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Sillen-Aurivillius intergrowth phases as templates for naturally layered multiferroics Samuel Yu Him Liu, Wojciech Miiller, Yun Liu, Max Avdeev, and Chris D. Ling Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 21 Sep 2012 Downloaded from http://pubs.acs.org on September 26, 2012

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Chemistry of Materials

Sillen-Aurivillius intergrowth phases as templates for naturally layered multiferroics Liu, S.,∗,† Miiller, W.,† Liu, Y.,‡ Avdeev, M.,¶ and Ling, C.D.† School of Chemistry, University of Sydney, Sydney 2006, Australia, Research School of Chemistry, Australian National University, ACT 0200, Australia, and The Bragg Institute, ANSTO, PMB 1, Menai 2234, Australia E-mail: [email protected]

Abstract The ferroelectric Sillen-Aurivillius phase Bi5 PbTi3 O14 Cl, a layered structure containing three-layer perovskite-type blocks, has been modified by substituting magnetic transition metal cations M 3+ = Cr3+ , Mn3+ or Fe3+ for

1 3

of the Ti4+ cations, accompanied by co-doping

of Bi3+ for Pb2+ . The aim of the modification was to produce naturally-layered ferroelectromagnetic compounds. Rietveld refinements against high-temperature synchrotron X-ray powder diffraction data show that the resulting new compounds Bi6 Ti2 MO14 Cl undergo noncentrosymmetric (P2an) to centrosymmetric (P4/mmm) ferroelectric phase transitions for Bi6 Ti2 CrO14 Cl at 974.6(2) K, Bi6 Ti2 MnO14 Cl at 913.5(6) K, and Bi6 Ti2 FeO14 Cl at 1044.8(1) K. Ferroelectric properties were measured on Bi6 Ti2 FeO14 Cl using piezoresponse force microscopy which showed typical ferroelectric hysteresis behaviour in the polarisation with varying field strength as well as a piezoelectric strain. Combined Rietveld refinements against X-ray and neutron powder diffraction data indicate a statistical 1:2 distribution of M 3+ and Ti4+ across all ∗ To

whom correspondence should be addressed University of Sydney ‡ The Australian National University ¶ Australian Nuclear Science and Technology Organistion † The

1 ACS Paragon Plus Environment


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three perovskite layers, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi5 PbTi3 O14 Cl) and pronounced spin-glass (cluster glass-type) behaviour below Tirr (0) = 4.46 K that we have characterised by detailed magnetic susceptibility and heat capacity measurements.

Keywords : Sillen-Aurivillius, cluster-glass, multiferroic, neutron diffraction

Introduction For a material to be classified as multiferroic, it must simultaneously exhibit a combination of ferroelectricity, ferromagnetism and/or ferroelasticity. 1 In a true multiferroic, these properties should not only coexist, but interact, creating the potential for applications in new types of switches, sensors, and memory storage devices. The greatest interest lies in combining ferroelectricity (FE) and ferromagnetism (FM) because the reversible domains of such magnetoelectrics could mimic those of magnetic data storage materials. 2,3 Unfortunately, the types of materials in which these ferroic properties arise are often mutually exclusive. Ferroelectricity requires a non-centrosymmetric crystal lattice, while ferromagnetism requires unpaired d (or f ) electrons that usually drive cations towards centrosymmetric environments. One of the rare cases of an oxide that overcomes this paradox is BiFeO3 in which the stereochemically active 6s2 lone pair on Bi3+ creates a ferroelectric distortion below TC (FE) ≈ 1100K while the Fe3+ cations undergo anti-ferromagnetic ordering via Fe-O-Fe superexchange below TN ≈ 640 K. 4,5 BiFeO3 grown epitaxially on SrTiO3 has been shown to exhibit magnetoelectric coupling behaviour. 4,6,7 BiMnO3 also shows magnetoelectric properties but not at room temperature: the promotion of ferroelectric order by the Bi 6s2 lone pair occurs below a TC (FE) of ∼700 K while the ferromagnetic transition temperature is much lower, TC (FM) ≈ 100 K. 8,9 Rare earth manganites RMnO3 , R = rare earth, also show interesting multiferroic behaviour. In these compounds, complex helical magnetic states and Dzyaloshinskii-Moriya interactions are invoked to explain the coexistence and interaction of ferromagnetism and ferroelectricity. 10 HoMnO3 , for example, has 2 ACS Paragon Plus Environment

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been shown to support coupled ferroelectric and ferromagnetic domain ‘switching’. 11 An alternative, arguably more simplistic, approach is to build ferroelectric and ferromagnetic layers into naturally-layered structures. The Aurivillius series of phases show particular potential here. Aurivillius phases consist of n perovskite layers ‘sandwiched’ between α-PbO-type [Bi2 O2 ]2+ layers. The n = 3 phase Bi4 Ti3 O12 for example, is ferroelectric below TC (FE) ≈ 943 K. 12,13 Using these multilayered phases as ‘scaffolds’ for new multiferroic materials, by doping magnetic transition metal ions onto the perovskite B site, has been tried before; e.g., Bi5 Ti3 CrO15 , an n = 4 Aurivillius phase synthesised by Giddings et al., 14 and Bi2-x Sr2+x (Nb/Ta)2+x M 1-x O12 , a series of n = 3 phases synthesised by Sharma et al. 15,16 The n = 3 case shows special promise because the octahedral coordination environment of the B-site cation in the central layer is more symmetric than those of the B-site cations in the outer layers, and should therefore be the preferential doping site for magnetic cations. Sharma et al. 15,16 verified this preference, but could only successfully replace half of the cations in the central layer with magnetic ones. The Sillen-Aurivillius phases are structurally very similar to the Aurivillius phases, but with an additional halide layer between the [Bi2 O2 ]2+ layers. The series can be described as [Bi2 O2 ] [An−1 Bn O3n+1 ] [Bi2 O2 ] [X m ] where n is the number of perovskite layers (A), and m the number of halide layers (X); abbreviated to AnXm. Potential for transition metal doping into the perovskite layers have been demonstrated by earlier researchers on A1X1-type phases 17,18 with AvilaBrande 18 achieving up to

1 3

magnetic transition metal substitution into the single perovskite layer.

