Stairlike Aurivillius Phases in the Pseudobinary Bi5Nb3O15

Dec 5, 2017 - model is embedded into one single superspace group description, a detailed structural analysis of these stairlike. Aurivillius phases wi...
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Stairlike Aurivillius Phases in the Pseudobinary Bi5Nb3O15−ABi2Nb2O9 (A = Ba and Sr) System: A Comprehensive Analysis Using Superspace Group Formalism Gwladys Steciuk,†,‡ Nicolas Barrier,† Alain Pautrat,† and Philippe Boullay*,† †

CRISMAT, Normandie Université, ENSICAEN, UNICAEN, CNRS UMR 6508, 6 Bd Maréchal Juin, F-14050 Caen Cedex 4, France Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Prague, Czech Republic

‡

S Supporting Information *

ABSTRACT: We report the possibility of extending the socalled stairlike Aurivilius phases in the pseudobinary Bi5Nb3O15−ABi2Nb2O9 (A = Ba and Sr) over a wide range of compositions. These phases are characterized by a discontinuous stacking of [Bi2O2] slabs and perovskite blocks, leading to long-period intergrowths stabilized as a single phase. When analyses from precession electron diffraction tomography and Xray and neutron powder diffraction are combined, the monoclinic incommensurately modulated structure with q = αa* + γc* previously proposed for the ABi7Nb5O24 composition could be generalized to the Bi5Nb3O15−ABi2Nb2O9 (A = Ba and Sr) compounds. Considering the compositions expressed as (A,Bi)1−xNbxO3−3x, the stacking sequence associated with compositions ranging from x = 2/5 to 3/8 is governed by the component γ of the modulation vector and can be predicted following a Farey tree hierarchy independently to the A cation. The length of the steps, characteristic of the stairlike nature, is controlled by the α component and depends on the substitution ratio A/Bi and the nature of A (A = Ba and Sr). This study highlights the compositional flexibility of stairlike Aurivillius phases.

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Aurivillius family. The first members with a composition ABi7Nb5O24 (A = Ba, Pb, and Sr) could be first seen as a mere combination of Bi5Nb3O15 (p = “1 + 2”)21 and ABi2Nb2O9 (p = 2) (A = Sr, Pb, and Ba).25 They actually possess an original modulated structure, leading to a layered structure built up with discontinuous [Bi2O2] slabs and perovskite blocks exhibiting nonusual ferrorelaxor properties.15 The average cell (a ∼ b ∼ ap√2 − 5.5 Å, c ∼ 5.2 Å, and β ≥ 90°) can be related to a fluorite-type structure with the existence of a complex occupational and displacive incommensurate modulation characterized by a modulation vector in the form q = αa* + γc*. We found that the stacking sequence (Aurivillius-like) is governed by the γ component of the modulation vector and is constant for all ABi7Nb5O24 compounds whatever the A cation. The length of the steps (stairlike) is governed by the α component, which varies with the nature of the A cation (A = Sr, Pb, and Ba). In the course of our investigation on stairlike Aurivillius phases, we noticed the possibility of stabilizing compounds with a cationic composition significantly different from that of ABi7Nb5O24. Despite these variations, all electron diffraction patterns presented the characteristic features of stairlike Aurivillius phases, i.e., similar cell parameters and

