Structural Transformation Pathways of Multiferroic BiFeO3 under High

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Structural Transformation Pathways of Multiferroic BiFeO3 under High Pressure Ye Wu, Xi Han, and Haijun Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00977 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

Structural Transformation Pathways of Multiferroic BiFeO3 under High Pressure

Ye Wu*, Xi Han, Haijun Huang*

School of Science, Wuhan University of Technology, Wuhan, Hubei 430070, China

*Corresponding author: [email protected] (Y. W.); [email protected] (H. H.)

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract High-pressure phase transitions in multiferroic BiFeO3 have been investigated by synchrotron radiation X-ray diffraction up to ~54 GPa at room temperature. BiFeO3 exhibits complex structural behavior under high pressure. Four phase transitions with the phase sequence R3c → OI and OIII → OI → Pnma → Pnm21 were observed at ~3 GPa, ~6 GPa, ~12 GPa, and ~45 GPa, respectively. Due to octahedral tilts and cation displacements under high pressure, the BiFeO3 presents large unit-cells and complex superstructures or domain structures between the stable R3c and Pnma phases. The OI and OIII phases have superstructure lattices √2apc × 3√2bpc × cpc and √2apc × 2√2bpc × 2cpc, respectively, and the OIII phase is isostructural with antiferroelectric PbZrO3 (space group: Pbam). The Pnma → Pnm21 transition at ~45 GPa indicates the occurrence of large unit-cell and an increase of the structural distortion. The Pnm21 phase with a superstructure lattice 2√2apc × 2bpc × √2cpc has a group-subgroup relation with the Pnma phase.


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Introduction Multiferroics, which exhibit both ferroelectric and magnetic order in a single phase, have attracted a great deal of interest.1-2 Among multiferroics, bismuth ferrite, BiFeO3, is one of the most promising multiferroics for both fundamental and applied research, as it exhibits both ferroelectric and magnetic properties very well at ambient conditions and above room temperature.3-4 In BiFeO3, the ferroelectricity is induced by the 6s2 lone pair electrons of Bi3+ ions until the ferroelectric Curie temperature TC = 1100 K. The incommensurate G-type antiferromagnetic order with cycloidal modulation occurs in BiFeO3 below the Néel temperature TN = 640 K.5 Due to the unique physical properties of BiFeO3, many experimental and theoretical efforts have been devoted to its structural characteristics and the nature of magnetoelectric coupling.6 A BiFeO3 crystal has a perovskite-type structure with space group R3c and lattice parameters a = 5.579 Å and c = 13.869 Å in hexagonal setting at ambient conditions.7 BiFeO3 exhibits interesting structural and physical behavior at low temperature8-10 and high temperature.11-13 Several high-temperature phases have been proposed but discrepancies still exist in the literature.11-13 Structural behavior of BiFeO3 at high pressure is also important for understanding its structural stability and physical properties. Gavriliuk et al.14-15 has reported that no phase transition occurs in BiFeO3 up to 70 GPa at room temperature, an anomaly of lattice parameters at 40-50 GPa is attributed to the high-spin to low-spin transition of Fe3+. Nevertheless, two phase transitions have been proposed by another group.16-17 The R3c phase transforms to a



monoclinic phase (space group: C2/m) at ~3 GPa and then to a GdFeO3-type structure (space group: Pnma) at ~12 GPa.17 Zhang et al.18 also observed the C2/m phase at pressures 5-11 GPa. The reported Pnma phase confirmed the results of theoretical calculations.19 Afterwards, Belik et al.20 identified three intermediate phases as orthorhombic (OI, OII, and OIII phases) instead of the monoclinic C2/m between the R3c and Pnma phases. The OIII phase has a PbZrO3-type structure (space group: Pbam) and is also identified by neutron powder diffraction.21 Later, Guennou et al.22-23 reported six phase transitions at pressures 0-60 GPa. There are three intermediate phases (Phase II, III, and IV) between the R3c and Pnma phases and the Pnma phase transforms to the Pnmm phase at ~38 GPa and then to the Cmcm phase at ~48 GPa. Generally, it is consistent that the Pnma phase is stable above ~11 GPa, however, structural behavior of BiFeO3 below ~11 GPa and above ~38 GPa are not well clarified. In this study, we investigated structural behavior of multiferroic BiFeO3 up to 54 GPa at room temperature using in-situ synchrotron radiation X-ray diffraction (XRD) and Diamond Anvil Cell (DAC). Four pressure-induced phase transitions of BiFeO3 were observed here due to its complex structural distortions. We discussed the structural transformation pathways of BiFeO3 under high pressure.

