1 cation increases with decreasing lanthanide ion size. The distortion of the TiO6 octahedra in the crystal structure of Bi2Ln2Ti3O12 is similar to that observed in the related triple layered Ruddlesden-Popper phase, Na2La2Ti3O10 (which is isostructural with the K2Ln2Ti3O10 precursor employed in this study). In this material, the TiO6 octahedra forming the inner TiO2 sheet of the perovskite-type layer are found to rotate by 12.58 about the <0 0 1> axis in order to reduce the

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Table 2 Selected bond lengths in Bi2Ln2Ti3O12 materials, with Ln ¼ La, Pr, Nd and Sm ˚) Bond (A

La

Ti1–O1 Ti1–O2

1.931 (5) 2.00 (1)

1.935 (3) 1.95 (1)

1.932 (3) 1.94 (1)

1.931 (5) 1.92 (1)

4 2

Ti2–O2 Ti2–O4 Ti2–O3

2.27 (1) 1.74 (2) 1.959 (2)

2.26 (1) 1.78 (2) 1.948 (2)

2.25 (1) 1.78 (1) 1.946 (2)

2.26 (2) 1.78 (2) 1.940 (2)

1 1 4

Ln–O1 Ln–O2 Ln–O3

2.76 (2) 3.07 (3) 2.716 (1) 2.536 (7)

2.68 (1) 3.11 (3) 2.703 (1) 2.508 (5)

2.68 (1) 3.12 (3) 2.700 (1) 2.501 (5)

2.66 (2) 3.11 (2) 2.695 (1) 2.493 (6)

2 2 4 4

Bi–O4 Bi–O5

2.882 (5) 2.297 (1)

2.856 (4) 2.288 (1)

2.855 (4) 2.289 (1)

2.845 (5) 2.286 (1)

4 4

Rotation of Ti1 octahedra (o)

7 (1)

Pr

Nd

10 (1)

Sm

10 (1)

Multiplicity

11 (1)

Fig. 2. Final profile fit for Bi2La2Ti3O12 showing the observed data and calculated profile (solid points and smooth line, respectively) with the difference profile shown below; the tick marks indicate the positions of the allowed Bragg reflections.

Table 3 Bond valence sums for cations in the Bi2Ln2Ti3O12 structure, with Ln ¼ La, Pr, Nd and Sm Atom

La

Pr

Nd

Sm

Ti1 Ti2 Ln Bi

4.1 4.2 3.0 2.8

4.3 4.2 2.9 2.9

4.3 4.2 2.7 2.9

4.4 4.3 2.4 2.9

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Fig. 3. Schematic structural representation of Bi2Ln2Ti3O12.

over-bonding of the Ti cation, with a concomitant increase of the in-plane Ti–O bond length of 2.5% [22]. In addition, these octahedra are found to be tilted, with the apical oxygen atoms displaced from the ideal 4e (0, 0, z) position, to a general 16n (x, 0, z) position. Notably, the isotropic thermal parameters of the O2 and O1 oxygen atoms co-ordinated to the Ti1 cation in Bi2Ln2Ti3O12 are relatively large. This may indicate a subtle tilting of the Ti1 octahedra, similar to that observed in Na2La2Ti3O10, which has not been resolved in the current study; this would be effective in further reducing the over-bonding of the Ti1 cation. Neutron powder diffraction studies, in order to determine more precisely the position of the oxygen atoms in the Bi2Ln2Ti3O12 structure are required, to clarify this point. The structural data summarised in Table 2 indicate that 18.1(3)% of the La/Bi cations are disordered over the alternative Bi/La cation sites in the respective fluorite- and perovskite-type layers of Bi2La2Ti3O12. Hervoches and Lightfoot reported that approximately 21(1)% of Bi/La cations were disordered in the crystal structure of Bi2La2Ti3O12, prepared by high temperature solid-state synthesis [6]. The estimated precision associated with the disorder of the Bi/La cations is not sufficient to resolve a significant difference in the level of Bi/La disorder between the two studies. This result is unfortunate, since it would suggest that the synthesis of Bi2La2Ti3O12 by the metathesis reaction summarised in Fig. 1 does not afford materials in which the level of Bi/La disorder is significantly suppressed. The

Fig. 4. Schematic representation of the asymmetric co-ordination environment about the Ti2 cation.

