High Pressure Crystal and Magnetic Phase Transitions in Multiferroic

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High Pressure Crystal and Magnetic Phase Transitions in Multiferroic Bi0.9La0.1FeO3 Christopher S. Knee,*,† Matthew G. Tucker,‡ Pascal Manuel,‡ Shengzhen Cai,† Johan Bielecki,§ Lars Börjesson,§ and Sten G. Eriksson† †

Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg SE 412 96, Sweden Rutherford Appleton Laboratory, ISIS Facility, Science and Technology Facilities Council, Oxfordshire OX11 0QX, United Kingdom § Department of Applied Physics, Chalmers University of Technology, Gothenburg SE 412-96, Sweden ‡

S Supporting Information *

ABSTRACT: The crystal and magnetic structures of multiferroic Bi0.9La0.1FeO3 have been studied using high resolution neutron powder diffraction in the pressure range 0−8 GPa. Two structural phase transitions are observed. The first, at ∼1 GPa, transforms the polar R3c structure to an antipolar PbZrO3-like √2ap × 2√2ap × 2ap perovskite superstructure; the second, at ∼5 GPa, results in a smaller, √2ap × √2ap × 2ap unit cell and a structure described with Ibmm (nonstandard setting of Imma) symmetry, in which the a−a−b0 octahedral tilt system is retained and the antipolar cation displacements lost. Accompanying the changes in the nuclear structure, the antiferromagnetic spin structure evolves from a cycloid, with a modulation length, λ ≈ 770 Å, to collinear arrangements with the moments aligned along the b-axis (Pbam) and the a-axis (Ibmm) of the orthorhombic unit cells. In comparison with BiFeO3 the transition from a rhombohedral to an orthorhombic structure is suppressed by ∼3 GPa, reflecting the dilution of the stereochemically active bismuth lone pair by lanthanum. A correlation between the cell contraction of Bi1−xLaxFeO3 (0.0 ≤ x ≤ 0.3) induced by chemical pressure and hydrostatic pressure on BiFeO3 is determined, with substitution of 1 mol % of La approximately equivalent to application of 0.05 GPa. Bi0.9La0.1FeO3 is found to have a higher bulk modulus than BiFeO3. temperature of 640 K5 that averages any potential ferromagnetism to zero. The impact of lanthanide substitution at the Bi site on the ferroelectric behavior is mixed. At low lanthanide levels enhanced dielectric properties have been reported due to a reduction in leakage currents. 6−8 However, at higher concentrations the activity of the Bi3+ lone pair becomes diluted and a general transition from the polar R3c symmetry to the centrosymmetric orthorhombic Pnma space group of GdFeO3 is observed.9−13 The exact nature of the structural changes that occur is dependent on the size and concentration of the lanthanide, and the emergence of an antipolar PbZrO3type intermediate for large to midsized lanthanides9,11 is of particular interest. Lanthanide substitution is believed to lead to the gradual collapse of the spin cycloid stabilized in the ferroelectric R3c phase.14 At higher substitution levels, a transition to a canted collinear AFM spin structure with a weak ferromagnetic component is inferred from magnetization data15,16 reflecting the evolving symmetry of the nuclear structure.

1. INTRODUCTION The perovskite BiFeO3 remains to date one of the few multiferroic materials to display both ferroelectric and magnetic order at RT and above. Furthermore, magnetoelectric coupling effects have been demonstrated in BiFeO3 thin films via the electric field control of the antiferromagnetic (AFM) domain structure.1 These unique properties are intimately linked to the presence of two structural distortions, ferroelectric displacements of the cations and tilts of the FeO6 octahedra. As noted recently by Guennou et al.2 the coincidence of these two structural instabilities is extremely rare in perovskites. In BiFeO3 the lone pair activity of the Bi3+ ion leads to a net polarization along the ⟨111⟩ primitive perovskite direction in the noncentrosymmetric R3c symmetry. The first demonstration of robust ferroelectric hysteresis loops came from thin films,3 but the intrinsic property has since been found in single crystals, which display an off-axis polarization consistent with a saturation polarization ∼60 μC/cm2.4 The magnetic properties, originating from the high spin d5, Fe3+ ions located in the distorted oxygen octahedra, are dominated by strong antiferromagnetic (AFM) couplings between all neighboring ions. The spin structure is incommensurate in nature with a cycloidal modulation of periodicity ∼620 Å below the Néel © 2013 American Chemical Society