Experimental Methods Synthesis and Characterization of BiFeO3. BiFeO3 sample was synthesized in a cubic press at Wuhan University of Technology. The starting material, a stoichiometric mixture of Bi2O3 (99.99 %) and Fe2O3 (99.99 %), was pressed into disc


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and placed in a Mo capsule. The pyrophyllite assembly with the sample capsule was cold-pressurized to 3 GPa and heated up to 800 ºC for 15 min. After heat treatment the sample was quenched to room temperature by turning off power directly, and the pressure was slowly released to ambient pressure. XRD patterns on the recovered sample showed that it was a single phase with space group R3c. X-ray diffraction experiments at high pressure. XRD experiments at high pressure were performed at the GeoSoilEnviroConsortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). A monochromatic X-ray beam was used with a wavelength of 0.3344 Å and a beam size of 3 × 5 µm2. Experiments were carried out using a symmetric-type DAC equipped with 300 µm culet size diamonds and a pre-indented rhenium gasket with the thickness of 30~40 µm. The BiFeO3 sample was loaded into the sample chamber of 120 µm in diameter drilled in the pre-indented Re gasket. A ruby small sphere and fine gold powder was placed next to the sample for pressure calibration.24-25 Neon gas was used as pressure transmitting medium. X-ray diffraction patterns on the sample and gold were recorded by a MAR-165 CCD. Typical exposure time was 10 s. Diffraction patterns were collected with increasing pressure up to ~54 GPa at intervals of 1-2 GPa. Two-dimensional diffraction patterns were integrated to two theta vs intensity data by FIT2D26 and then analyzed by the Model biased method implemented in the GSAS+EXPGUI software.27

Results and Discussion An overview of the experimental evidence for phase transitions is given in Figure 1,



where we present representative XRD patterns. The analysis of the diffraction patterns reveals four phase transitions at ~3 GPa, ~6 GPa, ~12 GPa, and ~45 GPa with the phase sequence R3c → OI and OIII → OI → Pnma → Pnm21. The high-pressure phases, OI, OIII, and Pnma, were reported previously17, 20-22 and confirmed by our experiments, but the structure of OI phase have not been determined.20 At pressures above ~3 GPa, the crystal symmetry was obviously different from the R3c phase marked by the emergence of new diffraction reflections and splitting of diffraction peaks (Figures 1, 2, and 3a). That indicates the emergence of a complex superstructure or domain structure. Between ~3 GPa and ~6 GPa, the diffraction reflections could not be indexed to one superstructure. Two orthorhombic reflections, (110)OI and (110)OIII, clearly indicate the existence of a mixture of two phases at pressures 3-6 GPa (Figure 4). Belik et al.20 reported that the R3c phase transforms to an orthorhombic phase OI at ~4 GPa and then to an orthorhombic OII at ~7 GPa during compression, while, the orthorhombic OIII phase appears at 2-3 GPa during decompression. Here we found that diffraction patterns at 3-6 GPa could be indexed to superstructure phases OI and OIII as reported by Belik et al.20. Between ~3 GPa and ~6 GPa, the diffraction reflection (110)OI strengthens with increasing pressure, however, the diffraction reflection (110)OIII weakens and finally disappears at pressures above ~6 GPa (Figure 4). It indicates the OIII phase disappears and the OI phase is stable above ~6 GPa. The diffraction reflections of the OI and OIII phases could be indexed to orthorhombic phases characterized by superstructure lattices √2apc × 3√2bpc × cpc and √2apc × 2√2bpc × 2cpc, respectively,


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where apc, bpc, and cpc are the lattice parameters of the pseudo-cubic (pc) perovskite subcell. The observed reflection conditions afford the maximum space group Cmmm for the OI phase and Pbam for the OIII phase, which are consistent with the results reported previously.20 The superstructure OI phase has a large unit-cell with Z = 6 and the OIII phase actually is isostructural with antiferroelectric PbZrO3. The structural transition from the polar rhombohedral R3c phase to the antipolar Pbam phase were also confirmed by Neutron powder diffraction.21 We did not observe the OII phase characterized by space group Ibam in the study of Belik et al.,20 since there is no split of the orthorhombic reflection (110)OI during compression in this study (Figure 5). The diffraction reflection (110)OII of the OII phase is supposed to appear at the left (smaller two theta) of reflection (110)OI upon compression. As illustrated in previous studies,20, 22 BiFeO3 shows rather complicated structural behavior at pressures below 11 GPa associated with the emergence of complex superstructures and domain structures, and a large hysteretic behavior was also reported upon compression and decompression. The OII phase may appear in a very narrow pressure range. Moreover, the monoclinic phase (C2/m) proposed by Haumont et al.17 is actually a rough structural model and cannot fit all superstructure reflections very well at pressures 3-12 GPa in this study. The OI phase transforms to the GdFeO3-type structure (space group: Pnma) above ~12 GPa accompanied by the characteristic (111) reflection (Figures 1 and 5a). The Pnma phase is characterized by a superstructure lattice √2apc × 2bpc × √2cpc and has been well-established in previous studies.17, 22-23 It is stable ranging from ~12 GPa to



