modulated by crystallographic shear planes.

Transcription

1 Anion-deficient (Pb,Bi) 1-x Fe 1+x O 3-y perovskites modulated by crystallographic shear planes. Artem M. Abakumov, a, * Dmitry Batuk, a Joke Hadermann, a Marina G. Rozova, b Denis V. Sheptyakov, c Alexander A. Tsirlin d, Daniel Niermann e, Florian Waschkowski e, Joachim Hemberger e, Gustaaf Van Tendeloo, a Evgeny V. Antipov b a Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium b Department of Chemistry, Moscow State University, Moscow, Russia c Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institut (PSI), CH-5232 Villigen, Switzerland d Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, Dresden, Germany e Institute of Physics II, University of Cologne, Zülpicher Str. 77, Cologne, Germany RECEIVED DATE TITLE RUNNING HEAD: Anion-deficient (Pb,Bi) 1-x Fe 1+x O 3-y perovskites * EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium, artem.abakumov@ua.ac.be 1

2 Abstract We demonstrate for the first time a possibility to vary the anion content in perovskites over a wide range through a long-range-ordered arrangement of crystallographic shear (CS) planes. Anion-deficient perovskites (Pb,Bi) 1-x Fe 1+x O 3-y with incommensurately modulated structures were prepared as single phases in the compositional range from Pb Bi Fe O to Pb Bi Fe O Using a combination of electron diffraction and high-resolution scanning transmission electron microscopy, a superspace model was constructed describing a periodic arrangement of the CS planes. The model was verified by refinement of the Pb Bi Fe O crystal structure from neutron powder diffraction data ((3+1)D S.G. X2/m( 0 ), X = [1/2,1/2,1/2,1/2], a = (2)Å, b = (2)Å, c = (2)Å, = (3) o, q = (7)a* (6)c* at T = 550K, R P = 0.043, R wp = 0.056). The (Pb,Bi) 1-x Fe 1+x O 3-y structures consist of perovskite blocks separated by CS planes confined to nearly the (509) p perovskite plane. Along the CS planes, the perovskite blocks are shifted with respect to each other over the 1/2[110] p vector that transforms the corner-sharing connectivity of the FeO 6 octahedra in the perovskite framework to an edge-sharing connectivity of the FeO 5 pyramids at the CS plane, thus reducing the oxygen content. Variation of the chemical composition in the (Pb,Bi) 1-x Fe 1+x O 3-y series occurs mainly due to a changing thickness of the perovskite block between the interfaces, that can be expressed through the components of the q vector as Pb 6 +2 Bi Fe 1+ - O The Pb, Bi, and Fe atoms are subjected to strong displacements occurring in antiparallel directions on both sides of the perovskite blocks, resulting in an antiferroelectric-type structure. This is corroborated by the temperature-, frequency-, and field-dependent complex permittivity measurements. Pb Bi Fe O demonstrates a remarkably high resistivity > 0.1 TΩcm at room temperature and orders antiferromagnetically below T N = 608(10)K. 2

3 Introduction BiFeO 3 is one of the rare perovskites combining ferroelectric and magnetic ordering in a single phase material. Being a room-temperature multiferroic, BiFeO 3 is a playground for the investigation of the physical phenomena behind multiferroicity and is also considered as a potential material for spintronic applications, particularly magnetically and electrically accessed memory devices [1]. The main instability resulting in ferroelectricity in BiFeO 3 is driven by a lone pair on the Bi 3+ cation [2, 3], similar to that in PbTiO 3 [4]. Due to the obvious similarity between the electronic structures of Bi 3+ and Pb 2+, the substitution of Pb for Bi provides a potential way to modify the electric and magnetic properties of the mixed lead-bismuth ferrites. In the Bi-rich region of the Bi 1-x Pb x FeO 3 solid solutions, the rhombohedrally distorted R3c perovskite structure is stable up to x = 0.05 and transforms to a cubic Pm 3 m perovskite existing between x = and 0.2 [5]. Both R3c and Pm 3 m phases co-exist in the 0.05<x<0.125 region. Above x 0.3, the appearance of the "Pb 2 Fe 2 O 5 " phase was reported [5]. In the limiting terms of the solid solution, BiFeO 3 and "Pb 2 Fe 2 O 5 ", the oxidation state of iron was proven to be +3 [6, 7]. In order to keep the charge balance, the heterovalent Bi 3+ Pb 2+ replacement should create oxygen vacancies in the structure. At large concentration, strong interaction between vacancies promotes their long range ordering [8]. "Pb 2 Fe 2 O 5 " was for a long time considered as an ordered anion-deficient perovskite with the brownmillerite-type structure where the oxygen vacancies reside in layers of FeO 4 tetrahedra alternating with layers of FeO 6 octahedra [7]. However, we have demonstrated that "Pb 2 Fe 2 O 5 " does not exist as an individual compound, but is in fact an intergrowth of perovskite-based structures, where anion deficiency is adopted by a periodic arrangement of translational interfaces, similar to crystallographic shear (CS) planes [9, 10]. At such interfaces, the blocks of perovskite structure are displaced with respect to each other by a vector 1/2[110] p that creates edge-sharing FeO 5 pyramids along the interfaces, thus reducing the oxygen content. Particularly important for the formation of such 3

