Structural changes in the homologous series of the Aurivillius phases Bin+1Fen−3Ti3O3n+3

Journal of Alloys and Compounds 528 (2012) 103–108

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Structural changes in the homologous series of the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 N.A. Lomanova a,∗ , V.G. Semenov b , V.V. Panchuk b , V.V. Gusarov a,c a b c

Ioffe Physical Technical Institute, 26 Polytekhnicheskaya Str., St. Petersburg 194021, Russian Federation St. Petersburg State University, 7-9 Universitetskaya Emb., St. Petersburg 199034, Russian Federation St. Petersburg State Technological Institute (Technical University), 26 Moskovsky Ave., St. Petersburg 190013, Russian Federation

a r t i c l e

i n f o

Article history: Received 14 October 2011 Received in revised form 7 March 2012 Accepted 9 March 2012 Available online 16 March 2012

a b s t r a c t The Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 have been synthesized via solid state reactions. Mössbauer spectroscopy and X-ray analysis have revealed the dependence of Fe ions distribution over the nonequivalent sites and of unit cell parameters on the number of perovskite-like layers in the Aurivillius phases. Said dependencies are shown to undergo abrupt changes when the perovskite-like block becomes 2 nm thick.

Keywords: Oxides Layered compounds Scanning electron microscopy X-ray diffraction Mössbauer spectroscopy

1. Introduction The perovskite-like compounds in the Bi2 O3 –TiO2 –Fe2 O3 system possess semiconductor, ferromagnetic, ferroelectric and catalytic properties [1–7] which determine the potential of their wide practical application. A homologous series of compounds with the general formula Bin+1 Fen−3 Ti3 O3n+3 is realized in the Bi2 O3 –TiO2 –Fe2 O3 system. These compounds have the Aurivillius phase type layered perovskite-like structure described for the first time in [8–10]. Usually, crystal structures of these phases are described as an intergrowth of {(Bi2 O2 )2+ }∞ layers and {(An−1 Bn O3n+3 )2− }∞ perovskite-like blocks, where A is a twelve coordinated cation, e.g., Na, K, Ca, Sr, Ba, Pb, Bi, Ln (Ln is a rare earth element), etc., and B is an octahedral cation such as Fe, Ti, Nb, Ta, Cr, etc. Here, n is the number of octahedral layers in the perovskite-like block. Thus, a unit cell of Bin+1 Fen−3 Ti3 O3n+3 shows that the fluorite-like layers of {(Bi2 O2 )2+ }∞ with a thickness f ≈ 4.08 A˚ [11] alternate with the perovskite-like layers of {(Bin+1 Fen−3 Ti3 O3n+1 )2− }∞ with an average thickness h = h(n) that depends on the number of octahedral layers in the perovskite-like block (see Fig. 1). The values of n may be integer or fractional. The latter ones correspond to the structures in which the perovskite-like blocks with an hi = h(ni )

∗ Corresponding author. E-mail address: [email protected] (N.A. Lomanova). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.040

© 2012 Elsevier B.V. All rights reserved.

thickness alternate with a different number of ni octahedral layers in a block. The two nonequivalent octahedral B cation sites identified in the perovskite-like block are B(1) inside the block and B(2) on the outer sides of the block (see Fig. 1). The nature of ions distribution over the nonequivalent sites determines many physical and chemical properties of compounds including their stability [12]. Therefore, the data on the dependence of ion distribution over sites on the charge, radius, site preference energy and other characteristics of ions, as well as on the composition and structure of compounds, may be useful for analyzing the possibility of obtaining new compounds and predicting their properties. Though the number of publications devoted to synthesis and studies of structural peculiarities and properties of the Aurivillius phase compounds is big, the literature sources with the data on the cation distribution over the octahedral sites B(1) and B(2) are relatively scanty. As a rule, the available works lack systematic approach oriented towards analyzing the dependence of cation distribution over octahedral sites on the Aurivillius phases composition and structural parameters. The distribution of octahedral cations over the nonequivalent sites in the perovskite-like block for the three-layer Aurivillius phases Bi2 SrNaNb2 TaO12 and Bi2 Sr2 Nb2.5 Fe0.5 O12 is investigated in [13]. The occupation of sites was determined by the Rietveld refinement of the powder X-ray diffraction data. It was demonstrated that despite the chemical similarity of Nb and Ta, their distribution

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f B(2): [Ti/Fe](1)O6 B(1): [Ti/Fe](2)O6 h