INTRODUCTION The Aurivillius family1,2 of Bi-layered oxides with the general formula [Bi2O2]2+[Ap−1Bp O3p+1]2− are intergrowth compounds built up from the regular stacking of [Bi2O2]2+ fluorite-like slabs and [Ap−1BpO3p+1]2− perovskite blocks, where p denotes the number of octahedral layers in thickness along the stacking direction. A is a 12-fold-coordination cation, like Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, and Bi3+ or a combination of them. Ferroelectric Aurivillius phases with low p values (1−4) can be stabilized with B cations having high oxidation state and empty d orbitals such as W6+, Nb5+, and Ti4+. They have been widely investigated as potential lead-free piezoelectric devices especially in applications under high-temperature and highfrequency conditions,3 ferroelectric nonvolatile memories,4,5 oxide ion conductors,6 and photocatalysts.7 A renewed interest in the solid-state chemistry of Aurivillius phases arose because of the possibility of obtaining room-temperature magnetoelectric properties for compounds with higher thicknesses of the perovskite blocks (p ≥ 4) stabilized by introducing B-site cations (Fe, Co, and Mn) with lower oxidation state.8−13 This search for new Bi-based layered multiferroics also includes works on Sillen−Aurivillius phases.14 In this context of blooming activities around Aurivillius phases, the recent discovery of so-called stairlike Aurivillius15 phases appears as a singular but interesting branch of the © XXXX American Chemical Society

Received: December 5, 2017

A

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

an upper-mounted Gatan ORIUS 200D CCD camera. PEDT data sets of nonoriented patterns were recorded at room temperature on several different thin crystals for different compositions. For all data collections (see Table 1), the precession angle was set to 1.2° with a goniometer tilt step below 1°. PEDT data sets were analyzed using the computer programs PETS,17 SUPERFLIP,18 and JANA200619 following a procedure resembling the one used in single-crystal X-ray diffraction. Details about the methodology to solve complex structures such as incommensurately modulated structures using PEDT can be found elsewhere.20−24 For each data set, the result is a list of hklm indices with associated intensities and estimated standard deviations based on counting statistics.

modulation vectors. This result strongly suggests that the structure can adapt to a significant deviation from the ABi7Nb5O24 stoichiometry, leading us to investigate further the existence of a possible solid solution within the pseudobinaries Bi5Nb3O15−ABi2Nb2O9 (A = Ba and Sr). In the following, we will present experimental proof of the existence of a large number of stairlike Aurivillius phases not limited to the ABi7Nb5O24 composition. We will then show how their stacking sequences can be seen as different uniform ordering sequences dependent on the composition that can be obtained from a simple Farey tree analysis.16 Finally, when this model is embedded into one single superspace group description, a detailed structural analysis of these stairlike Aurivillius phases will be performed.

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RESULTS AND DISCUSSION Analysis of XRPD diagrams for Bi5Nb3O15−BaBi2Nb2O9 compounds shows a great similarity with that of the BaBi7Nb5O24 compound previously reported.15 Notably, the presence of a peak of high intensity at low angle [∼8° (2θ)], together with two very close peaks exhibiting the highest intensities at 28−29°, is a signature of the stairlike Aurivillius phases15 (Figure S1). The change in the nominal composition is reflected in XRPD by a smooth and continuous evolution of the patterns from 1 to 17-11-54 (close to Bi5Nb3O1521) to 3-119-42 (close to BaBi2Nb2O925). The monoclinic lattice parameters and modulation vector of BaBi7Nb5O2415 were used as initial starting values to model (via the Lebail method) the XRPD patterns obtained for several compounds going from Bi5Nb3O15 to BaBi2Nb2O9 (Figure S1). This first fit allows us to evidence a progressive decrease of the monoclinic angle and the c parameter concomitant with an increase of the a and b parameters (Figure S2). The XRPD patterns also exhibit satellite reflections up to the order 6, with a clear change in their positions depending on the composition and an attenuation of their intensities upon going closer to BaBi2Nb2O9 (Figure S1). To further confirm the formation of stairlike Aurivillius phases, single-crystal PEDT data have been acquired on some of these compounds (Table 1) in order to have a better view of the reciprocal space and ease analysis of these modulated structures including symmetry. The 3D PEDT reconstruction of the reciprocal space confirms the choice of a monoclinic cell with lattice parameters a ∼ b ∼ ap√2 − 5.5 Å, c ∼ 5.2 Å, and β ≥ 90° and a modulation vector in the form q = αa* + γc* for all of the synthesized compositions. It also highlights a progressive evolution of the components α and γ of the modulation vector (Table 1) from α = −0.0443(3) and γ = 0.2433(3) for 1-30-17-94 to α = −0.0238(3) and γ = 0.2200(3) for 3-11-9-42. Referring to ABi7Nb5O24 structures,15 an increase in the absolute value of α leads to a shorter length of the steps (stairlike), while an increase of γ should reflect the change in the stacking sequence (Aurivillius-like) upon going from p = 2 (BaBi2Nb2O9) to p = “1 + 2” (Bi5Nb3O15). Regarding the symmetry, the sections (h0lm)* are presented for the two compositions 1-17-11-54 and 3-11-9-42 equidistant of BaBi7Nb5O24 and located close to the limit of the explored Bi5Nb3O15−BaBi2Nb2O9 line (Figure 2). For 1-17-11-54, h = 2n + 1 reflections are clearly noticeable, whereas for 3-11-9-42, the sections (h0lm)* and (h1lm)* evidence a condition h0lm: h = 2n associated with an a glide plane. At this stage, we decided to solve the structures ab initio (SUPERFLIP18) for the two limit compositions 1-30-17-94 and 3-11-9-42 from PEDT data considering the superspace group (SSG) X21(α0γ)0 with 1 1 1 X = (0, 2 , 2 , 2 ). The result is presented as an extended (010) projection of the 3D electrostatic potential map where the