These types of compounds have also been shown to adopt non-centrosymmetric space groups exhibiting ferroelectric properties. 19,20 The objective of the present work has been to synthesise transition metal-doped versions of the Sillen-Aurivillius phase Bi5 PbTi3 O14 Cl first reported by Aurivillius 21 and characterised as ferroelectric by Kusainova et al. 22 The n = 3 phase has been chosen for its potential to drive preferential doping of magnetic cations on the central octahedral layer, as demonstrated in n = 3 Aurivillius phases by Sharma et al. 15,16 Substituting stoichiometric amounts of M = Cr3+ , Mn3+ and Fe3+ for Ti4+ in the B site also requires charge compensation which we aimed to achieve



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by replacing Pb2+ with Bi3+ . The synthesis of Bi6 Ti2 FeO14 Cl was reported by Ackerman, 23 however thorough structural and physical property analysis for this A3X1-type and its variants are still lacking. Our goal was to synthesise Bi6 Ti2 CrO14 Cl, Bi6 Ti2 MnO14 Cl and Bi6 Ti2 FeO14 Cl, and investigate their potential as multiferroics.

Experimental Section A ∼5 gram sample of polycrystalline Bi5 PbTi3 O14 Cl was prepared by weighing stoichiometric amounts of Bi2 O3 (Aithaca, 99.999%, dried at 1023 K for 20 hours before use), BiOCl (Sigma Aldrich, 99%), TiO2 (Aithaca, 99.995%), and PbO (Aithaca, 99+%). The reagents were ground to a homogenous powder with a mortar and pestle. Homogeneity was improved by the addition of acetone ground into the mixture. Allowing the acetone to evaporate, the unreacted powder was then pressed into a 20 mm diameter pellet and then broken to ∼6 mm pieces and placed into a 7 mm diameter quartz tube. The tube was evacuated and sealed using an H2 /O2 blowtorch, placed in a muffle furnace and heated to 1093 K for 20 hours yielding a sample of yellow Bi5 PbTi3 O14 Cl pellets. The polycrystalline product was analysed by X-ray powder diffraction (XRD) for purity. The doped series followed a similar procedure. Bi6 Ti2 CrO14 Cl and Bi6 Ti2 FeO14 Cl were prepared with Cr2 O3 (Aithaca, 99.999%) and Fe2 O3 (Aithaca, 99.999%) respectively replacing

1 3

of

the TiO2 and additional Bi2 O3 replacing the PbO. The Cr- and Fe-doped samples underwent an initial heat of 1093 K for 20 hours before being ground, pressed, sealed and heated several times with XRD patterns taken in between each cycle until the sample was as pure as possible. A final annealing was performed with slow cooling; the samples were heated to 1093 K for 12 hours, then cooled to 873 K over 168 hours. The resulting pellets were green-brown in colour for Bi6 Ti2 CrO14 Cl and orange-brown for Bi6 Ti2 FeO14 Cl. Polycrystalline Bi6 Ti2 MnO14 Cl was prepared as above with Mn2 O3 (Aithaca, 99.999%) replacing Cr2 O3 . Unlike the Cr and Fe analogues, the sample was subjected to a single long heat to 1043 K for 100 hours and cooled slowly to 823 K over 168 hours. After heating, the tube was opened and the pellets of black Bi6 Ti2 MnO14 Cl product were retrieved


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and analysed by XRD. X-ray powder diffraction (XRD) was used to monitor the progress and purity of the reactions. XRD data were obtained using a PANalytical X’pert PRO diffractometer in Bragg-Brentano geometry using a sealed-tube source of Cu-Kα radiation (λα1 = 1.5405 Å, λα2 = 1.5443 Å). Data were collected in the range 5° ≤ 2θ ≤ 80° with a step size of 0.1313°. Synchrotron X-ray diffraction (SXRD) data were obtained at the Australian Synchrotron, Melbourne, on the Powder Diffraction beamline. Samples were ground into a very fine powder and placed in 0.2 mm quartz capillaries for high-temperature measurements, or 0.2 mm boron glass capillary for low-temperature measurements. Data were collected in Debye-Sherrer geometry at λ = 0.95296 Å using a cold/hot air blower between 200 K and 1173 K (20 K steps, 5 K/min ramp) with 2 × 120 sec (2 detector positions) acquisition time at each step from 4° ≤ 2θ ≤ 84°. Neutron powder diffraction (NPD) data were obtained at the Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), using the Echidna (high-resolution) diffractometer. Powdered samples were sealed in 8 mm diameter vanadium cans and low, high and roomtemperature data were collected between 3 and 1073 K with 2 hour data collection at each step. Data were collected between 2.75° ≤ 2θ ≤ 164° where λ = 2.4395 Å. The starting models used in this project were adapted from Kusainova et al.’s 22 structure for Bi5 PbTi3 O14 Cl. Rietveld refinements against XRD, SXRD and NPD data were carried out using the GSAS 24 program with the EXPGUI 25 interface. Physical property measurements were carried out on the samples. Magnetisation and magnetic susceptibility were measured using the Vibrating Sample Magnetometer (VSM) option on a Quantum Design Physical Property Measurement System (PPMS). Powdered samples (∼15 mg) were put into a polyethylene container and data were collected in the temperature range 2 K ≤ T ≤ 400 K with fields of up to 9 T. Tests using an alternating magnetic field (AC) with amplitude B0 = 0.0002 T and a range of frequencies (0.1–10 kHz) were used to investigate time-dependent magnetic properties. 26 The sample was placed into a gel capsule and sealed with epoxy glue. Heat capacity measurements were conducted using the relaxation technique in the Heat Capacity option.