Bi O Ti/Fe

Fig. 1. Bi5 FeTi3 O15 structure.

over sites in the triple-layered perovskite slabs differs from random. The B(2) sites (terminal octahedra) are preferentially occupied by Nb (Nb/Ta: 74/26) and the B(1) sites (central octahedra) by Ta (Nb/Ta: 52/48). According to [14], a reference to which is made in [13], a similar distribution of Nb and Ti cations over the sites is observed for the three-layer Aurivillius phases of the formula Bi2 Sr2 Nb2 TiO12 . The neutron Rietveld refinement for the threelayer Aurivillius phases Bi1.8 Sr2.2 Nb2.2 Ti0.8 O12 , has determined a slight preference of Nb for the B(2) sites (Nb/Ti: 78/22) in comparison with the inner B(1) site (Nb/Ti: 64/36) [15]. According to [13], Fe cannot be found at the B(2) site (Nb/Fe: 100/0) in the Aurivillius phases of the formula Bi2 Sr2 Nb2.5 Fe0.5 O12 , since Fe occupies the B(1) site together with Nb (Nb/Fe: 50/50). The distribution of Ti and Cr over sites in the four-layer Aurivillius phases of the formula Bi5 Ti3 CrO15 was determined in [16]. The Rietveld analysis of high resolution neutron diffraction data demonstrated a significant disorder of Ti and Cr over the available nonequivalent octahedral sites. Ti and Cr ions were found to preferentially occupy the outer B(2) (Ti/Cr: 87.1/12.9) and inner B(1) (Ti/Cr: 62.9/37.1) sites, respectively. The distribution of Ti and Fe over sites in the four-layer Aurivillius phases of the formula Bi5 Ti3 FeO15 was determined in [15] using the powder neutron diffraction data and the 57 Fe Mössbauer data on Bi5 Ti3 FeO15 . The neutron diffraction data allowed concluding that the Fe/Ti distribution over the two distinct B(1) and B(2) sites was perfectly random within the experimental error. An analysis of Mössbauer spectroscopy data has shown that Fe occupies two octahedral sites with the 51% and 49% ratios, that is, these data were found to be in agreement with the Rietveld refinement. It is noted in [15] that the Mössbauer spectroscopy data on the components distribution over the sites, on the isomer shift and quadrupole splitting allow concluding that both sites occupied by Fe3+ ions are octahedral, though with different degrees of distortion. In [17], the Rietveld technique was applied to refine the structure of n = 6 Aurivillius phases Bi7 Ti3 Fe3 O21 and the experimental data were found to agree well with the supposed random distribution of Ti and Fe over sites.

The most systematic investigation of Fe ions distribution over sites is reported in [18], where the Mössbauer spectroscopy was used to analyze the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 (n = 4, 6, 7 and 8). The obtained results show that only for the eight-layer Aurivillius phases Bi9 Ti3 Fe5 O27 the distribution of Fe3+ over the nonequivalent sites within the experimental error can be regarded as accidental. The n = 4, 6 and 7 Aurivillius phases demonstrated a deviation from the disordered distribution of Fe3+ over sites which is beyond the error limits mentioned by the authors. The performed analysis shows that, as a rule, the nonsystematic nature of the data on cations distribution over the octahedral sites in the perovskite-like block and the inconsistency of data from different published sources do not make it possible to find regularities in the distribution of cations over sites determined by the Aurivillius phases composition and structural parameters. Of interest is the revealing of regularities between cation distribution over nonequivalent sites, structural peculiarities and properties of compounds. Such regularities are easier to reveal by investigating compounds from one and the same homologous series. The present paper considers Fe distribution over the nonequivalent sites in the perovskite-like block for a large group of the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 and analyzes the Fe ions fraction and condition as a function of the octahedral layers number (n) in the perovskite-like block. A relation between stability and structural changes in the Aurivillius phases and the condition of Fe ions and the rate of nonequivalent octahedral sites occupation by them is investigated. 2. Experimental methods Compounds were synthesized from the pure-grade bismuth oxide, analyticalgrade iron(III) oxide and extrapure-grade titanium(IV) oxide. Said oxides were mixed taking stoichiometry of Bin+1 Fen−3 Ti3 O3n+3 (n = 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 8, 9) and of BiFeO3 into account. The initial substances were ground and mixed in a chalcedony vibratory mill with the chalcedony spherical milling body, pressed into pellets and fired in air with a stepwise increase of temperature. After each firing step, the samples were ground in the vibratory mill, repressed and fired at a higher temperature. A detailed description of the technique of the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 and the perovskite-like BiFeO3 synthesis is provided in [19,20]. The phase composition of samples was analyzed by X-ray diffraction (XRD) on DRON 3 diffractometer with CuK␣ radiation. Besides, the phase composition of samples and phases elemental composition were examined by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) using a CamScan MV2300 equipped with an Oxford Link microprobe attachment. The Mössbauer examination of samples employed a WISSEL spectrometer. The measurements were performed in the absorption geometry at room temperature with 57 Cо/Rh source. The isomer shift (IS) values are referenced relative to ˛-Fe. The experimental spectra were folded and fitted by Lorentz functions using the computer program MossFit [21]. The Mössbauer data-based calculation of Fe distribution over the inner B(1) and outer B(2) sites of the perovskite-like block supposed similarity of Mössbauer coefficients for different conditions.