EXPERIMENTAL SECTION

Polycrystalline samples with nominal compositions AaBibNbcOd further abbreviated as a-b-c-d were synthesized by conventional solid-state reactions. Stoichiometric amounts of Bi2O3, Nb2O5, and ACO3 (A = Ba and Sr) were used as raw materials, mixed, pressed into pellets, and calcined in air at 1000 °C for 48 h. After regrinding and repressing, a second thermal annealing for 48 h at 1025 °C (A = Ba) or 1050 °C (A = Sr) was applied. Most A = Ba compositions were chosen between Bi5Nb3O15 and BaBi2Nb2O9. Two compositions out of this line were tested in order to investigate extra chemical flexibility within the system BaO−BiO1.5−NbO2.5 (Figure 1). Some Bi5Nb3O15− SrBi2Nb2O9 compositions were also synthesized to study the influence of the A cation.

Figure 1. (a) Ternary diagram of the BaO−BiO1.5−NbO2.5 solid-state solution showing compositions related to Bi5Nb3O15,21 BaBi2Nb2O9,25 and BaBi7Nb5O24.15 (b) Nominal compositions for Bi5Nb3O15− BaBi2Nb2O9 compounds (blue dots) and compounds out of the Bi5Nb3O15−BaBi2Nb2O9 line (green dots) are given as a-b-c-d corresponding to BaaBibNbcOd. X-ray powder diffraction (XRPD) patterns were recorded in the 2θ range of 5−150° [scan step: 0.009° (2θ)] with a Bruker D8 Advance diffractometer [Cu Kα1 ∼ 0.013° (2θ)] equipped with a Lynx-Eye detector and in the 2θ range of 5−120° (step: 0.013°) with an X’Pert MPD Pro diffractometer (Cu Kα1/Kα2) equipped with a PIXcel detector. For 1-12-8-39 and 2-9-7-33 compositions (A = Ba), neutron powder diffraction (NPD) experiments were performed on the D2B diffractometer (Institue Laue-Langevin, Grenoble, France) at ambient temperature using a wavelength λ = 1.594 Å. For neutron experiments, the powdered samples (∼8 g) were put into a vanadium container (⌀ = 8 mm). For transmission electron microscopy investigations, a small quantity of powder ( 3σ(I). The nominal compositions are summarized as Baa−Bib−Nbc−Od.