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Polycrystalline samples (∼10 mg) were pressed into flat pellets and mounted on the measurement platform using thermal grease. Data were collected in the range of 2 ≤ T ≤ 120 K with a high vacuum maintained throughout the experiment. A piezoresponse force microscope (PFM) based on an atomic force microscope system (Cypher, Asylum Research) was used to identify the existence of ferroelectricity in this Sillen-Aurivillius phase compound. To apply the voltage to the sample, a platinum-coated silicon cantilever (AC240TM) was used as a conductive tip, which has a force constant of 2 N m−1 , a resonant frequency of 70 kHz, and a tip radius ∼28 nm. The driving frequencies were chosen near the contact resonance to boost the piezoresponse (PR) signals via a dual-frequency resonance-tracking technique. 27 During the hysteresis loop measurements, an AC voltage was superimposed onto a triangular squarestepping wave (f = 0.2 Hz and bias of ±30V). The sample surface was well polished before PFM characterisation.

Results and discussion Rietveld refinements against XRD data(Figure 1) show that these samples were largely pure with only a very small percentage (∼2%) of impurities. We have tentaively identified Bi4 Ti3 O12 and Bi24 Cl10 O31 , but as these have large unit cells and are present in insufficient quantities to be included in the refinements, these are not definative assignments. Comparably small quantities of transition metal-rich impurities are presumably also present, but do not contribute significant diffraction intensity. Cell parameters, metal atom positions, and atomic displacement parameters (ADPs) (which were globally constrained) were refined.


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λ = 1.5405 Å, 1.5443 Å M = Mn(III)

λ = 1.5405 Å, 1.5443 Å M = Cr(III)

Intensity (a.u.)

Intensity (a.u.)

λ = 1.5405 Å, 1.5443 Å Bi5PbTi3O14Cl

7

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(°)

λ = 1.5405 Å, 1.5443 Å M = Fe(III)

(°)

Figure 1: Final room-temperature sealed-tube Cu-Kα X-ray Rietveld refinements for Bi5 PbTi3 O14 Cl, Bi6 Ti2 CrO14 Cl, Bi6 Ti2 MnO14 Cl and Bi6 Ti2 FeO14 Cl in the P2an space group (#30). Data points are represented as red crosses with the calculated fit as the overlaying solid line. Reflection-markers and the difference curve are provided under each fit. ACS Paragon Plus Environment


Sequential Rietveld refinements of SXRD patterns were run for each temperature step allowing only the cell parameters and global ADPs to refine. According to Kusainova et al., 22 Bi5 PbTi3 O14 Cl transforms from the orthorhombic P2an space group symmetry to the tetragonal P4/mmm. The same transformation was seen in our high-temperature SXRD data (Supplementary Figure S1). Higher temperature refinements were adequately refined using this tetragonal space group. The orthorhombic distortion ( ab − 1) was used as an order parameter to determine the P2an to P4/mmm transition temperature (Figure 2). A free fit of

a b

− 1 = C(TC − T )1/B yielded values

of B close to 4 for all three compounds, which was subsequently fixed to an integer value of 4 according to the expectations of Landau theory. 28 The fits then yielded TC (FE) values of 974.6(2) K for Bi6 Ti2 CrO14 Cl, 913.5(6) K for Bi6 Ti2 MnO14 Cl, and 1044.8(1) K for Bi6 Ti2 FeO14 Cl. These are all higher than for the undoped parent compound Bi5 PbTi3 O14 Cl, for which TC (FE) = 863 K. The observed trend in TC (FE) (Bi6 Ti2 MnO14 Cl < Bi6 Ti2 CrO14 Cl < Bi6 Ti2 FeO14 Cl) does not

Bi6Ti2CrO14Cl

0.0070

Bi6Ti2MnO14Cl Bi6Ti2FeO14Cl

0.0060 0.0050

(a/b) -1

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0.0040 0.0030 0.0020 0.0010 0.0000 700

750

800

850

900

950

1000

1050

T (K)

Figure 2: Plot of orthorhombic distortion for Bi6 Ti2 CrO14 Cl, Bi6 Ti2 MnO14 Cl and Bi6 Ti2 FeO14 Cl as a function of temperature. follow atomic number (Cr < Mn < Fe). This may be due to Jahn-Teller effects in the high-spin d states of these M 3+ ions. The Mn3+ sample (d 4 ) should have a significant Jahn-Teller distortion compared to Cr3+ (d 3 ) and Fe3+ (d 5 ), giving it a lower TC (FE) than the Cr and Fe samples. In other ferroelectric systems, it has been suggested 13 that the degree of octahedral tilting calculated


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from the tolerance factor corresponds to relative shifts in TC (FE), where an increase in t leads to a decrease in TC (FE). However, this does not appear to be the case here, given that the effective ionic radii of high spin Mn3+ and Fe3+ ions in octahedral environments are very similar (∼0.645 Å). 29 Neutron powder diffraction (NPD) data are more sensitive than XRD data to relatively light O and Cl atoms in the presence of heavy metals such as Bi, and therefore provide better structural information for compounds such as these. However, our refinements were subject to some unusual limitations. For M = Fe3+ , the positions of the B site cations could not be well determined by NPD data alone because the weighted average neutron scattering length of 32 Ti (B = -3.438 fm) and 31 Fe (B = 9.45 fm) is 0.858 fm, which is much smaller than the scattering lengths of O (5.805 fm), Bi (8.532 fm) and Cl (9.579 fm). 30 Combined room-temperature refinements against both XRD and NPD data were therefore used in structure determination for all the doped compounds. In the case of M = Mn3+ , similarities in scattering lengths (B(Mn) = -3.73 fm, B(Ti) = -3.438 fm) led to unrefinable fractional occupancy parameters, which were therefore fixed at 13 M, 23 Ti across both sites. Because of these issues, as well as the presence of local disorder associated with the mixed nature of the B site, reliable bond-valence sums (BVS) could not be obtained. Nevertheless, Rietveld refinements against room temperature data clearly supported our structural models. Refinement of global atomic displacement parameters (ADPs), sample-dependent Lorentzian profile parameters, background, scale, and zero-offset gave adequate fits and models. The fractional occupancies of Bi6 Ti2 FeO14 Cl and Bi6 Ti2 CrO14 Cl refine very closely to the