3. Results and discussion The XRD, SEM and EPMA data on samples of the synthesized compounds Bin+1 Fen−3 Ti3 O3n+3 and BiFeO3 are presented in Figs. 2 and 3 and Table 1. Samples 1–7 can be regarded as singlephase ones with a high degree of accuracy. Together with the main synthesized compounds (i.e., Aurivillius phases), samples 9 and 10 show that the Bi2 Fe4 O9 impurity is the present in a small quantity (up to 1%). Along with the main phase of BiFeO3 , sample 11 also contains small quantities of ␥Bi2 O3 and Bi2 Fe4 O9 as impurities. The presence of ␥-Bi2 O3 and Bi2 Fe4 O9 impurities in BiFeO3 produced by the solid state reactions is discussed in [20] and explained by the deviation of the BiFeO3 – based phase from the stoichiometric composition at the change of temperature. It should be noted that small quantities of

N.A. Lomanova et al. / Journal of Alloys and Compounds 528 (2012) 103–108

BiFeO3 γ-Bi2O3 Bi2Fe4O9 Bin+1Fen-3Ti3O3n+3

11

10 9 8 7 6 5 4 3 2 1

20

25

30

35

40

45

50

55

60

2Θ, deg Fig. 2. X-ray diffractograms of samples. The curve number corresponds to the sample number (Table 1).

Fig. 3. Sample 5 SEM image. Dark fields correspond to sample pores.

impurity phases in samples 9–11 cannot have a significant influence on the results of investigations of structural changes in compounds Bin+1 Fen−3 Ti3 O3n+3 that occur with the increase of layers number in the perovskite-like block.

105

The Mössbauer spectra of samples of the synthesized compounds Bin+1 Fen−3 Ti3 O3n+3 and BiFeO3 are presented in Fig. 4. The Mössbauer spectra parameters are given in Table 2. An analysis of the Mössbauer spectra has found the compounds Bin+1 Fen−3 Ti3 O3n+3 to be paramagnetic at room temperature at 3.5 ≤ n ≤ 7 (samples 1–8). Two doublets identified within the Mössbauer spectra of all samples evidence the presence of two nonequivalent sites for Fe ions (FeI and FeII ). It should be noted that the obtained values of the isomer shifts ISFeI , ISFeII and of the quadrupole splittings QSFeI , QSFeII (Fig. 5a and b) are characteristic of Fe3+ ion in the octahedral surrounding. Since the octahedral surrounding of Fe3+ ions by oxygen ions over the outer B(1) sites of the perovskite-like block is less symmetrical than that over the inner B(2) sites, then, QSFeI < QSFeII allows a conclusion that FeI site corresponds to B(1), while FeII corresponds to B(2). A similar conclusion about the Fe3+ ions characteristic in the Aurivillius phases structure was made on the basis of the Mössbauer spectra in [15,18]. The Mössbauer spectra of samples 9 and 10 that correspond to the synthesized n = 8, 9 Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 , have shown the presence of a sextet in addition to two doublets. A sextet evidences that a part of iron is in the magnetically ordered state; its fraction is close to 30% (Table 2), i.e., according to the X-ray diffraction, exceeds the content of impurity phases by over an order of magnitude. In this relation, the magnetically ordered state can be attributed only to iron in the perovskitelike blocks of the Aurivillius phases with a big thickness of the perovskite-like block. The presence of both doublet and sextet in the Mössbauer spectra is apparently associated with the possible irregular mixed-layering of the perovskite-like blocks in Bin+1 Fen−3 Ti3 O3n+3 compounds with a high n value, i.e., with the coexistence of differently thick perovskite-like blocks in the same compound. Such a possibility is confirmed by the widening of Xray diffraction lines in these samples (Fig. 2). It should be noted that sextet parameters in samples 9 and 10 are similar to those of the sextet in the BiFeO3 – based perovskite-like phase (Table 2). A somewhat lower values of the effective magnetic field (Beff ) in the Aurivillius phases in comparison with the perovskite-like BiFeO3 may be due to a size of the perovskite-like block that is smaller than parameters of pure BiFeO3 in these compounds, and also due to the partial replacement of Fe ions by Ti ions in them (Table 2) Based on the ISFeI and ISFeII values it may be concluded that the effective charge of Fe3+ ions in B(1) is smaller than that of Fe3+ ions in B(2), which may be related to a higher concentration of Ti4+ ions that surround Fe3+ ions at B(2). Such a situation is possible if Ti4+ ions preferentially occupy the B(2) site. The occupation of B(1) and B(2) sites by Fe3+ ions (A %) was determined from the doublet lines integral intensities ratio (Table 2). The corresponding values for samples 9 and 10 are given in Table 2 in parentheses. The ordered distribution of Fe3+ and Ti4+ ions over B(1) and B(2) sites is confirmed by a comparison of the Mössbauer spectra with