Figure 2. Reciprocal sections reconstructed from PEDT data in the PETS program. (a) (h0lm)* plane from the 1-17-11-54 compound. (b) (h0lm)* and (h1lm)* planes from the 3-11-9-42 compound.

structures adapt to the composition. In order to understand the structural relationship between these phases, it is necessary to take a closer look at the way in which they are described in the superspace approach and see how, using a fluorite average cell, it is possible to construct a structure having an Aurivillius-type stacking. Starting from a hypothetical fluorite BiO2 structure (space group Fm3̅m and aF ∼ 5.4 Å), if one out of three Bi layers (and its surrounding O2 layers) is periodically replaced by one BO2 layer (and apical O layers), it is possible to form an Aurivillius sequence p = 1 corresponding to the compound Bi2BO6 (B = W26,27 or Mo28,29), as illustrated in Figure 3a. The substitution ratio is linked to the proportion of perovskite B sites over the total amount of cationic sites such as x = B/(B + Bi) = 1/3. Following this example, an Aurivillius sequence

stairlike Aurivillius structure is confirmed for both compositions (Figure S3). By examination of some of de Wolff’s sections of the potential map obtained around the cationic atomic domains (Figure S3), the average position and displacive modulation affecting the A/Bi and Nb sites look similar for these two limit compositions. The different stacking sequence is actually a consequence of a subtle change in the length of the crenel functions defining the cationic atomic position. A more pronounced difference in the average positions between the compositions 1-30-17-94 and 3-11-9-42 can be seen on de Wolff’s sections obtained around the oxygen atomic domains (Figures S4 and S5, respectively). The pseudobinary Bi5Nb3O15−BaBi2Nb2O9 system appears to be constituted of a series of stairlike Aurivillius phases whose C

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Starting with a hypothetical BiO2 fluorite-like compound, the scheme illustrates how to build (A,Bi)1−xBxO3x−3 Aurivillius layer stacking in the cases of Bi2BO6 (p = 1) with x = 1/3 and an intergrowth Bi5B3O15 (p = 1 + 2) with x = 5/8. (b) Farey tree series, followed by the compounds with (A,Bi)1−xBxO3x−3 compositions lying between Bi5Nb3O15 and BaBi2Nb2O9.

1)d, these phases are described using a model with only five atomic positions (Bi/Ba, Nb, and three O), where Bi/A and Nb share the same average atomic position, discriminated using a crenel function (see Table S1 as an illustration). The lengths of the crenel functions are composition-dependent and are expressed as ΔNb = x and Δ(A,Bi) = ΔO = 1 − x. Not just on paper, the dependency of the widths of crenel functions to the composition is confirmed from the section of the electrostatic potential obtained from PEDT data analyses (Figure S3b). As was already stated, looking at the XRPD data, a continuous evolution of the lattice parameters is evidenced for compositions lying on the Bi5Nb3O15−ABi2Nb2O9 line for both A = Ba (Figures S1 and S2) and Sr (Figure S7). The lattice parameters a, b, c, and β show an evolution of less than 1% with a global increase of the unit cell volume with the amount of Ba/Sr inserted on the Bi sites. The main change occurs for the two components α and γ of the modulation vector (45.5% and 9.7%, respectively, for A = Ba). These two components turn out to be key, explaining how the structure adapts to compositional changes. Regarding the γ component (Figure S6a), as was just explained, only a very few deviations are observed from the relationship γ = 1 − 2x and shall reflect small composition variations from the nominal ones. However, for A = Sr, the XRPD pattern of 3-11-9-42 no longer possesses the characteristics of a stairlike phase with α = 0 and γ = 0.2 but shows similarities with SrBi2Nb2O9, indicating that the stairlike structure can adapt the addition of Sr in the structure to a lesser extent than in the A = Ba system. It is still noteworthy that our model reproduces the “classic” Aurivillius layered structure when α = 0 and is thus effective in describing the structure of the limit SrBi2Nb2O9 composition. The evolution of the α component varies linearly with x and is dependent on the chemical species of the A cation because the slopes of α versus x are clearly different for A = Ba and Sr (Figure S6b). Note that for x = 3/8, i.e., Bi5Nb3O15, the trend lines for A = Ba and Sr converge to a value α = −0.05. While the orthorhombic Bi5Nb3O15 (type IV) is not a stairlike Aurivillius phase,21,31 this is actually consistent with the