4 interfaces is the role of the Pb 2+ cations, which help to relieve the stress at the interface by the spatial localization of their lone pairs [11, 12]. We proposed that the mechanism of accommodation of anion deficiency through CS planes would be common for anion-deficient perovskites containing lone pair A cations. Indeed, similar perovskites with Mn (or mixed Mn and Fe) are also known [13-15]. In this contribution, we demonstrate that the anion-deficient perovskites in the Pb-Bi-Fe-O system (particularly in the large Pb content region) adopt structures with long-range-ordered crystallographic shear planes. A number of single-phase (Pb,Bi) 1-x Fe 1+x O 3-y compounds have been isolated. Their incommensurately modulated structure is described with a generalized superspace model and refined from neutron powder diffraction data for the Pb Bi Fe O composition. We demonstrate that cooperative cation displacements are intrinsic in these compounds and result in an antiferroelectric-type structure. Experimental Section Synthesis. The (Pb,Bi) 1-x Fe 1+x O 3-y samples were prepared by solid state reaction from PbO, Bi 2 O 3, and Fe 2 O 3. The sample compositions are given in Table 1. The initial materials were mixed, ground in an agate mortar under acetone, pressed into pellets, placed into alumina crucibles, and fired in air at 790 o C for 20 h and at 830 o C for 80 h with intermediate regrindings. The samples were furnace cooled. For the neutron powder diffraction experiment, a separate batch of ~ 7 g with the Pb Bi Fe O composition was prepared. Outside the compositional range given in Table 1, a formation of the Bi 1-x Pb x FeO 3- Pm 3 m phase [5] and the "Pb 2 Fe 2 O 5 "-like disordered phase [7, 9] was observed for the Bi-rich and Pb-rich regions, respectively. Powder X-ray and neutron diffraction. Powder X-ray diffraction (PXD) data were collected on a Huber G670 Guinier diffractometer (CuK 1 -radiation, curved Ge monochromator, 4

5 transmission mode, image plate). Powder neutron diffraction (PND) data were collected with the high-resolution powder diffractometer HRPT at ETH Zurich and Paul Scherrer Institut (PSI), Switzerland with the use of a radiation-type furnace at high temperatures up to 700 K. The datasets at T = 700K taken with the wavelengths = , , and Å were simultaneously used for the Rietveld refinement in order to combine a large range of diffraction vectors with sufficient resolution. Crystal structure refinement was performed with the JANA2006 program [16]. Transmission electron microscopy. Electron diffraction patterns and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were obtained on a FEI Tecnai G 2 transmission electron microscope. High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL 4000EX microscope. The cation compositions of the samples were confirmed by EDX analysis (Table S1 of Supporting Information) performed with a JEOL 5510 scanning electron microscope equipped with the Oxford INCA system. Dielectric measurements. The measurements of the dielectric permittivity were carried out in a pseudo-four-probe geometry employing a frequency response analyzer with high-impedance interface (Novocontrol) in a commercial 4 He magneto-cryostat (Oxford) within the temperature range 5 K < T < 300 K and a frequency range between 1 Hz and 1 MHz. In the same temperature range, measurements with an electric field stimulation up to 250 V/mm were carried out using a high-voltage amplifier-module (Novocotrol) in two-probe geometry in order to study possible non-linear response behavior. The samples under investigation were disc-like pellets with a diameter of typically D 5 mm and a thickness of d 1.2 mm. For higher frequencies up to 1 GHz and in temperatures up to 400K a strip-line based transmission setup in a PPMS-System (QuantumDesign) was used. Here the complex permittivity was determined using a LCR-Meter for lower frequencies (Agilent HP4284A) and for higher frequencies by evaluating the transmission-coefficient S 21 with a vector network analyzer (Agilent PNA-X). 5

6 Band structure and electron localization function. Density functional theory (DFT) band structure calculations were performed for a commensurate approximant representing basic features of the incommensurately modulated (Pb,Bi) 1-x Fe 1+x O 3-y structures. The calculations were done in the TB-LMTO-ASA code [17] and cross-checked with the full-potential FPLO approach [18]. A local density approximation (LDA) exchange-correlation potential [19] was supplied with a mean-field (LSDA+U) correction to account for the correlation effects in the Fe 3d shell. Further details of the computational procedure and representative energy spectra are given in the Supporting Information. Results Reciprocal lattice. The electron diffraction patterns of the (Pb,Bi) 1-x Fe 1+x O 3-y samples consist of reflections arranged into linear arrays (Fig. 1). The centers of mass of the reflection arrays roughly correspond to the nodes of a perovskite pseudocubic reciprocal lattice with a p 3.9Å (subscript p denotes the perovskite subcell). The presence of such satellite arrays is a characteristic feature of structures modulated by periodically spaced translational interfaces [20, 21]. Along the translational interface, parts of the basic structure are displaced with respect to each other by a fraction of the lattice translation of the basic structure. The periodic arrangement of such interfaces results in step-like occupational modulations, which, being coupled with strong atomic displacements due to structure relaxation at the interfaces, give rise to a modulated structure where the satellite reflections can be as strong as the main reflections. A body-centered monoclinically distorted perovskite-type lattice of the main reflections was chosen, with a b c a p, the angle close to 90 o and satellite arrays lying in the a*-c* plane. A complete indexation of the ED patterns was performed with the diffraction vector H = ha* + kb* + lc* + mq, q = a* + c*. The parameters determined from the ED data were applied for indexation of the PXD patterns. They can be indexed consistently assuming a presence of satellites up to 3 rd 6