1 3

distribution and this random distri-

bution was therefore assumed for Bi6 Ti2 MnO14 Cl. Refined parameters and R-factors are reported in Tables 1, 2 and 3. The final refined structure of Bi6 Ti2 CrO14 Cl is shown in Figure 3 and the corresponding Rietveld fit in Figure 4. The combined Rietveld refinements yielded an essentially random distribution of magnetic cations across the three perovskite layers. This is different to the case in the doped n = 3 Aurivillius phases Bi2-x Sr2+x (Nb/Ta)2+x M 1-x O12 studied by Sharma et al. 15,16 who found all the magnetic dopants ordered into the central perovskite layer. The driving force behind ordering in that case was the size of the dopant, M 4+ , which was significantly smaller than Ti4+ , even at higher temper9 ACS Paragon Plus Environment


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c a

b

Figure 3: Final room-temperature refined structure for Bi6 Ti2 CrO14 Cl. Bi sites are purple, Cl sites are green, O sites are red, and mixed Ti/Cr sites are proportionally black/blue.


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Intensity (a.u.)

X-ray diffraction λ = 1.5405 Å, 1.5443 Å Bi6Ti2CrO14Cl

(°)

Neutron diffraction λ = 2.4395 Å Bi6Ti2CrO14Cl

Intensity (a.u.)

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(°)

Figure 4: Final combined Rietveld fits to room-temperature sealed-tube XRD (Cu-Kα ) and NPD (λ = 2.4395 Å) data for Bi6 Ti2 CrO14 Cl in the P2an space group (#30). Data points are represented as red crosses with the calculated fit as the overlaying solid line. Reflection-markers and the difference curve are provided under the fit.



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Table 1: Final refined parameters for Bi6 Ti2 CrO14 Cl. Space group P2an (#30), a = 5.4937(2), b = XRD 5.4741(2), c = 22.0913(8) Å, V = 664.35(6)Å3 . Rtotal = 0.0535, Rtotal p wp = 0.0749, RBragg = 0.0337, 2 RNPD Bragg = 0.0761, GOF [χ ] = 2.609. Site x(a) y(b) z(c) Bi1 0.983(5) 0.0106(17) 0.41188(13) Bi2 0.995(5) 0.5066(16) 0.09390(13) 0.035(5) 0.5205(11) 0.29608(12) Bi3 Ti/Cr1 1.000(12) 0 0 Ti/Cr2 0.486(7) 0.508(8) 0.8200(4) O1 0.921(5) 0.051(5) 0.0909(9) O2 0.958(6) 0.963(5) 0.2633(6) 0.673(6) 0.243(4) 0.9956(12) O3 O4 0.248(8) 0.245(5) 0.3602(9) O5 0.149(6) 0.328(4) 0.1873(9) 0.240(8) 0.232(5) 0.8439(8) O6 O7 0.729(7) 0.737(5) 0.3680(9) 1 1 Cl 0 2 2

100Uiso (Å2 ) Occ. 1.75(10) 1 1.75(10) 1 1.75(10) 1 1.75(10) 0.60(3)/0.40(3) 1.75(10) 0.699(14)/0.301(14) 3.12(17) 1 3.12(17) 1 3.12(17) 1 3.12(17) 1 3.12(17) 1 3.12(17) 1 3.12(17) 1 3.12(17) 1

Table 2: Final refined parameters for Bi6 Ti2 MnO14 Cl. Space group P2an (#30), a = 5.5150(4), b = XRD 5.4939(4), c = 22.0436(13) Å, V = 667.89(9)Å3 . Rtotal = 0.0730, Rtotal p wp = 0.1088, RBragg = 0.0663, 2 RNPD Bragg = 0.0932, GOF [χ ] = 4.827. Site Bi1 Bi2 Bi3 Ti/Mn1 Ti/Mn2 O1 O2 O3 O4 O5 O6 O7 Cl

x(a) y(b) z(c) 100Uiso (Å2 ) Occ. 0.986(6) 0.013(3) 0.4124(2) 1.84(16) 1 0.990(6) 0.513(3) 0.0951(2) 1.84(16) 1 0.018(7) 0.5215(19) 0.2969(2) 1.84(16) 1 2 1 0.931(9) 0 0 1.84(16) 3/3 2 1 0.447(9) 0.499(9) 0.8177(7) 1.84(16) 3/3 0.884(6) 0.049(6) 0.0947(14) 2.7(3) 1 0.926(7) 0.959(7) 0.2609(10) 2.7(3) 1 0.635(8) 0.256(4) 0.999(2) 2.7(3) 1 0.241(10) 0.252(7) 0.3722(14) 2.7(3) 1 0.075(10) 0.337(5) 0.1874(12) 2.7(3) 1 0.240(10) 0.222(6) 0.8329(12) 2.7(3) 1 0.725(9) 0.705(6) 0.3594(15) 2.7(3) 1 1 1 0 2.7(3) 1 2 2


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Table 3: Final refined parameters for Bi6 Ti2 FeO14 Cl. Space group P2an (#30), a = 5.5186(2), b = XRD 5.4851(2), c = 22.1800(7) Å, V = 671.39(6)Å3 . Rtotal = 0.0518, Rtotal p wp = 0.0706, RBragg = 0.0500, 2 RNPD Bragg = 0.0884, GOF [χ ] = 2.404. Site x(a) y(b) z(c) Bi1 0.984(4) 0.0108(17) 0.41252(12) 0.997(5) 0.5125(19) 0.09440(11) Bi2 0.034(4) 0.5091(16) 0.29727(10) Bi3 Ti/Fe1 0.015(11) 0 0 0.515(8) 0.8185(4) Ti/Fe2 0.506(7) O1 0.931(5) 0.066(4) 0.0913(8) 0.957(6) 0.962(4) 0.2683(6) O2 O3 0.675(5) 0.233(4) 0.0048(10) 0.239(7) 0.236(4) 0.3603(9) O4 O5 0.128(6) 0.345(4) 0.1868(8) O6 0.251(7) 0.268(5) 0.8403(7) O7 0.720(6) 0.766(5) 0.3662(8) 1 1 Cl 0.0 2 2