corresponding to p = 2 is formed by substituting two out of five Bi layers (x = 2/5) and an intergrowth |p = 1|p = 2| by substituting three out of eight Bi layers (x = 3/8; Figure 3a). If m every n Bi layers are replaced by BO2 layers, taking also into account the surrounding O layers, compositions in the form nBiO2−mBiO4 + mBO2 + (n − m)O, i.e., Bin−mBmO3n−3m, are obtained. We do recover the description of an Aurivillius phase in t h e f o r m o f a B - si t e d e fic i e n t p e r o vs k i t e 3 0 Bi1−xBx□1−2xO3−3x, where x = m/n and 1 − 2x represents the number of “faults” per perovskite unit expressed from a fluorite average cell. Assuming that the substitution of Bi layers in BiO2 is uniform, a unique stacking sequence will exist for a given composition and can be deduced through a Farey tree series, knowing x30 (Figure 3b). In this description, ABi2B2O9 (x = 2 /5) and Bi5B3O15 (x = 3/8) represent the two limit members of the branch Bi5Nb3O15−BaBi2Nb2O9 that we explored. For instance, the stacking sequence for x = 5/13, i.e., (A,Bi)8Nb5O24, can be determined considering that in the Farey tree x = 5/13 is the median of 3/8 and 2/5, with 3/8 being the median of 2/5 and 1 /3 (Figure 3b). 5/13 can be decomposed as 1/3 + 2/5 + 2/5, which corresponds to the stacking sequence along c of perovskite blocks p = 1 + 2 + 2. For all x included between 2 /5 and 3/8, the stacking sequence can be determined using the Farey tree hierarchy and is related to the component γ of the modulation vector q by the formula γ = 1 − 2x. While this description only models the layer stacking (Aurivillius nature linked to γ) without taking into account the existence of shearing mechanisms (stairlike nature linked to α), it already gives important features that shall be applicable for all of the Bi5Nb3O15−BaBi2Nb2O9 compounds. The γ component of the modulation vector is dependent on x, i.e., on the ratio of Nb over the total number of cations, which implies that γ shall not be dependent on the chemical nature of A. Experimentally, both the dependency of γ on x and its invariance to changes in the chemical species of A can be verified (Figure S6a). The model used previously for ABi7Nb5O24 (x = 5/13) compounds15 can be generalized to any (AyBi1−y)1−xBxO3−3x stairlike Aurivillius phases having a composition x ∈ [2/5, 3/8], where y = (−3 + 8x)/(1 − x) considering the charge balance. In (3 + D

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. NPD Rietveld refinement diagrams of 1-12-8-39 and 2-9-7-33 compounds showing measured (black), calculated (red), and difference (blue) curves. Black and green ticks correspond to fundamental and satellite reflections, respectively.

report31,32 of a minority monoclinic “stepped” Bi5Nb3O15 (type IV*), whose structure can be described using the proposed model for stairlike Aurivillius phases. To confirm the possibility of generalizing our compositiondependent model, structure refinements were conducted from the PEDT and XRPD data available for compositions on the Bi5Nb3O15−ABi2Nb2O9 lines for both A = Ba and Sr. Because of the large amount of data, only part of the results will be presented here and illustrated for some A = Ba compositions. The results of XRPD Rietveld refinements and PEDT data analyses will be illustrated with 1-17-11-54 and 3-11-9-42 compositions (Tables S1 and S2). While PEDT refinements in the kinematical approximation lead to high values for the reliability factors, we consider that, starting from an almost complete data set, the refined parameters are relevant enough to discuss the structural evolution when supported by XRPD