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Fig. 5. Variation of cation disorder in Bi2Ln2Ti3O12 with lanthanide ionic radii for Ln ¼ La, Pr, Nd and Sm (ionic radii are for eight-fold co-ordination, from [24]).

mechanism of the reaction summarised in Fig. 1 does not, therefore, appear to be topotactic in nature, since a topotactic mechanism would be expected to preserve the cation ordering present in the K2La2Ti3O10 precursor, leading to a significant suppression of disorder in the Bi2La2Ti3O12 product. Thus, the reaction summarised in Fig. 1 does not appear to offer an effective means of exercising synthetic control over the extent of cation disorder in Bi2Ln2Ti3O12 phases. Interestingly, however, a significant suppression of cation disorder in the Bi2Ln2Ti3O12 system does occur as the size of lanthanide ion decreases, as shown in Fig. 5. Bond valence sum calculations reveal that the Bi3þ ion is slightly under-bonded in the Bi2Ln2Ti3O12 structure and that the Ln3þ ion becomes progressively more under-bonded as the size of the lanthanide ion decreases. The Goldschmidt tolerance factor, t, defined by Eq. (1), gives an indication of the degree of mismatch between the A–O and B–O bonds in an ABO3 perovskite-type layer [23]: rAO (1) t ¼ pffiffiffi 2 rBO where rA–O and rB–O represent the ideal A–O and B–O bond lengths, respectively, as given by the sum of the appropriate ionic radii [24]. In agreement with the bond valence sum calculations described above, Eq. (1) indicates that the perovskite-type layer in Bi2Ln2Ti3O12 becomes progressively less stable as the size of the lanthanide ion decreases. Using tolerance factor arguments, Armstrong and Newnham [13] suggested that considerable strain exists at the interface between the fluorite- and perovskite-type layers in the structure of Aurivillius phases. These authors suggest that the ‘‘ideal’’ a-parameter for the Bi2O2 fluorite-type layer is ˚ , with the ‘‘ideal’’ a-parameter for the perovskite-type layer, ap, given by Eq. (2): af ¼ 3:80 A ap ¼ 1:33rB þ 0:60rA þ 2:36 —

(2)

where rA and rB denote, respectively, the ionic radii of the A ¼ Ln and B ¼ Ti cations in the perovskite-type layer. For Ln ¼ La, Pr, Nd and Sm, the ideal value of ap afforded by Eq. (2) is greater ˚ , indicating that the fluorite- and perovskite-type layers are under than the ideal value of af ¼ 3:80 A tensile and compressive strain, respectively (ionic radii are for eight-fold co-ordination, from [24]).

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Bond valence sum calculations (Table 3) indicate that the lanthanide cation becomes progressively less stable in the perovskite-type layer, as the size of the lanthanide cation decreases. The effect of bismuth substitution in the 12-co-ordinate A-site, due to cation disorder, is to increase the effective size of this cation site in the perovskite layer, since rBi > rLn , according to Eq. (2). Thus, cation disorder is expected to increase the under-bonding of the lanthanide cation in the perovskite layer. Since the stability of the lanthanide cation in the 12-co-ordinate A-site already decreases with the size of the lanthanide cation, according to bond valence sum calculations, cation disorder is expected to decrease with the size of the rare earth cation, as shown in Fig. 5. This is in accordance with the observed stability range of the Bi2Ln2Ti3O12 structure, which is stable only for trivalent lanthanide cations larger than Gd, the smaller lanthanide cations being unable to satisfy the requirements of 12-fold co-ordination [12]. Arguments similar to those outlined above have been proposed to explain the trend of decreasing cation disorder with alkaline earth size in the double layered Bi2ANb2O10 Aurivillius system, with A ¼ Ba, Sr and Ca [8]. In this system, cation disorder acts to reduce over-bonding of the alkaline earth cation in the perovskite-type layer, which increases with the size of the alkaline earth cation.

4. Conclusions The synthesis of Bi2Ln2Ti3O12 (Ln ¼ La, Pr, Nd and Sm) Aurivillius phases via the reaction of a Ruddlesden-Popper precursor, K2Ln2Ti3O10, with a stoichiometric amount of BiOCl, has been studied. This reaction does not appear to be topotactic in nature and, therefore, does not provide an effective method for synthetic control of cation disorder in the Bi2Ln2Ti3O12 system. Rietveld analysis of X-ray powder diffraction data has allowed the degree of cation disorder in the Bi2Ln2Ti3O12 system to be investigated. Bond valence sum calculations indicate that the lanthanide cation becomes increasingly under-bonded as the size of the lanthanide cation is reduced. Cation disorder effectively increases the size of the 12-co-ordinate A-site in the perovskite layer, leading to further under-bonding of the lanthanide cation. Consequently, cation disorder in the Bi2Ln2Ti3O12 system (Ln ¼ La, Pr, Nd and Sm) is suppressed as the size of the lanthanide ion is reduced. This offers a potential strategy for the chemical control of cation disorder in the Bi2Ln2Ti3O12 system.

Acknowledgements N.C.H. is grateful to Dr. L. Ranson, Dr. R.R. Schwarz, Mr. M.E.J. Birch and Mr. J.A. Wright at the London & Scandinavian Metallurgical Co Limited, for support and encouragement. We thank EPSRC for financial support under grant reference GR/L50365/01. References [1] [2] [3] [4] [5]

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