7 order (Fig. 2). The lattice parameters of the basic structure and components of the modulation vector for the (Pb,Bi) 1-x Fe 1+x O 3-y samples are listed in Table 1. The only reflection condition observed from the ED and PXD patterns is h + k + l + m = 2n suggesting the most symmetric superspace group X2/m( 0 ), where X stands for a centering vector [1/2, 1/2, 1/2, 1/2] (B2/m( 0), no [22] in a standard setting). Since the q vector has only and components, the interfaces are confined to the (h0l) p planes, where / = h/l and the spacing between the interfaces is estimated as 2 2 d a p / 2 (the factor 2 reflects that the length of the q vector is inversely proportional to the doubled spacing between the interfaces due to the centered lattice). Decreasing x in the (Pb,Bi) 1-x Fe 1+x O 3-y formula does not significantly change the orientation of the modulation vector, but decreases noticeably the spacing between the satellite reflections (Fig. 1). The h/l ratio stays close to 0.56 for all compositions (Table 1). Thus the orientation of the interfaces with respect to the axes of the basic structure changes only slightly, whereas the thickness of the perovskite blocks between the interfaces increases substantially with decreasing x. High-resolution electron microscopy: the structure of the interfaces. The low magnification HRTEM images demonstrate that the (Pb,Bi) 1-x Fe 1+x O 3-y structures consist of a long-range-ordered sequence of interfaces separating uniformly thick blocks of the parent perovskite structure (Fig. 3). More details on the atomic structure of the interfaces were obtained from HAADF-STEM images (Fig. 4). Projected atomic columns on such images appear as dots of different brightness, roughly proportional to Z 2 (Z the average atomic number along the column). Being viewed along the [010] zone axis (coinciding with a cubic perovskite direction), the brightest dots correspond to the Pb-Bi columns (Z = 82, 83). These dots form a prominent square pattern between the interfaces. Faint dots centering these squares correspond to the projections of the Fe-O columns (Z = 26, 8). Series of HAADF-STEM images for (Pb,Bi) 1- xfe 1+x O 3-y with different x clearly reveal the tendencies expected from the ED study: decreasing x 7

8 increases the thickness of the perovskite blocks, only slightly changing the orientation of the interfaces. Due to the peritectic character of the melting, we did not succeed in growing single crystals suitable for structure analysis. In order to perform Rietveld refinement from powder diffraction data, a starting model is necessary, appropriately describing the occupational and displacive modulation caused by the interfaces. We constructed a generalized description of perovskites modulated by translational interfaces in the (3+1)-dimensional space. There are many examples of perovskite-based layered complex oxides, where the stacking sequence of the atomic layers in the 3D structure is reduced to occupancy modulations imposed on the basic perovskite structure. For such perovskite-based homologous series as the Bi 2 A n-1 B n O 3n+3 Aurivillius series [23, 24], the A n B n O 3n+2 series with {110} p -shaped perovskite blocks separated by extra oxygen layers [25, 26], and the hexagonal perovskites [27, 28], the sequences of atomic layers along the direction of the modulation wave propagation can be easily identified. However, the possibility to describe the (Pb,Bi) 1-x Fe 1+x O 3-y structures with arbitrarily oriented (h0l) p interfaces as a stacking of atomic layers is not obvious. We will describe any arbitrary (h0l) p interface as a combination of short fragments of low-index "parent" interfaces. Looking at the details of the atomic structure of the interface on the HAADF-STEM image (Fig. 5) and at the corresponding arrangement of the cation columns, one can identify that the (h0l) p interface is composed of (001) p fragments of constant length and (101) p fragments of variable length. Assuming these fragments to be uniformly distributed along the interface plane, the interface can be represented as a combination of (101) p fragments with length n, (101) p fragments with length n+1 and (001) p fragments with a length 1. The length here is expressed as the number of repeat periods of the perovskite unit cell along either the [ 1 01] p direction for the (101) p fragments or along the [100] p direction for the (001) p fragments. For the interface in Fig. 5, n is equal to 1. If is the fraction of the (101) p fragments with the length n and 1- is the fraction of the (101) p fragments with the length n+1, the crystallographic orientation of the interfaces is expressed through the average number <n> 8

9 as (<n>, 0, <n>+1). One can demonstrate that the parameters <n> and defining the sequence of the low-index "parent" interfaces and, hence, the structure of the interface, can be expressed through the components of the modulation vector as <n> = and = 1, where {} denote the fractional part of (see p.s3 of Supporting Information for derivation). This allows us to define the parameters of the occupational modulation functions for the atoms in the basic perovskite structure through the components of the modulation vectors (see pp.s3-s10 of Supporting Information for derivation). The set of such expressions (Table S2 of Supporting Information) form a generalized (3+1)-dimensional model of the perovskite structure modulated by the (h0l) p translational interfaces. The idealized chemical composition, calculated from the (3+1)-dimensional model is expressed by the formula A 1- + B The x and y in the (Pb,Bi) 1-x Fe 1+x O 3-y formula have simple relations to the components of the modulation vector: x =, y = 3 +. In the particular case of the Pb-Bi-Fe-O system, an extra restriction is imposed by the requirement to keep the oxidation state of Fe equal to +3. Indeed, both limiting compounds "Pb 2 Fe 2 O 5 " and BiFeO 3 are proven to contain Fe 3+ exclusively [6, 7]. Adjusting the Bi 3+ /Pb 2+ ratio to keep the Fe oxidation state constant, one arrives at the chemical formula Pb 6 +2 Bi Fe 1+ - O This formula is verified by comparison of the bulk sample compositions (Table 1) with the compositions calculated from the components of the modulation vector. Good correspondence was found between the and values, experimental and calculated x in the (Pb,Bi) 1-x Fe 1+x O 3-y formula, and the Bi/Pb ratio (Fig. 6). The Pb Bi Fe O crystal structure. In order to verify the proposed superspace model, a Rietveld refinement of the Pb Bi Fe O structure was performed. The atomic displacements were taken into account with a linear (sawtooth) displacement modulation function with superimposed harmonic modulation waves [29]. No atomic displacements along 9