Table 1 Investigated samples microanalysis data. Sample No.

Nominal composition

1 2 3 4 5 6 7 8 9 10 11

Bi9 Ti6 FeO27 Bi5 FeTi3 O15 Bi11 Fe3 Ti6 O33 Bi6 Fe2 Ti3 O18 Bi13 Fe5 Ti6 O39 Bi7 Fe3 Ti3 O21 Bi15 Fe7 Ti6 O45 Bi8 Fe4 Ti3 O24 Bi9 Fe5 Ti3 O27 Bi10 Fe6 Ti3 O30 BiFeO3

Layers number, n 3.5 4 4.5 5 5.5 6 6.5 7 8 9 ∞

Phase composition, microanalysis data Bi8.9±0.1 Ti5.8±0.2 Fe0.9±0.1 O27 Bi4.9±0.1 Fe0.9±0.1 Ti3.1±0.2 O15 Bi11.1±0.2 Fe2.7±0.3 Ti6.4±0.2 O33 Bi5.8±0.2 Fe1.8±0.2 Ti2.7±0.3 O18 Bi13.4±0.1 Fe4.9±0.2 Ti6.1±0.1 O41.1±0.4 Bi7.4±0.3 Fe3.0±0.2 Ti3.0±0.2 O21.5±0.2 Bi14.7±0.3 Fe6.5±0.5 Ti5.4±0.3 O45 Bi8.0±0.1 Fe4.2±0.2 Ti2.8±0.3 O23.7±0.3 Bi8.9±0.1 Fe4.9±0.1 Ti3.2±0.2 O27 Bi10.1±0.3 Fe5.9±0.1 Ti3.0±0.2 O30.0±0.2 BiFe1.1±0.1 O3.1±0.1

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N.A. Lomanova et al. / Journal of Alloys and Compounds 528 (2012) 103–108 a

b

1.00

1

0.99

1.00

1.00

2

0.98

9

0.98

1.00

3

0.98

4

5

0.98 0.96 1.00 0.98 0.96

6

Relative transmission

Relative transmission

0.96 1.00 0.98 0.96 1.00

1.00

10

0.98

0.96 1.00

1.00 0.98 0.96 1.00 0.98 0.96 0.94

7

11

0.98

8 0.96 -4

-2

0

2

-10

4

-5

Velocity [mm/s]

0

5

10

Velocity [mm/s]

Fig. 4. Mössbauer spectra for samples. The curve number corresponds to the sample number (Table 1).

Table 2 Mössbauer spectra parameters of samples. Sample No.