100Uiso (Å2 ) Occ. 2.34(10) 1 2.34(10) 1 2.34(10) 1 2.34(10) 0.726(15)/0.274(15) 2.34(10) 0.637(8)/0.363(8) 2.89(17) 1 2.89(17) 1 2.89(17) 1 2.89(17) 1 2.89(17) 1 2.89(17) 1 2.89(17) 1 2.89(17) 1

atures above TC (FE). A plausible explanation for disordered doping in the present case is that M 3+ have very similar ionic radii to Ti4+ . Synthesis was carried out above the TC (FE) for all doped compounds, where there was no driving force for ordering of magnetic and non-magnetic cations. A long heating cycle at temperatures just below TC (FE) might address this issue. This was attempted, but other than a slight reduction in the intensities of the minor impurity peaks, no change was observed by XRD. Another approach, presently being explored, is to attempt to synthesise these compounds at lower temperature using alternate chimie douce or hydrothermal methods. Locally, the mixture of magnetic and non-magnetic cations leads to a highly strained structure below TC (FE) with competition between high symmetry and distorted bond lengths for magnetic and non-magnetic cations. Figure 5 shows a local model of a random mixture of Fe and Ti cations in Bi6 Ti2 FeO14 Cl. Structural strain would arise in this model due to clusters of FeO6 octahedra in high symmetry environments and TiO6 in more distorted environments. This clustering of magnetic cations is responsible for the spin glass behaviour observed in magnetic and heat capacity measurements described below.



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c b

a

Figure 5: A local model of the perovskite tri-layer (Bi cations not shown) illustrating a statistical distribution of Fe cations in the B sites. FeO6 octahedra are brown and TiO6 are blue.

Magnetic properties The temperature dependence of the magnetic susceptibility χ(T ) and its reciprocal collected in magnetic field of B = 1 T for all three compounds is depicted in Figure 6. At high temperatures χ(T ) of all samples obeys Curie-Weiss (CW) law (shown in red):

χ(T ) =

2 µeff 8(T − ΘCW )

(1)

where µeff stands for an effective magnetic moment and ΘCW for the characteristic Curie-Weiss temperature. All these values are collected in Table 4. The obtained values of effective moment Table 4: Curie-Weiss fit results for Bi6 Ti2 MO14 Cl compounds and theoretical values of spin-only exp th ) are theoretical values. effective moments for comparison, where (µeff ) are experimental and (µeff M Cr Mn Fe

exp th (µ ) Θ µeff (µB ) µeff B CW (K) 3.50 3.38 -78.53 4.75 4.9 (HS) 39.3 6.75 5.92 -427

for Cr and Mn compounds are close to those predicted using µeff = 2(S(S + 1))0.5 µB for d 3 and high-spin d 4 configurations respectively. The effective moment value obtained for the Fe sample is somewhat higher than theoretically expected for d 5 ions. Negative characteristic ΘCW temperatures in Cr and Fe samples point to the domination of antiferromagnetic coupling in these materials, 14 ACS Paragon Plus Environment

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(a)

(b)

(c)

Figure 6: Magnetic susceptibility and its reciprocal collected for (a) Bi6 Ti2 FeO14 Cl, (b) Bi6 Ti2 CrO14 Cl and (c) Bi6 Ti2 MnO14 Cl. For details see the text.



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whereas ΘCW > 0 in Mn sample suggests FM coupling between Mn ions. As no anomaly was found in χ(T ) of Cr and Fe samples at B = 1 T magnetic field, the Mn sample reveals some sort of magnetic transition at T = 45 K. However, because this sample was found to be strongly inhomogeneous (based on XRD and NPD results), one cannot exclude the possibility that this anomaly originates from magnetic impurities. No distinct anomalies were found in the magnetic susceptibility of Cr and Fe samples, suggesting that they are paramagnetic down to 2 K. However, strongly nonlinear behaviour of the reciprocal susceptibility below 100 K and 300 K in Cr and Fe samples, respectively, may suggest a more complex magnetic state than simple paramagnetism. To shed more light on the magnetic properties of Bi6 Ti2 CrO14 Cl and Bi6 Ti2 FeO14 Cl, we measured magnetic susceptibility at lower magnetic fields. We found characteristic field cooling (FC) zero field cooling (ZFC) branching in the Fe sample, an obvious hallmark of a magnetic transition, whereas the Cr sample did not reveal any anomaly at magnetic fields down to 0.002 T and 2 K temperature (not shown).

Susceptibility measurements on the Fe-doped compound Based on the crystal structure, Bi6 Ti2 FeO14 Cl is not a magnetically dense system - the (Ti,Fe)O4 plane seems to be strongly disordered, with only

1 3

of available transition metal positions occupied

by Fe3+ ions. One should then expect a ground state with short-range rather than long-range magnetic order. To test if Bi6 Ti2 FeO14 Cl is simply paramagnetic, superparamagnetic or shows spinor cluster-glass behaviour we conducted a series of DC and AC magnetic measurements. Figure 7 shows examples of temperature dependencies of the magnetic susceptibility at low temperatures and several different magnetic fields. The most obvious feature is the (FC)-(ZFC) susceptibility branching, a hallmark of magnetic irreversibility. As one can see, the irreversibility is suppressed with magnetic field, shifted below 2 K for B ≥ 1 T. The down-arrows indicate the strongly fielddependent characteristic temperature Tirr , the so called irreversibility temperature, associated with the onset of the blocking of magnetically ordered clusters/spins by the formation of a spin-glass state. Tirr depends strongly on applied magnetic field, magnetic cluster size, anisotropic properties


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(a.u.)