10 the x 2 axis are allowed by symmetry. A common linear displacement coefficient along the x 1 axis was refined for all atoms because these coefficients were the same in the range of one standard deviation if refined individually. Since all atoms are located at inversion centers, only odd components of the harmonic displacive modulation functions can have non-zero values. The harmonic displacement modulation functions were taken in the orthogonalized form to avoid correlations of their coefficients due to restricted existence intervals of atoms defined by the width of their occupational domains. The relationship between the harmonic and orthogonalized functions is defined in Tables 3 and 4. The value of the modulation function coefficient was set to zero if it does not exceed its standard deviation. The harmonics up to 4 th order were taken into account. However, we realized that including the satellites up to 6 th order noticeably improves the agreement between the calculated and experimental profiles. Probably, the step-like occupational modulations rather than the displacive modulations make the overwhelming impact into the intensities of the higher-order satellites. The acentric subgroups of X2/m( 0 ) were also tested in the refinement. However, removing the inversion center results in strong correlations between the modulation function parameters and does not improve the reliability factors. According to the temperature evolution of the NPD pattern, Pb Bi Fe O is in the antiferromagnetically ordered state at room temperature. The Neel temperature has been estimated from the temperature dependence of the intensity of magnetic reflections as T N = 608(10)K, so that the refinement was performed for the data measured at T = 700K. The scattering lengths of Pb (9.405) and Bi (8.532) are close and this does not allow us to investigate possible ordering of these cations. Random occupation of the A-position with the occupancy factor g(pbbi) = 0.68Pb Bi was assumed. The occupancy factor was calculated from the components of the q vector using the Pb 6 +2 Bi Fe 1+ - O formula. Individual isotropic atomic displacement parameters (ADPs) were refined for all atoms, except for the PbBi position. For this position the ADP was refined in an anisotropic approximation that resulted in a U 22 component noticeably larger than U 11 and U 33. Selected crystallographic data and reliability 10

11 factors for Pb Bi Fe O are provided in Table 2. Atomic positions, ADPs, and modulation parameters are listed in Table 3 and 4. Experimental, calculated, and difference NPD profiles are shown in Fig. 7. Interatomic distances as a function of the internal coordinate t (t = x 4 - qr av ) are shown in Fig. 8 and listed in Table S3 of Supporting information. Dielectric Properties. The dielectric properties of the Pb Bi Fe O sample have been studied by means of the temperature-, frequency- and field-dependent complex permittivity ε*. As it will be discussed below, this compound exhibits an antiferroelectric alignment of the polar components at the interfaces of the perovskite-like building blocks. Fig.9 displays the temperature-dependent complex permittivity ε*(t) and the real part of the conductivity σ =ωε 0 ε for frequencies between 1Hz and 1GHz. The real part of the dielectric permittivity ε (T) increases monotonously towards higher temperatures (Fig. 9a). A weak dispersion can be monitored throughout the complete frequency range under investigation pointing towards the relaxation of polar entities possibly located in domain walls or interfaces, respectively. The dielectric loss ε (T) (Fig. 9b) in this regime is significantly smaller than the real part. Only at higher temperatures depending on frequency an additional relaxation contribution leads to an enhancement of the loss ε (T) and of the conductivity σ (T) (Fig. 9c). This feature is strongly frequency dependent; the step shifts to higher T with increasing frequency. Such behavior could be due to heterogeneities within the sample or at the sample surface. Such interfaces (e.g. between different perovskite blocks, grains or even between sample and electrodes) give rise to resistive contributions together with highly capacitive depletion layers [30]. This results in an effective RC-element in series to the intrinsic resistivity. The effective conductivity is thermally activated conductivity (see Fig. 9c towards higher temperatures) and gives a contribution to the dielectric loss ε ~σ /ω and an additional capacitive contribution to ε. The underlying timeconstant τ=rc is strongly temperature dependent due to the thermally activated semiconducting characteristic of the resistivity. However, for high frequencies (or at low enough temperatures) these additional contributions are shortcut and the bulk properties dominate. The data measured 11

12 at 1GHz do not show such additional contributions and also no other anomalies which could be indicative of any phase transition within the complete temperature range up to 400K. At low temperatures, a relatively high value of ε 20 can be found which rises up to ε 40 at 400K. Although this behavior does not point directly towards the presence of dielectric order, it is compatible with an order scenario setting in at higher temperatures, as suggested by the structural investigations. The overall resistivity of the material is remarkably high: from the lowest frequency measured (1Hz), one can estimate the DC-value of the resistivity to be well above 0.1 TΩcm at room temperature. Such high values for the resistivity could be indicative of a locally polar ordered state. Coupling of the antiferroelectric state to the antiferromagnetic order in this compound can not be evidenced from the results shown in Fig.9. Neither permittivity nor conductivity are influenced by an external magnetic field of 5 T as demonstrated for the 10 khz curves. Fig. 10 displays measurements of the dielectric permittivity in electric fields up to E=200V/mm at temperatures 10K, 250K, and 300K. Only at 300K, where the interface contributions start to appear, a slight electric field dependence of the dielectric loss ε can be found (Fig. 10b). All other data shown in Fig. 10 do not reveal any electric field dependent feature. Also no hysteresis effects can be found in the ε*(e) curves. The absence of nonlinearities (at least for the electric field range E 200V/mm) also could be demonstrated by evaluating the higher harmonic contributions of the polarization response due to harmonic stimulation (see Fig. S6 of Supporting information). While these findings are not compatible with a ferroelectric ground state, an antiferroelectrically ordered state could show linear behavior for electric fields below the critical field at which one sublattice polarization is flipped indeed. In summary, the dielectric characterization of the samples corroborates our conclusion on the antiferroelectric order, derived from the structural study. Electronic structure. Despite the unusual structural organization, the electronic structure of (Pb,Bi) 1-x Fe 1+x O 3-y is rather typical for ferrites. The LDA spectrum (Fig. S5 of Supporting 12