Layers number, n

Component

Isomer shift, IS ± 0.02 (mm/s)

Quadrupole splitting, QS ± 0.03 (mm/s)

1

3.5

Doublet1 Doublet2

0.37 0.16

0.59 0.64

– –

60 40

2

4

Doublet1 Doublet2

0.37 0.17

0.60 0.58

– –

76 24

3

4.5

Doublet1 Doublet2

0.37 0.14

0.61 0.59

– –

74 26

4

5

Doublet1 Doublet2

0.36 0.27

0.56 0.63

– –

78 22

5

5.5

Doublet1 Doublet2

0.38 0.20

0.57 0.62

– –

74 26

6

6

Doublet1 Doublet2

0.38 0.24

0.57 0.61

– –

74 26

7

6.5

Doublet1 Doublet2

0.38 0.28

0.57 0.64

– –

78 22

8

7

Doublet1 Doublet2

0.38 0.31

0.57 0.61

– –

73 27

9

8

Doublet1 Doublet2 Sextet

0.36 0.35 0.37

0.54 0.85 0.16

– – 47.7

58 (80) 14 (20) 28

10

9

Doublet1 Doublet2 Sextet

0.37 0.37 0.40

0.53 0.78 0.13

– – 47.3

57 (77) 17 (23) 26

BiFeO3

∞

Doublet1 Doublet2 Sextet

0.24 0.36 0.40

0.91 0.41 −0.12

– – 49.5

3 6 91

the Fe3+ ions content at the nonequivalent sites calculated assuming the random distribution of Fe3+ and Ti4+ ions over B(1) and B(2) sites. Corresponding data are presented in Fig. 6 as a dependence of the relative occupation of B(1) and B(2) sites by Fe ions (A %) on the number of layers (n) in the perovskite-like block. The numerical dependencies obtained assuming random distribution of Fe3+ ions over B(1) and B(2) sites are plotted as dotted lines. The A(n) dependencies plotted on the experimental points are represented by solid lines. A comparison of numerical and experimental data

Effective magnetic field, Beff ± 0.3 (T)

Integral intensities ratio, A (%)

on the occupation of B(1) and B(2) sites by Fe ions shows that the content of Ti4+ ions at the outer sites of the perovskite-like block is above the values which could be expected supposing the random distribution of Fe3+ and Ti4+ ions over B(1) and B(2) sites. At the inner sites B(1), concentration of Fe3+ is above the values that may be expected from the random distribution of Fe3+ and Ti4+ over sites. It should be noted that the ordered distribution of Cr3+ ions over sites was observed in [16] for the Aurivillius phases Bi5 CrTi3 O15 . In this case, Cr3+ ions preferentially occupied B(1) sites,

N.A. Lomanova et al. / Journal of Alloys and Compounds 528 (2012) 103–108

а

b

0.45

107

0.90 0.85

0.40

0.80

QS [mm/s]

IS [mm/s]

0.35 0.30 0.25 0.20

0.75 0.70 0.65 0.60

0.15

0.55 0.10

3

4

5

6

7

8

9

3

n

4

5

6

7

8

9

n I

II

Doublet Fe Doublet Fe

Doublet Fe data [18]

II I

Doublet Fe data [18] Sextet Fig. 5. Mössbauer spectra parameters of samples.

Doublet Fe Doublet Fe

A[%]

100 90 80 70 60 50 40 30 20 10 0

B(1) and B(2) sites equalize. Therefore, a shift in electron density in Fe3+ ions, that is determined by the surrounding of Fe3+ ions by Ti4+ ions, should become the same for Fe3+ ions at both B(1) and B(2). When the values of isomer shifts ISFeI and ISFeII converge as n keeps increasing, then the values of effective charges of Fe3+ ions at B(1) and B(2) sites converge, while the values of quadrupole splittings QSFeI and QSFeII depend on n oppositely. QSFeI and QSFeII do not differ much in the 3.5 ≤ n ≤ 4.5 range, while in the case of n ≥ 5 QSFeI decreases sharply and QSFeII remains at the same level (Fig. 5b). It should be noted that the difference between QSFeI and QSFeII values increases almost two times and remains at the same level for all 5 ≤ n ≤ 7. At n > 7, a sharp increase in difference between QSFeI and QSFeII is also observed. However, in such a case, the error of quadrupole splitting parameters increases sharply because of the appearance of sextet lines (due to magnetic ordering) in the Mössbauer spectra of samples. Abrupt changes in symmetry around Fe ions at B(1) should obviously find a reflection in structural parameters of the Aurivillius phases. In this respect, indicative is the dependence of the unit cell volume (V) in Bin+1 Fen−3 Ti3 O3n+3 compounds on n. The V(n) dependence can be presented as two linear dependencies (Fig. 7a), that is V = V0 + V n (V0 = 151.9 ± 0.2, V = 130.7 ± 0.2 for 3.5 ≤ n ≤ 5, and V0 = 28.2 ± 0.3, V = 107.4 ± 0.1 for 5 < n ≤ 9). To analyze similar changes at n ≈ 5, i.e., when the perovskitelike block thickness h is approximately 2 nm, a dependence c˜ = c˜ (n) = h/n (Fig. 7b). At n < 5, a considerable growth of c˜ value at the increase of n is observed, while at n > 5 the growth of c˜ slows down abruptly with the increase of n. In the latter case, c˜ approaches the value of the corresponding parameter of the BiFeO3 – based perovskite-like monoblock (˜c (n) → c˜BiFeO3 = 3.47Å ).