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Figure 7: Low temperature susceptibility of Bi6 Ti2 FeO14 Cl collected at various fields. Right and left arrows indicate FC and ZFC conditions, respectively. Down-arrows mark the irreversibility temperature Tirr and up-arrows indicate Tmax .



of the material and measurement time. According to the Thouless - De Almeida model of spin glass transition in the mean field approximation 31 it should follow the relation Tirr = Tirr (0) + β B2/3 . To estimate Tirr we used the 1%

4.5

4.0

Tirr (K)

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3.5

3.0

2.5

2.0 0.0

0.2

0.4

B

2/3

0.6

0.8

2/3

(T

)

Figure 8: Thouless-de Almeida scaling of Tirr (B2/3 ). For details see the text. criteria, i.e., we take the point in Tirr where the relative difference between χFC and χZFC is 1%. As shown in Figure 8, Bi6 Ti2 FeO14 Cl follows the above relation with zero-field freezing temperature Tirr (0) = 4.46 K and β = 12.78 KT−2/3 . Based on the latter analysis, one can calculate a frustration parameter f =| ΘCW | /Tirr (0)= 96. The extremely large value of this empirical parameter points to the presence of very strong magnetic frustration in Bi6 Ti2 FeO14 Cl. The second characteristic temperature, Tmax , is given by the maximum in the ZFC branch of magnetic susceptibility (marked with an up-arrow in Figure 7), observed below Tirr . Contrary to spin-glass systems, in cluster glasses a distribution of dipole moments due to the cluster size is very often present. As exchange interactions between magnetic clusters depend on the size of magnetic moment, its distribution leads to a distribution of freezing temperatures on a microscopic scale, which broadens and extends the ZFC maximum. The irreversibility temperature Tirr then points to the start of the freezing process and Tmax indicates when most of the clusters are blocked. With increasing magnetic fields, Tmax shifts towards lower temperatures. Although the presence 18 ACS Paragon Plus Environment

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of a Tmax feature in χZFC (T ) in the low fields, evolving into a broad maximum at higher fields, below Tirr is a typical feature of both spin glass classes, a monotonic increase of χFC (T ) below Tirr observed at the lowest fields is characteristic for a cluster-freezing transition. 32,33 Figure 9 shows magnetic field dependencies of isothermal magnetisation M(B), collected at various temperatures. At low temperatures we observe a typical Brillouin function-like dependence of the magnetisation, but with very low induced magnetic moments, M(2K,9T)= 0.94 µB per Fe3+ ion. This is a much smaller value than expected for independent S = 5/2 systems (5 µB ), due to very strong antiferromagnetic coupling in the system. Linear M(B) dependence is recovered at temperatures above 40 K. The opening of the hysteresis loop below 4 K is another cluster glass-like feature, observed in Bi6 Ti2 FeO14 Cl. The hysteresis loop is narrow, with very low remnant magnetisation MR (2K)= 0.01 µB /Fe and coercive field Bc = 0.04 T. These observations classify Bi6 Ti2 FeO14 Cl as a very soft magnetic material, which is a common feature of spin glasses.

/f.u.)

0.04

M (

B

1.00

0.75

0.02

T=2 K

0.00

-0.02

/f.u.)

-0.04 -0.10

-0.05

T

B

M (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.50

0.00

B (T)

0.05

0.10

0.25

0.00

0

1

2

3

4

5

6

7

8

9

B (T) Figure 9: Field dependencies of magnetisation M collected at various temperatures. In the inset is shown the opening of hysteresis loop at the lowest temperatures. One of the most striking experimental features of a spin glass system is the time-dependence of magnetisation. 34 For the measurement of time-dependent remnant magnetisation (MR ), a powdered sample was zero-field-cooled from 150 K to the desired temperature. A magnetic field of 2 T was then applied for 10 min before being switched off. The time at which the field was switched off 19 ACS Paragon Plus Environment


is taken as t = 0 and the decay of MR was recorded as a function of time. Figure 10 shows the time dependence of MR collected at 2 and 3 K, well below Tirr . The reduction of magnetisation with time is very distinct. Many functions have been proposed to describe time decay of remnant magnetisation, but in this case we found the most suitable form to be the one with logarithmic and exponential terms:

-2

10

(

B

/Fe)

T = 2K

M

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T = 3K -3

10

10

100

1000

10000

t (s)

Figure 10: Time-dependent remnant magnetisation, collected at 2 and 3 K temperature. Solid line is a fit to Equation 2. −t

MR = MR (0) + α ln(t) + β e( τ

)

(2)

A least-square fit yields MR (0) = 0.0081 µB , α = −4.7 · 10−4 µB , β = 4.75 · 10−4 µB , τ = 299.3 at T = 2 K and MR (0) = 0.0015 µB , α = 7.2 · 10−5 µB , β = 1.2 · 10−4 µB , τ = 515 s at T = 3 K. According to Fischer and Hertz 35 magnetic relaxation effects may be explained based on the model assuming many possible magnetic configurations separated by barriers of varying heights in spin glass systems. AC susceptibility measurements can be used to investigate the dynamical properties of spin glass systems. Figure 11 shows the real part of magnetic susceptibility χ 0 measured with different frequencies ω/2π. The maximum in χ 0 (T ), is related to the cluster blocking (‘freezing’) phenomena, T f , ∼3.4 - 3.6 K, close to Tmax in χZFC collected at the lowest magnetic field (0.002 T). T f shifts towards lower temperatures as frequency increases, which is typical for materials with 20 ACS Paragon Plus Environment

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1.00 0.100 kHz 0.333 kHz 0.576 kHz 1.000 kHz

0.95 max

3.330 kHz 5.760 kHz

'/ '

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10.00 kHz

0.90

3.0

3.5

4.0

4.5

5.0

T (K) Figure 11: Temperature dependencies of the real component of ac susceptibility, χ 0 , collected at various frequencies. short-range magnetic order. In spin glasses, the frequency dependence of T f follows the empirical Vogel–Fulcher formula: 36 ω = ω0 exp