13 Information) reveals Fe 3d states at the Fermi level and O 2p states between 7 and 1.5 ev. Pb orbitals contribute to the bands around 8 ev (6s) and above 2 ev (6p). Similar features have been reported for other transition-metal oxides with Pb 2+ [31,32]. The metallic nature of the spectrum originates from the lack of strong correlations of the Fe 3d shell in LDA. The meanfield treatment of such correlations via the LSDA+U method (on-site Coulomb repulsion and exchange U = 5 ev and J = 1 ev, respectively [33]) opens a gap of 1.0 ev in the energy spectrum (Fig. S5 of Supporting Information). This assigns (Pb,Bi) 1-x Fe 1+x O 3-y to wide-gap semiconductors and supports the high resistivity of our samples. The band gap can be understood in terms of the Mott-Hubbard mechanism which is typical for transition-metal compounds. A comparison of different spin configurations shows that an antiferromagnetic (AFM) order is preferred over the ferromagnetic (FM) one. The energy difference is about 60 mev/fe, i.e., the effective exchange coupling is J ~ 20 K where we assumed 6 couplings per Fe site and spin-5/2 of Fe 3+ (see Supporting information). This energy scale is rather typical for anion-deficient Fe 3+ perovskites [32,34] and corroborates the experimentally observed AFM ordering with the high Neel temperature of 608 K. Discussion A projection of the Pb Bi Fe O structure along the b-axis is shown in Fig. 11. According to the q vector, the interfaces are close to the (509) p plane. The interfaces, marked with arrows in Fig. 11, can be easily discerned by pairs and quadruples of FeO 5 tetragonal pyramids. The pyramids share edges with each other and share corners with the neighboring FeO 6 octahedra of the perovskite blocks. Pyramids and octahedra together form six-sided tunnels along the interfaces, where double Pb,Bi columns are located. Formally the X = [1/2, 1/2, 1/2, 1/2] centering vector implies that the displacement vector for the interfaces should be R = 1/2[111] p. However, the connectivity scheme of the Fe-O polyhedra 13

14 suggests splitting this vector into two parts R = R 0 + R 1, where R 0 = 1/2[110] p and R 1 = 1/2[001]. When the part R 0 is applied to the perovskite structure, this changes the corner-sharing connectivity of the FeO 6 octahedra in the perovskite framework to edge-sharing connectivity of the FeO 5 pyramids at the interface plane. This operation is similar to the shear operation in ReO 3 -type oxides, which induces anion deficiency due to crystallographic shear (CS) planes [35, 36]. Using obvious analogies with the CS planes in the ReO 3 -type oxides (changing connectivity of the metal oxygen polyhedra from corner-sharing to edge-sharing, the ability to adopt variable orientation and to form long-range ordered structures), the interfaces in (Pb,Bi) 1-x Fe 1+x O 3-y can also be classified as CS planes. In the (Pb,Bi) 1-x Fe 1+x O 3-y structures, the CS planes serve as a tool to reduce the oxygen content upon heterovalent Bi 3+ Pb 2+ replacement. The decreasing oxygen content is mainly taken up by the decreasing thickness of the perovskite blocks between the CS planes, whereas the orientation of the CS planes does not vary significantly. The thickness of the perovskite block can be expressed as the average number of FeO 6 octahedra between two successive CS planes along the a axis and can be calculated from the width of the occupational domain where the O(1) and O(2) atoms co-exist as N = Upon decreasing oxygen content, N decreases from 12.1 for Pb Bi Fe O to 4.2 for Pb Bi Fe O (Table 1). This behavior is drastically different from that observed in the (M,W)O 3-x (M = Ti, Nb, Ta) oxides, where not only the spacing between CS planes changes with oxygen content, but also the orientation varies significantly from {102} towards {001} trough {10l} planes with higher l indexes [37]. Another significant difference is the absence of the A-cations in the ReO 3 -type structure and the complete occupation of all available A-positions in (Pb,Bi) 1-x Fe 1+x O 3-y. Therefore, the (Pb,Bi) 1-x Fe 1+x O 3-y structures represent the first example of the possibility to vary the anion content in perovskites over a wide range through a long range ordered arrangement of crystallographic shear planes. 14

15 The R 1 part does not change the connectivity of the iron-oxygen polyhedra and can be considered as a relaxation part, relieving stress introduced by the periodic arrangement of the CS planes and optimizing the interatomic distances at the interfaces. The actual atomic displacements are, however, a result of the combination of R 1 with the displacements imposed by the linear and harmonic modulations. The actual relaxation displacements appear to be smaller than could be expected from the R 1 = 1/2[001] vector. This is easy to see by tracing the mutual displacements along the c axis of the octahedral Fe positions in the perovskite blocks on both sides of the interfaces (Fig. 11). A similar character of the relaxation was also observed for the CS planes in perovskite-type compounds in the Pb-Fe-O system [9, 10]. The basic perovskite structure of Pb Bi Fe O is only slightly distorted at the central part of the perovskite block (Fig. 12a). However, on going along the a axis from the center of the perovskite block towards the interfaces, the perovskite structure acquires a gradually increasing polar distortion: the Pb, Bi, and Fe atoms are cooperatively displaced along the c axis with respect to the oxygen sublattice. The octahedral coordination of the Fe atoms at the center of the perovskite block (d(fe-o(1)) = 2.015(3)Å 2, d(fe - O(2)) = 2.091(9)Å 2, d(fe - O(3)) = (5)Å 2) gradually evolves towards a distorted tetragonal pyramid. One Fe-O(2) distance shrinks to 1.84(3)Å, whereas another Fe-O(2) distance elongates up to 2.70(3)Å (Fig. 8). The Pb and Bi atoms shift from the center of the perovskite cube towards its face, acquiring an 8-fold coordination. These shifts are driven by the lone-pair localization, as confirmed by the attractors of the electron localization function (ELF). No attractors in the vicinity of the Pb6 atom at the central part of the perovskite block are found (in Fig. 12d, this can be seen as the lack of an isosurface near this Pb atom). In contrast, the distorted local environment induces attractors which are evidenced by ELF isosurfaces. The six-sided tunnels confine the lone pairs of Pb1 and Pb8, similar to the structure of Pb 2 Fe 2 O 5 [9]. The polar distortions in (Pb,Bi) 1-x Fe 1+x O 3-y are similar to those observed in the PbTiO 3 and PbVO 3 structures (Fig. 12b). The maximal Fe displacement in Pb Bi Fe O is in 15