~45 GPa in this study. With increasing pressure, the (111) reflection of the Pnma phase disappears and new reflections appear, for example, (311), (400) and (341) reflections (Figure 5). Above 45 GPa, the crystal symmetry was obviously different from the Pnma phase. Instead of the phase transitions Pnma → Pnmm at 38 GPa and Pnmm → Cmcm at 48 GPa reported by Guennou et al.,22 the Pnma phase undergoes a transition to the Pnm21 phase at about 45 GPa in this study (Figures 1 and 5b), which provides a good fitting with the diffraction patterns. The Pnm21 phase, with a superstructure lattice 2√2apc × 2bpc × √2cpc, actually is a commensurate modulated structure of the Pnma phase along the a-axis with the modulation wave vector q = 0.5a*.28 The Pnmm phase proposed by Guennou et al.22 has the same superstructure lattices with the Pnm21 phase. As we observed, the Pnm21 phase is stable up to 54 GPa and does not transform to the Cmcm phase, which has a superstructure of 2√2apc × 4bpc × √2cpc.22 The Pnm21 phase was also found in Bi1-xCaxMnO3 (x = 0.4 and 0.45) perovskite compounds at room and low temperatures (150 K) and at ambient pressure.29 Unit-cell volume per formula and pseudo-cubic lattice parameters in BiFeO3 with increasing pressure are plotted in Figure 6. There is no noticeable drop for the volume during the R3c → OI and OIII → OI transitions (Figure 6a). Nevertheless, the pseudo-cubic lattice parameters show step-like changes during the R3c → OI and OIII transitions typical for first-order phase transitions. While, the pseudo-cubic lattice parameters change slightly during the OI and OIII → OI transition. Both volume per formula and pseudo-cubic lattice parameters change abruptly at the phase transition


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OI → Pnma, because the Pnma phase appears more distorted than the original R3c structure. A fit of the volume per formula in the stability region of the Pnma phase with a third-order Birch-Murnaghan equation of state (EOS) leads to V0 = 59.8 (Å3), K0 = 194(7) GPa, and K0’ = 2.1(2), which are in good agreement with results published previously.22 During the transition Pnma → Pnm21 at ~45 GPa, the unit-cell volume per formula and pseudo-cubic lattice parameters show slight reductions rather than abrupt discontinuities. Additionally, Pnma and Pnm21 phases present a group-subgroup relation. Therefore, the transition Pnma → Pnm21 should be a second-order transition. As an important member of multiferroic perovskites, BiFeO3 shows rich phase transitions under high pressure. Both experimental and theoretical studies propose that the R3c phase transforms to the Pnma phase at pressures above 11 GPa with different intermediate structures or without,22, 30 even different experimental methods are used, for example, XRD, Raman and Infrared spectroscopy.16-17, 31 Discrepancies exist about the intermediate structures between the stable R3c and Pnma phases among previous studies. In this study, we demonstrate that the R3c phase transforms to orthorhombic superstructures √2apc × 3√2bpc × cpc and √2apc × 2√2bpc × 2cpc (corresponding to OI and OIII phases, respectively) rather than monoclinic phase (C2/m) at pressures below 11 GPa. The suggested OI (Cmmm, Z = 6) and OIII (Pbam, Z = 8) phases in this study were also reported previously.20, 21 The OIII phase with the Pbam space group is isostructural with antiferroelectric PbZrO3, and its detailed structural model has been determined by means of neutron powder diffraction in previous study.21 Due



to the limited resolution of the diffraction patterns from high-pressure experiments and the complex superstructures or domain structures, the precisely structural model of the OI phase cannot be solved in this study. This also is the case for the superstructure Pnm21 phase presented in this study. Our study verifies that the stable Pnma phase transforms to an orthorhombic superstructure above 40 GPa, although, the suggested phase transition Pnma → Pnm21 at ~45 GPa in this study is different from the transitions Pnma → Pnmm → Cmcm at 38 and 48 GPa proposed by Guennou et al..22 The Pnm21 phase has the same superstructure lattice 2√2apc × 2bpc × √2cpc with the Pnmm phase. Different transition pressures may be attributed to different hydrostatic conditions, and limit of diffraction pattern resolution may generate different space groups of superstructures. Thus, high-resolution X-ray diffraction experiments on (single-crystal) BiFeO3 at high pressures are still required to solve the detailed crystal structures of high-pressure phases. According to previous literature,20 although significant hysteretic behavior was found, phase transitions of BiFeO3 below 10 GPa at room temperature were reversible during decompression. There are no experiments to evidence the (ir)reversibility of phase transitions in BiFeO3 above 10 GPa at room temperature. Even so, it’s noteworthy that high-pressure phases of BiFeO3 have been synthesized at different pressure-temperature conditions or by different substitutions. The Pnma phase of BiFeO3 is stable ranging from ~12 GPa to ~45 GPa. This phase was also found in other BiMO3 perovskites at high pressures and room temperature, such as BiInO3 above 2 GPa,32 BiCrO3 above 1 GPa,33 BiScO3 above 4 GPa,33 BiMnO3 above 6