Rietveld refinements. Note that the recent possibility of accurately analyzing the PEDT data using the dynamical theory of diffraction33−35 is not yet applicable to modulated structures. NPD Rietveld refinements where performed for only two selected compositions, i.e., 1-12-8-39 and 2-9-7-33, to obtain notably better insight of the oxygen parameters. The structure parameters obtained from NPD refinements are summarized in Tables S3 and S4 (Figure 4). The compounds were refined in the SSG X21(α0γ)0 and

(

1

1

X = 0, 2 , 2 ,

1 2

) except for 3-11-9-42, which was refined in

the SSG X21/a(α0γ)00 (see Table S2). The main difference observed following the composition is in terms of the oxygen atomic positions and modulation parameters. As was already pointed out by PEDT (see Figures S4 and S5), NPD refinements confirm the presence of stronger octahedral E

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (010) and (001) structure projections in a supercell of (a) 1-12-8-39 and (b) 2-9-7-33 compounds with average stacking following [001]. In order to show the octahedral distortions within the slabs and the lengths of the p = 1 and 2 perovskite blocks, the (001) and (010) projections are represented for (c) 1-17-11-54, (b) 1-12-8-39, (e) 2-9-7-33, and (f) 3-11-9-42.

Upon going from x = 3 / 8 (Bi 5 Nb 3 O 15 ) to x = 2 / 5 (BaBi2Nb2O9), the empirical Goldschmidt tolerance factor t evolved with x from t = 0.936 to 1.004 and can be associated with an evolution of the dielectric properties2,36 from a “normal” ferroelectric (Bi5Nb3O1537) to a relaxor ferroelectric (BaBi2Nb2O938). Focused on structural analysis of the newly found stairlike Aurivillius phases, it is interesting here to follow the structural changes undergone by the compounds (Ba,Bi)1−xNbxO3−3x in the range x ∈ [2/5, 3/8]. The main structural evolution is the attenuation of NbO6 octahedral tilting in the perovskite blocks (Figure 5) concomitant with the weakening of the Bi−O bonds between [Bi2O2] and the perovskite parts (Figure 6).39 As a general tendency for “normal” Aurivillius ferroelectric phases, all Bi atoms possess a

rotations in the perovskite blocks for 1-12-8-39 (x = 0.381; %Ba/cations = 4.8%) than for 2-12-9-39 (x = 0.389; %Ba/cations = 11.1%) (Figure 5). As a general trend, p = 1 blocks are more distorted than p = 2 for a given composition. This tendency is also observed from PEDT refinements on 1-17-11-54 (x = 0.379, %Ba/cations = 3.44%) and 3-11-9-42 (x = 0.391, %Ba/cations = 13%). Moreover, following %Ba/cations, only the length of p = 2 blocks increases, which supports the idea that Ba goes preferentially within p = 2 blocks.15 Accordingly, the curve representing the bond valence for the Ba/Bi atomic position versus the internal parameter t shows a minimum closer to 2+ for the compositions 3-11-9-42 and 2-9-7-33, where 13% and 11.1% of Bi3+ sites are substituted by Ba2+, rather than for 1-128-39 (4.8%) and 1-17-11-54 (3.4%) (Figure S8). F

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. M−O connectivity for M = A/Bi atoms in the [Bi2O2] slabs for (a) 1-17-11-54, (b) 1-7-5-24, and (c) 2-9-7-33. In a normal ferroelectric, all Bi atoms possess a 4 + 2 coordination with four short Bi−O distances below 2.5 Å and two in the range of 2.5−2.85 Å.