16 between the Ti displacement in PbTiO 3 (short and elongated Ti-O distances of 1.77 and 2.39Å, respectively) and V displacement in PbVO 3 (short and elongated V-O distances of 1.67 and 3.00Å, respectively) [31, 38]. However, in contrast to PbTiO 3 and PbVO 3, the off-center displacements in Pb Bi Fe O do not lead to a polar structure. The off-center displacements on the left and right sides of the perovskite blocks are antiparallel and the local dipoles compensate each other resulting in an antiferroelectric-type structure. However, small ferroelectric-type displacements in Pb Bi Fe O could also occur along the b axis. The larger U 22 component of the anisotropic ADP for the PbBi position indicates that the PbBi atom might be randomly shifted from the mirror plane along the b axis. Chemical compositions and temperature conditions which could favor freezing these displacements and result in a polar structure are currently under investigation. It is worth to discuss whether the antiferroelectric-type displacements in Pb Bi Fe O occur solely due to the lone pair activity of Pb 2+ and Bi 3+ or whether they are the consequence of a structure relaxation at the interfaces. A conclusion can be drawn from the comparison of the Pb Bi Fe O and PbBaFe 2 O 5 structures. In the latter structure the interfaces are confined to the (101) p plane and the A-positions in the perovskite blocks are occupied only by Ba 2+ cations, so that the influence of the lone pair can be completely excluded [11]. The (101) p fragments of the interfaces in both structures are shown in Fig. 13 along with the adjacent part of the perovskite block. The polar displacement is very remarkable in Pb Bi Fe O and practically absent in PbBaFe 2 O 5. This strongly suggests that the displacements are promoted by the lone pairs of the A-cations in the perovskite blocks of Pb Bi Fe O Acknowledgment. The authors acknowledge financial support from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference ESTEEM. This work is based on experiments performed at the Swiss 16

17 spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. D.B. and J.Hadermann acknowledge financial support from the Research Foundation Flanders (FWO G N). D.N. and J.Hemberger. were supported by the DFG through SFB608 (Cologne). A.T. acknowledges the funding by Alexander von Humboldt Foundation. Supporting Information Available: The results of the EDX analysis for all samples, derivation of the superspace model, list of the metal-oxygen distances (Å) for Pb Bi Fe O 2.675, temperature dependent dielectric non-linearity for Pb Bi Fe O 2.675, crystallographic information file for Pb Bi Fe O This material is available free of charge via the Internet at References 1. Calatan, G.; Scott, J.F. Adv. Mater. 2009, 21, Hill, N.A. J. Phys. Chem. 2000, 104, Khomskii, D.I. J.Magn. Magn. Mater. 2006, 306, Cohen, R.E. Nature, 1992, 358, Chaigneau, J.; Haumont, R.; Kiat, J. M. Phys. Rev. B 2009, 80, Biran, P.A.; Montano, U.; Shimony, J. Phys. Chem. Solids, 1971, 32, Grenier, J.-C.; Pouchard, M.; Hagenmuller, P. Rev. Chim. Miner. 1977, 14, Stølen, S.; Bakken, E.; Mohn, C.E. Phys. Chem. Chem. Phys., 2006, 8,

18 9. Abakumov, A.M.; Hadermann, J.; Bals, S.; Nikolaev, I.V.; Antipov, E.V.; Van Tendeloo, G. Angew. Chem. Int. Ed. 2006, 45, Hadermann, J.; Abakumov, A.M.; Nikolaev, I.V.; Antipov, E.V.; Van Tendeloo, G. Solid State Sci. 2008, 10, Nikolaev, I.V.; D Hondt, H.; Abakumov, A.M.; Hadermann, J.; Balagurov, A.M.; Bobrikov, I.A.; Sheptyakov, D.V.; Pomjakushin, V.Yu.; Pokholok, K.V.; Filimonov, D.S.; Van Tendeloo, G.; Antipov, E. V. Phys. Rev. B 2008, 78, Abakumov, A.M.; Hadermann, J.; Van Tendeloo, G.; Antipov, E.V. J. Am. Ceram. Soc. 2008, 91, Bougerol, C.; Gorius, M.F.; Grey, I.E. J. Solid State Chem. 2002, 169, Lepoittevin, C.; Hadermann, J.; Malo, S.; Perez, O.; Van Tendeloo, G.; Hervieu, M. Inorg. Chem. 2009, 48, Malo, S.; Lepoittevin, C.; Perez, O.; Hebert, S.; Van Tendeloo, G.; Hervieu, M. Chem. Mater. 2010, 22, Petricek, V.; Dusek, M. The crystallographic computing system JANA2000; Institute of physics: Praha, Czech Republic, Andersen, O.K.; Pawlowska, Z.; Jepsen, O. Phys. Rev. B 1986, 34, Koepernik, K.; Eschrig, H. Phys. Rev. B 1999, 59, Perdew, J.P.; Wang, Y. Phys. Rev. B 1992, 45, Van Landuyt, J.; De Ridder, R.; Gevers, R.; Amelinckx, S. Mater. Res. Bull. 1970, 5,