I

I

Doublet Fe data [18]

I II

Doublet Fe data [18] Sextet Doublet Fe 3

4

5

6

n

7

Doublet Fe

9

8

II II

Fig. 6. Fe distribution over B(1) and B(2) sites.

similarly to Fe3+ ions. An analysis of data obtained within the framework of this work and from the published sources shows that when ions are distributed over B(1) and B(2) sites in the ordered manner, the outer B(2) sites of the perovskite-like block are preferentially occupied by ions with a bigger charge. The ordered distribution of ions over B(1) and B(2) sites decreases with the increase in the perovskite-like block thickness. At n ≥ 7, the distribution of Fe3+ and Ti4+ ions over the inner and outer sites of the perovskite-like block tends to become random. The isomer shift value of Fe3+ ions at B(1) practically does not change with the increase of n in the Bin+1 Fen−3 Ti3 O3n+3 homologous series (Fig. 5a). It evidences a weak dependence of the effective charge of Fe3+ ions at B(1) sites on n. ISFeII , the isomer shift value of Fe ions at B(2), grows with the increase of n and approaches the ISFeI value at n = 8–9. Thus, an increase in BiFeO3 content in the perovskite-like block is accompanied by a decrease in the effective charge of Fe3+ ions located at the perovskite-like block inner sites. A possible explanation can be that when the value of n increases up to 8–9, the distribution of Fe3+ and Ti4+ ions over B(1) and B(2) sites becomes close to random, i.e., concentrations of Fe3+ ions at

4

a 800

3.5

ĉ, Å

600 V, Å3

b

3

400

2.5

200

2

3

4

5

6

n

7

8

9

3

4

5

6

n

7

8

9

∞

Fig. 7. Dependence of (a) the unit cell volume (V) and (b) the perovskite-like block size along the c axis on the value of n.

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N.A. Lomanova et al. / Journal of Alloys and Compounds 528 (2012) 103–108

The change in the c˜ (n) dependence at n ∼ 5 (see Fig. 7b) indicates structural changes in the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 . This change correlates with the data on the QSFeI (n) dependence and with those on the stability of Bin+1 Fen−3 Ti3 O3n+3 compounds, which (at n > 5) also show a sharp change in the dependence of the compounds decay temperature on n [22]. Obviously, it may be concluded from the foregoing structural analysis and that of the Mössbauer spectroscopy data that structure of the perovskite-like inner layers changes at n > 5 and makes this block structurally closer to the perovskite-like BiFeO3 , and it apparently causes a reduction in thermal stability of the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 at n > 5. The loss of stability by the Aurivillius phases Bin+1 Fen−3 Ti3 O3n+3 correlates with the distribution of Fe3+ and Ti4+ ions over B(1) and B(2) sites that approaches random. For instance, a compound with n = 9, in which the observed distribution of Fe3+ and Ti4+ ions over the outer and inner sites of the perovskite-like block is the closest to the maximally random, happens to be the last member of homologous series of Bin+1 Fen−3 Ti3 O3n+3 compounds. It should be noted that the perovskite-like block with the disordered distribution of Fe3+ and Ti4+ ions over the outer and inner sites has a thickness of ∼3.5 nm. 4. Conclusion A study of structural peculiarities of the layered perovskite-like compounds Bin+1 Fen−3 Ti3 O3n+3 in the Bi2 O3 –TiO2 –Fe2 O3 system has shown that structural parameters of a perovskite-like block composed mainly of BiFeO3 undergo sharp changes when its thickness increases up to ∼2 nm (n ≈ 5). Said changes correlate with a sharp decrease of the distortion of the octahedral oxygen surrounding of Fe ions at B(1) sites. The Aurivillius phases become unstable when thickness of the perovskite-like block increases up to ∼3.5 nm (n ≈ 9). It correlates with the disordered distribution of Fe3+ and Ti4+ ions over the outer B(2) and inner B(1) sites of the perovskite-like block. In this case, the effective charges of Fe ions at B(1) and B(2) sites equalize. A decrease in the Aurivillius phases stability also correlates with the

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