−Ea T f − T0

(3)

where ω0 /2π is the intrinsic relaxation frequency, Ea is the activation energy and T0 is the characteristic Vogel-Fulcher temperature. When all three parameters were varied freely, we did not obtain a good fit, so we fixed the characteristic frequency. As ω0 /2π varies from 108 in clusterglasses to 1013 Hz in spin-glass systems, 32 moderate ω0 /2π = 1010 Hz was a reasonable choice. The least-square fit shown in Figure 12(a) yields Ea /kB = 12 K and T0 = 2.72 K. The relative shift of the freezing temperature per frequency decade, δ T f = ∆T f /(T f ∆ log(ω/2π)) = 0.032, is much less than 0.1 (the value predicted for superparamagnets), but in agreement with 0.004 - 0.05 range typical for spin- and cluster-glass systems. While empirical Vogel–Fulcher efficiently describes the Bi6 Ti2 FeO14 Cl frequency-dependent magnetic response, dynamic scaling theory 37 is in general considered to be most appropriate to describe a spin glass transition. 38 The conventional result of dynamical scaling relates the critical relaxation time τ0 to the correlation length ξ as τ = τ0 ξ zν . τ0 is a microscopic flipping time of spins related to the characteristic frequency ω0 /2π , being the



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(a)

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(b)

Figure 12: (a) Vogel-Fulcher and (b) dynamical scaling theory fits for ω/2π(T f ). An arbitrary value of ω0 /2π = 1010 Hz was used. shortest time-scale in the system, z is the dynamical exponent and ν is the usual critical exponent for the correlation length. Figure 12(b) shows the fit to a model representing the conventional critical slowing down given by the expression:

ω/2π = ω0 /2π(

T f − Tg zν ) Tg

(4)

where Tg is the ‘true’ spin glass transition temperature, according to dynamical scaling theory. For the same ω0 /2π = 1010 Hz, the least-square fit yields zν = 7.3 and Tg = 3.13 K. The values of zν falls into the typical range for spin glasses, 7 - 12. One should note that zν = 2 and zν = 4 is predicted for 3D and 2D Ising systems undergoing phase transitions to long-range-ordered ground states at T = 0.

Heat capacity measurements Of the three reported Bi6 Ti2 MO14 Cl comsapounds, only Bi6 Ti2 FeO14 Cl seems to have an ordered ground state. To shed more light on the cluster-glass properties we decided to perform heat capacity measurements on Bi6 Ti2 FeO14 Cl and as a reference, Bi5 PbTi3 O14 Cl was treated as a nonmagnetic isostructural counterpart (without d electrons). The nonmagnetic compound Bi5 PbTi3 O14 Cl shows 22 ACS Paragon Plus Environment

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typical heat capacity behaviour – a monotonic decrease with decreasing temperature – as shown in Figure 13. Below T = 7 K one can fit C p to the formula: C p (T ) = γT + β T 3 + δ T 5

(5)

yielding γ = 0, β = 1.38 · 10−3 J mol K−4 and δ = 5.78 · 10−5 J mol K−6 . The first linear term stands for electronic contribution. Its zero value points to the nonmetallic character of Bi5 PbTi3 O14 Cl, whereas β and δ describes the lattice fraction of specific heat. According to the Debye model 39 in the low temperature limit, β is related to the characteristic Debye temperature ΘD by the relation: β=

12π 4 Rn 5Θ3D

(6)

where R and n stands for the gas constant and the number of atoms per formula unit. This yielded a value for Bi5 PbTi3 O14 Cl ΘD as 323.4 K, a reasonable value for a complex oxide.

Bi5PbTi3O14Cl Bi6Ti2FeO14Cl

Figure 13: Temperature dependence of the heat capacity C p collected for Bi6 Ti2 FeO14 Cl and Bi5 PbTi3 O14 Cl. Solid line is a fit to Equation 5. The low temperature heat capacity of Bi6 Ti2 FeO14 Cl behaves quite differently to its nonmagnetic counterpart. An additional contribution of magnetic origin emerges at temperatures below



10 K, as shown in Figure 13. It is worth noting that the magnetic contribution to specific heat is significant at temperatures well above the anomalies in DC and AC susceptibility. This additional contribution may be calculated as ∆C p = C p (Bi6 Ti2 FeO14 Cl) - C p (Bi5 PbTi3 O14 Cl). Excess magnetic specific heat, ∆C p , shown in Figure 14, forms an extended bump with a maximum at about 4.4 K, close to the irreversibility temperature at the lower magnetic fields. The extended character of the maximum is not explained by λ -like anomalies, as they very often accompany transitions to states with long-range-order. Assuming the linear dependence of ∆C p expected for spin glasses at the low temperature limit 40,41 (solid line in Figure 14), one can estimate the amount of magnetic entropy associated with the ordered state using the formula: Z T

Smag =

0

(∆C p /T )dT

(7)

3.0 2.5

(J/mol K)

2.0

2.0

1.5

1.0

S

-1

-1

(Jmol K )

2.5

0.5

1.5

0.0 0

Cp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

4

6

8

10

T (K) 1.0

0.5

0.0 0

2

4

6

8

10

12

T (K) Figure 14: Magnetic part of heat Bi6 Ti2 FeO14 Cl heat capacity, ∆C p . The inset shows estimated temperature dependence of magnetic entropy Smag .