19 21. Van Dyck, D.; Broddin, D.; Mahy, J.; Amelinckx, S. Phys. Status Solidi A 1987, 103, Janssen, T.; Janner, A.; Looijenga-Vos, A.; de Wolff, P. M. (1999). International Tables for Crystallography, Vol. C, edited by A. J. Wilson, pp Dordrecht: Kluwer Academic Publishers. 23. Boullay, Ph.; Trolliard, G.; Mercurio, D.; M. Perez-Mato, J.M.; Elcoro, L. J.Solid State Chem. 2002, 164, Boullay, Ph.; Trolliard, G.; Mercurio, D.; M. Perez-Mato, J.M.; Elcoro, L. J.Solid State Chem. 2002, 164, Elcoro, L.; M. Perez-Mato, J.M.; Withers, R.L. Acta Cryst. B 2001, 57, Elcoro, L.; Zuniga, F.J.; Perez-Mato, J.M. Acta Cryst. B 2004, 60, Boullay, Ph.; Teneze, N.; Trolliard, G.; Mercurio, D.; Perez-Mato, J.M. J.Solid State Chem. 2003, 174, Perez-Mato, J.M.; Zakhour-Nakhl, M.; Weill, F.; Darriet, J. J. Mater. Chem. 1999, 9, Petricek, V.; Gao, Y.; Lee, P.; Coppens, P. Phys. Rev. B. 1990, 42, Lunkenheimer, P.; Bobnar, V.; Pronin, A.V.; Ritus, A.I.; Volkov, A.A.; Loidl, A. Phys. Rev. B 2002, 66, Shpanchenko, R.V.; Chernaya, V.V.; Tsirlin, A.A.; Chizhov, P.S.; Sklovsky, D.E.; Antipov, E.V.; Khlybov, E.P.; Pomjakushin, V.; Balagurov, A.M.; Medvedeva, J. E.; Kaul, E. E.; Geibel, C. Chem. Mater. 2004, 16,

20 32. Abakumov, A.M.; Hadermann, J.; Batuk, M.; D Hondt, H.; Tyablikov, O.A.; Rozova, M.G.; Pokholok, K.V.; Filimonov, D.S.; Sheptyakov, D.V.; Tsirlin, A.A.; Niermann, D.; Hemberger, J.; Van Tendeloo, G.; Antipov, E.V. Inorg. Chem., doi: /ic101233s. 33. Leonov, I.; Yaresko, A.N.; Antonov, V.N.; Korotin, M.A.; Anisimov, V.I. Phys. Rev. Lett. 2004, 93, Spiel, C.; Blaha, P.; Schwarz, K. Phys. Rev. B 2009, 79, Magneli, A. Acta Chem. Scand , 2, Gado, P.; Magneli, A. Acta Chem. Scand. 1965, 19, Tilley, R.J.D. J. Solid State Chem., 1976, 19, Nelmes, R. J.; Kuhs, W. F. Solid State Commun. 1985, 54,

21 Table 1. Chemical compositions, lattice parameters, components of the modulation vector, h/l ratio for the (h0l) p CS planes and average number N of the FeO 6 octahedra between two successive CS planes along the a axis in (Pb,Bi) 1-x Fe 1+x O 3-y. Formula a, Å b, Å c, Å, o h/l N Pb Bi Fe O (1) (9) (1) (3) (7) (7) Pb Bi Fe O (7) (6) (9) (2) (4) (4) Pb Bi Fe O (9) (8) (1) (3) (5) (6) Pb Bi Fe O (1) (9) (1) (3) (3) (5) Pb Bi Fe O (9) (9) (1) (3) (6) (6) Pb Bi Fe O (1) (1) (2) (3) (6) (8) Pb Bi Fe O (1) (1) (2) (5) (3) (6) Pb Bi Fe O (1) (1) (2) (4) (5) (6) Pb Bi Fe O (1) (1) (5) (3) (6) (6) Pb Bi Fe O (1) (1) (2) (3) (6) (6) Pb Bi Fe O (2) (2) (3) (4) (6) (7)

22 Table 2. Selected parameters from Rietveld refinement for Pb Bi Fe O Formula Pb Bi Fe O Space group X2/m( 0 ) X = [1/2,1/2,1/2,1/2] a, Å (1) b, Å (8) c, Å (1), deg (2) q (4)a* (3)c* V, Å (4) Calculated density, g/cm T, K 700 Radiation Neutron, combined datasets with = , , Å 2 range, step, deg ; 0.05 Parameters refined R P, R wp 0.036, GOF 2.41 For the = Å set: R F (all reflections) R F (main reflections) R F (satellites of order 1, 2, 3, 4, 5 6) 0.036, 0.037, 0.036, 0.034, 0.034,

23 Table 3. Atomic positions, ADPs and modulation parameters for Pb Bi Fe O Atom x 1 x 2 x 0 3 x u 4 0, x u 0, z U eq/iso, Å 2 PbBi* / (1) (1) (8)** U 11 = 0.023(1), U 22 = (8), U 33 = 0.026(2), U 13 = (2), U 12 = U 23 = 0 U x,1 = (6), U z,1 = (4), U x,3 = 0.043(1), U z,3 = (1), U x,5 = 0.019(1), U z,5 = 0.008(1) Fe = u 0, x (PbBi) (5) (2) U x,1 = (4), U z,1 = (3), U x,3 = (5), U z,3 = (5) O(1) 1/ = u 0, x (PbBi) 0.153(2) (9) U x,3 = 0.015(2), U z,3 = (2) O(2) 0 0 1/ = u 0, x (PbBi) 0.129(2) (5) U z,2 = (2) O(3) 0 1/ = u 0, x (PbBi) 0.229(3) (5) * g(pbbi) = 0.68Pb Bi U x,1 = (7), U z,1 = (8), U x,3 = (8), U z,3 = (8) ** U eq 1 / 3 i j U ijai * a j *a i a j The anisotropic displacement factor has the form exp ( 2 2 i j U a * a * h h ) ij i j i j Modulation functions for the parameter defined in a restricted interval are given by the following: U ( x k 4 ) U, northo n ( x4 ) n 0, where the orthogonalized functions, obtained through a Schmidt k k 4 o n 4 n 4 ) n 1 n 1 orthogonalization routine, are given by ortho ( x ) B A sin(2 nx ) B cos(2 nx. i 23