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4 + 2 coordination with four short Bi−O distances below 2.5 Å and two in the range of 2.5−2.85 Å. Because of the incommensurately modulated nature of the stairlike phases, the Bi coordination is not unique but periodically evolves from 4 (encircled atoms) to 4 + 1 and 4 + 2 along the a direction whatever the x value (Figure 6). Such a weakening of the Bi−O bonds between [Bi2O2] and the perovskite parts is usually related to the apparition of a relaxor behavior.40 The fact that the choice of a centrosymmetric versus a noncentrosymmetric SSG involves only subtle structural changes (NbO6 octahedral rotations), with a weak experimental signature (the presence or not of h = 2n + 1 rows in the h0lm section), also indicates that the establishment of a long-term polar structure in these stairlike Aurivillius phases is unclear. In our previous study,15 BaBi7Nb5O24 (x = 3/13) was accordingly identified as a relaxor ferroelectric while described with the noncentrosymmetric SSG X21(α0γ)0. Referring to the usual structure−property relationships in ferroelectric Aurivillius phases, the presence of octahedral distortions in these stairlike Aurivillius phases (Figure 5), together with strong Bi−O bonds (Figure 6), would likewise suggest the existence of ferroelectric properties.2,41 The stairlike Aurivillius phases are incommensurately modulated compounds, and their crystal chemistry is different from that of the classic Aurivillius phases. For one given composition, octahedral distortions (Figure 5) and cation site mixing are diverse and vary periodically within the structure because of the incommensurately modulated nature of the compounds. For ABi7Nb5O24 compositions (A = Sr and Ba), it was associated with the existence of unusual relaxor ferroelectric properties.15 Considering the structural similarities between all of the stairlike Aurivillius phases, we expect such a relaxor behavior to be a generality rather than an exception for all of the compounds presented in this work.

CONCLUSIONS

From detailed analysis of the compounds (A,Bi)1−xNbxO3−3x, x ∈ [2/5, 3/8] (A = Ba and Sr), we proposed a unique (3 + 1)d structural model to describe the so-called stairlike Aurivillius phases.15 From this comprehensive analysis using a SSG formalism, the relationship between the structure parameters of different compounds of the same family is highlighted. Such a description also provides a useful tool to predict the structures of new members. Not limited to compositions on the Bi5Nb3O15−ABi2Nb2O9 line, this model offers enough flexibility to adapt to stairlike Aurivillius phases whose compositions are off this line (Figures 1 and S9). The study of these offline compounds could certainly be the subject of a separate article because it becomes clear that these phases seem capable of being stabilized over a wide range of compositions and would require a further understanding of their chemistry (Figure S6a). The results presented here show the existence of a wide range of compounds exhibiting a so-called stairlike Aurivillius structure, which, compared to conventional Aurivillius phases, possesses an enhanced compositional flexibility. Following our previous report,15 the stairlike Aurivillius family made up of three members at the beginning (ABi7Nb5O24 with A = Ba, Sr, and Pb) is enlarged in significant proportions. This is exemplified here for A = Ba and Sr, but there is little doubt that numerous stairlike Aurivillius phases can be produced using other divalent cations (Ca, Pb, etc.) but also monovalent (Na, K, etc.) or trivalent (La, etc.) cations. This opens a route for solid-state chemists to explore an exotic but interesting class of materials with notably the potential to finely tune their dielectric properties. This was not the main subject of this article, but there is undoubtedly much to be done. G

DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03026. Crystallographic parameters from XRPD and NPD Rietveld and PEDT refinements, pattern fitting (Lebail method) of XRPD diagrams, lattice parameters versus the Nb ratio x, projections of the electrostatic potential maps, sections of the electrostatic potential map around O sites, modulation components α and γ versus the Nb ratio x, XRPD diagrams for four compositions in the system SrO−BiO1.5−NbO2.5, Bi and Nb bond valence graphics, and XRPD profile refinement diagrams for compositions out of the Bi5Nb3O15−ABi7Nb5O24 (A = Ba and Sr) line (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Philippe Boullay: 0000-0002-2867-8986 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Emmanuelle Suard (ILL) for NPD data collection and Sylvie Collin for her help in sample preparation during the initial stage of this work.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03026 Inorg. Chem. XXXX, XXX, XXX−XXX