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GPa,33 and BiNiO3 above 4 GPa.34 Moreover, Bi1-xGdxFeO3 (x > 0.1) crystallizing in the Pnma phase were synthesized at high temperature and ambient pressure.35 The similar case Bi1-xNdxFeO3 (x = 0.15 and 0.2) has a PbZrO3-type structure (space group: Pbam, corresponding to the OIII phase of BiFeO3 in this study),36 and Bi1-xCaxMnO3 (x = 0.4 and 0.45) is the Pnm21 phase at room and low temperatures (150 K) and at ambient pressure.29 Stabilization of high-pressure phases of BiFeO3 at ambient conditions by various of substitutions opens possibilities to investigate their physical properties that cannot be studied at high pressure. While a more interesting thing is to synthesize pure BiFeO3 high-pressure structures at high pressure and relative low temperature, and these high-pressure phases can be quenchable for properties measurements. Therefore, our study not only confirm the rich pressure-induced phase transition in multiferroic BiFeO3, but also shed light on the structure-property relationship in multiferroic perovskites.

Conclusions In summary, we have investigated the structural behaviors of the multiferroic BiFeO3 up to 54 GPa at room temperature. BiFeO3 undergoes a series of phase transition with the phase sequence of R3c → OI and OIII → OI → Pnma → Pnm21. Between the stable R3c and Pnma phases, two orthorhombic phases, OI and OIII, were characterized by superstructures or domain structures with large unit-cells, implying complex octahedral tilts and cation displacements triggered by the Bi3+. The Pnma phase is stable at pressures 12-45 GPa and transforms to the Pnm21 phase above 45 GPa, which has a group-subgroup relation with the Pnma phase.



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Acknowledgments We thank Sergey N. Tkachev for loading gas medium. We acknowledge the financial support from the National Natural Science Foundation of China (41602036 and 41322028) and 973 Program of China (2014CB845904). High-pressure XRD experiments were performed at GSECARS of the APS, ANL. GSECARS is supported by

the

NSF-Earth

Sciences

(EAR-1128799)

and

the

DOE-Geosciences

(DE-FG02-94ER14466). APS is a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by ANL under contract DEAC02-06CH11357.

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Table of Contents Graphic



Figure 1 Representative X-ray diffraction patterns of BiFeO3 at high pressures and room temperature with phases: R3c (black), OI and OIII (orange), OI (magenta), Pnma (blue), Pnm21 (olive). 174x113mm (300 x 300 DPI)


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Figure 2 X-ray diffraction patterns of BiFeO3 at 2.6 GPa and room temperature. The Bragg reflections are indicated by tick marks. The refined lattice parameters are given. 156x96mm (300 x 300 DPI)



Figure 3 X-ray diffraction patterns of BiFeO3 at 4.1 GPa (a) and 7.3 GPa (b). The Bragg reflections for the corresponding phases are indicated by tick marks. The refined lattice parameters and Miller indices of some reflections are given. 284x321mm (300 x 300 DPI)


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Figure 4 Fragment (from 3.2 to 4.0º in 2θ) of X-ray diffraction patterns of BiFeO3 with increasing pressure at room temperature, emphasizing the phase transitions of R3c to Pbam (OIII) and Cmmm (OI) at pressures lower than 10 GPa. 170x230mm (300 x 300 DPI)



Figure 5 X-ray diffraction patterns of BiFeO3 at 28.2 GPa (a) and 50.4 GPa (b). The Bragg reflections for the corresponding phases are indicated by tick marks. The refined lattice parameters and Miller indices of some reflections are given. 284x321mm (300 x 300 DPI)


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Figure 6 Pressure-dependence of unit-cell volume per formula (a) and pseudo-cubic (pc) lattice parameters (b) in BiFeO3 during increasing pressure at room temperature. The volume per formula calculated from the equation of state (EOS) determined for the Pnma phase (dotted line) has been extrapolated over the full pressure range as a guide to the eye. Phase boundaries are marked by dashed lines. 280x311mm (300 x 300 DPI)


Structural Transformation Pathways of Multiferroic BiFeO3 under High

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