24 Table 4. Coefficients of the orthogonalized functions. ortho i B 0 A 1 A 2 A 3 A 4 PbBi ortho 0 1 ortho ortho ortho Fe, O(3) ortho 0 1 ortho ortho O(1) ortho 0 1 ortho O(2) ortho 0 1 ortho

25 Figures. Figure 1. Electron diffraction patterns of (Pb,Bi) 1-x Fe 1+x O 3-y for x = (a), 0.041(b), (c) and (d). The enlarged rows of satellite reflections with indicated modulation vector are shown at the bottom. Figure. 2. Powder X-ray diffraction pattern of the Pb Bi Fe O sample. 25

26 Figure. 3. [010] HRTEM image of the Pb Bi Fe O compound. The interfaces are visible as equidistant stripes inclined to ~ 45 o to the c-axis. 26

27 Figure 4. HAADF-STEM images of (Pb,Bi) 1-x Fe 1+x O 3-y for x = (a), 0.041(b), (c) and (d). 27

28 Figure 5. Detailed view of the HAADF-STEM image of Pb Bi Fe O at the interface region. Distribution of the cation columns is shown below. The PbBi columns are shown as black spheres (correspond to the brightest dots on the image), the Fe columns are shown as green spheres (corresponding to the less bright dots on the image). Representation of the interface as a combination of the (101) p (green) and (001) p parts is shown. 28

29 Figure 6. Compositional dependence of the modulation vector components based on the sample compositions (Table 1, triangles) and calculated from the Pb 6 +2 Bi Fe 1+ - O formula (squares). 29

30 Figure. 7. Experimental, calculated and difference neutron powder diffraction profiles ( = Å, T = 700K) for Pb Bi Fe O

31 Figure 8. Interatomic Fe-O and PbBi-O distances vs. internal coordinate t (t = x 4 - qr av ) in Pb Bi Fe O Symmetry codes for oxygen atoms: Fe-O distances: O(1): x 1,x 2,x 3,x 4 ; O(2): x 1,x 2,x 3,x 4 ; O(3): x 1,x 2,x 3,x 4 ; O(1) 1 : x 1-1,x 2,x 3,x 4 ; O(2) 1 : x 1,x 2,x 3-1,x 4 ; O(3) 1 : x 1,x 2-1,x 3,x 4 ; O(3) 2 : x 1-1/2,x 2-1/2,x 3 +1/2,x 4 +1/2; O(3) 3 : x 1 +1/2,x 2-1/2,x 3 +1/2,x 4 +1/2; PbBi-O distances: O(1) 1 : x 1-1,x 2,x 3-1,x 4 ; O(1) 2 : x 1-1/2,x 2-1/2,x 3-1/2,x 4 +1/2; O(1) 3 : x 1-1/2,x 2 +1/2,x 3-1/2,x 4 +1/2; O(1) 4 : x 1-1/2,x 2-1/2,x 3 +1/2,x 4 +1/2; O(1) 5 : x 1-1/2,x 2 +1/2,x 3 +1/2,x 4 +1/2; O(3) 1 : x 1-1/2;x 2-1/2;x 3-1/2;x 4 +1/2; O(3) 2 : x 1,x 2-1,x 3-1,x 4 ; O(3) 3 : x 1,x 2,x 3-1,x 4 ; O(3) 4 : x 1 +1/2;x 2-1/2;x 3-1/2;x 4 +1/2; O(3) 5 : x 1-1/2,x 2-1/2,x 3 +1/2,x 4 +1/2; O(3) 6 : x 1 +1/2,x 2-1/2,x 3 +1/2,x 4 +1/2; O(2) 1 : x 1-1/2,x 2-1/2,x 3-1/2,x 4 +1/2; O(2) 2 : x 1 +1/2,x 2-1/2,x 3-1/2,x 4 +1/2; O(2) 3 : x 1 +1/2,x 2 +1/2,x 3-1/2,x 4 +1/2. 31

32 Figure 9. Temperature dependence of the complex dielectric permittivity and conductivity for Pb Bi Fe O The data were measured for frequencies between 1Hz and 1GHz logarithmically spaced by a factor 100. The solid lines display measurements in zero magnetic field, the data presented by open circles (o) was measured in H = 5T at a frequency of 10 khz revealing no significant differences to the zero field results. 32

33 Figure 10. Electric-field dependence of the complex permittivity for Pb Bi Fe O The data was taken applying the DC-bias field E and an AC-stimulus of 1 V rms /mm. 33

34 Figure 11. The Pb Bi Fe O crystal structure projected along the b-axis. The interfaces are marked with arrows. The Pb,Bi atoms are shown as brown spheres. The Fe atoms inside the polyhedra are shown as small orange spheres. 34

35 Figure 12. A perovskite slab in the Pb Bi Fe O structure limited by two six-sided tunnels from both sides (a). Comparison of the atomic displacements in a fragment of the perovskite slab at the close proximity to the interface (b) with the atomic displacements in the PbVO 3 structure (c). ELF isosurfaces ( = 0.75) superimposed on the perovskite slab (d). The numbering of the Pb atoms follows the crystallographic data of the commensurate approximant (Table S4 of Supporting Information). 35

36 Figure 13. Comparison of atomic displacements at the (101) p parts of the interfaces in the Pb Bi Fe O (a) and PbBaFe 2 O 5 structures (b). 36

37 TOC figure. A perovskite slab in the Pb Bi Fe O structure. 37