Yielded values of Smag are shown in the inset of Figure 14. Magnetic entropy at T = 10 K is estimated to be 2.5 J/mole K. A linear interpolation of Smag (T ) between 0 and 2 K perfectly fits the higher temperature data, confirming the ∆C p ∼ T assumption made earlier. The calculated value 24 ACS Paragon Plus Environment

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of the magnetic entropy Smag removed at Tirr = 4.46 K corresponds to 2 J mol−1 K−1 , only 14% of the theoretical R ln 6 predicted for the S = 5/2 system undergoing a long-range magnetic transition. As discussed above, the investigated Sillen-Aurivillius phases are not magnetically dense materials. Their layered structures place them somewhere in between 2 and 3 dimensional systems, as the interlayer coupling through O-Bi-O layers should be much weaker than interlayer superexchange. In general, there is one condition driving the formation of the spin glass state - disorder. For all our compounds with magnetic cations M, no order within (Ti,M)O layers were observed, so this criterion appears to be fulfilled. However, sufficiently strong magnetic correlations for magnetic cluster formation above 2 K were only observed in the M = Fe compound. Since a material’s tendency to magnetically order depends on both the spin moment S and the mean exchange parameter J (represented by a characteristic Curie-Weiss temperature, ΘCW ), both these values are highest in Bi6 Ti2 FeO14 Cl and, as a result, short range magnetic order below about 4.5 K is observed. It is worth noting that the concentration of magnetic Fe3+ ions remains below the percolation threshold and only interactions via finite clusters at low temperatures emerge, leading to a cluster glass state.

Ferroelectric measurements The ferroelectric property measurement of the bulk material generally requires the application of a high voltage to the sample. Such a high voltage can induce a large leakage current and further facilitate the electrical breakdown if the sample was not dense enough. The density of Sillen-Aurivillius phase compound synthesised in this work is lower than 90%, which is too low for such a measurement. The PFM technique was therefore used to determine the ferroelectric character and associated electrical domain switching of the doped Sillen-Aurivillius compounds. Measurements were conducted on localized regions by placing the probing cantilever tip on the sample surface. Instrument limitations precluded piezoresponse (PR) measurement of the entire sample. However, to ensure that a representative ferroelectric switching was obtained, several cycles were run for each point and over 10 points were randomly selected on the sample surface 25 ACS Paragon Plus Environment


to exclude the contingency during measurement. Figure 15 shows a PR and phase hysteresis loops of the Fe-doped Sillen-Aurivillius compound collected by PFM. The PR varies nonlinearly with respect to an external applied voltage, exactly following the same trend as a typical polarisation hysteresis loop observed in ferroelectric materials. Most importantly, the PR variation corresponds to a ∼180° phase change of the electric domains. This clearly demonstrates that the observed PR hysteresis loop is directly related to the electrical domain switching that originates from the domain wall motion under an applied electric field, and hence the ferroelectric character of these doped Sillen-Aurivillius compounds. 2 0 0 P R ,

P h a s e 1 5 0

P ie z o r e s p o n s e (a .u .)

1 0 0 5 0

o P h a s e ( )

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0 -5 0 -3 0

-2 0

-1 0

0 1 0 V o lta g e (V )

2 0

3 0

Figure 15: The piezoresponse -voltage (dot) and phase-voltage (cross) hysteresis loops of the Fedoped Sillen-Aurivillius compound measured by PFM provides clear evidence for the existence of the ferroelectric characteristic in this compound.

Conclusion A new series of transition metal-doped Sillen-Aurivillius phases, Bi6 Ti2 MO14 Cl, where M 3+ = Cr3+ , Mn3+ or Fe3+ , have been characterised using XRD and NPD techniques. Rietveld refinement against variable temperature SXRD patterns revealed a ferroelectric transition from the noncentrosymmetric P2an space group to the tetragonal P4/mmm at 974.6(2) K for Bi6 Ti2 CrO14 Cl, 913.5(6) K for Bi6 Ti2 MnO14 Cl, and 1044.8(1) K for Bi6 Ti2 FeO14 Cl. The doped compounds were all higher in TC (FE) than the undoped parent compound, Bi5 PbTi3 O14 Cl. The structure was con26 ACS Paragon Plus Environment

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firmed by combined Rietveld refinements against XRD and NPD patterns and the structure solution showed an essentially random distribution of magnetic cations over all the B sites of the n = 3 perovskite block. It appears that the effective ionic radii of the magnetic M 3+ dopant cations are too similar in size to the Ti4+ they are replacing to provide a driving mechanism for order at the synthesis temperature. Nevertheless, complete

1 3

doping was achieved for the first time in an

Aurivillius-related phase. The resulting doped compounds have highly strained and magnetically frustrated structures. For Bi6 Ti2 FeO14 Cl, the cluster-glass type magnetism was characterised in detail with FC-ZFC conditions showing typical branching at a critical temperature Tirr . Furthermore, Tirr shifts were observed with variable field strength, a hallmark of distributed magnetic clusters. Differing cluster sizes of FeO6 octahedra scattered in the perovskite layers lead to time-dependent phenomena - decay of remnant magnetisation and frequency-dependent AC susceptibility. The presence of excess magnetic heat capacity below 10 K combined with relatively low magnetic entropy indicates that any associated magnetic order must be short-range in character. The Fe-doped compound was shown to be ferrolectric, as demonstrated by PFM, in which the piezoresponse was observed with changing field strength, which followed the typical shape of a hysteresis loop and is directly related to the electric domain switching. While these materials do not exhibit the desired long-range magnetic order within the central n = 3 perovskite layer, our findings have revealed interesting cluster-glass-type properties alongside ferroelectricity. Investigations of any potential spin glass - ferroelectric coupling are underway.

Supporting Information Available Figure S1 and Crystallographic information files (CIFs) for the final Rietveld refined structures of Bi6 Ti2 CrO14 Cl, Bi6 Ti2 MnO14 Cland Bi6 Ti2 FeO14 Cl at room-temperature are included supplementary files. This information is available free of charge via the Internet at http://pubs.acs.org/.



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Acknowledgement This work was supported by the Australian Research Council – DP110102662 and the Australian Institute of Nuclear Science and Engineering. This work was, in part, performed at the powder diffraction beamline at the Australian Synchrotron with the assistance from beamline scientist Dr. Justin Kimpton. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Australian Centre for Microscopy & Microanalysis, The University of Sydney.

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Bi6Ti2FeO14Cl TiO6 FeO6 c a


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