Received: October 28, 2013 Revised: December 17, 2013 Published: December 19, 2013 1180

dx.doi.org/10.1021/cm403558j | Chem. Mater. 2014, 26, 1180−1186

Chemistry of Materials

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Given the presence of two structural instabilities combined with room temperature multiferroic behavior, BiFeO3 provides a fascinating system to study competing order parameters. As described above, the role of chemical pressure in altering the properties and driving structural phase transitions via substitution at the A-site6−13,17 and B-sites18−20 has been the focus of many studies. In comparison, fewer reports focusing on the effect of pressure on BiFeO3 are to be found. The majority of pressure studies have used diamond anvils and synchrotron XRD,2,21−23 and although some consensus exists in relation to the high pressure region, i.e., a transition to Pnma (GdFeO3type) occurs for P > 10 GPa [ref 2 and references therein], the details of the structures stabilized at P < 10 GPa remain open. The present study focuses on the application of hydrostatic pressure on Bi0.9La0.1FeO3 and uses high resolution neutron powder diffraction to monitor the effects on both crystal and magnetic structures. A sequence of phase transitions from polar R3c to antipolar Pbam and paraelectric Ibmm (Imma) crystal structures accompanied by correlated changes in the spin structure are revealed. A striking similarity between hydrostatic and chemical pressure for the Bi1−xLaxFeO3 series is also demonstrated and the implications of this correlation for optimizing the multiferroic properties are discussed.

Table 1. Refined Structural and Magnetic Parameters of Bi0.9La0.1FeO3 at Selected Pressures and Ambient Temperature P (GPa) space group Z a (Å) b (Å) c (Å) cell volume (Å3) Bi0.9/La0.1(1) x y z Bi0.9/La0.1(2) x y z Fe x y z O(1) x y z O(2) x y z O(3) x y z O(4) x y Z O(5) x y z χ2 Rwp (%) Rp (%) μFe (μB) Rmag (%)

2. EXPERIMENTAL SECTION 2.1. Synthesis. Bi0.9La0.1FeO3 was prepared via a coprecipitation route starting from the nitrates of bismuth, lanthanum, and iron. After filtration the precursor was dried at 180 °C and fired for 2 h at 500 °C. The brown/red powder was then heated at 600, 700, 800, and finally 850 °C for 2 h durations with accompanying regrinds between each step. The sample was judged to be single phase based on Rietveld analysis24 of long scan PXRD data collected using a Bruker AXS D8 diffractometer operating with CuKα1 radiation. Samples of BiFeO3, Bi0.8La0.2FeO3, and Bi0.7La0.3FeO3 were prepared following an identical route, and their normalized cell volumes were estimated based on Rietveld analysis of PXRD data. 2.2. Neutron Diffraction. Neutron diffraction measurements were made on the WISH diffractometer25 at the ISIS neutron and muon facility, Oxfordshire U.K. Approximately 100 mg of sample was loaded into an encapsulated TiZr gasket,26 along with a small amount of Pb to act as a pressure marker. Fully deuterated methanol−ethanol was then added as the pressure transmitting medium to attain near hydrostatic compression over the studied pressure range. The sample was loaded in the VX4 Paris-Edinburgh press preloaded with approximately 70 bar pressure, and 1 h of data collected. Further data sets were collected at approximately steps of 1.1 GPa up to 7.65 GPa for durations ranging from 2 to 3 h. Separate ambient pressure data on BiFeO3, Bi0.9La0.1FeO3, and Bi0.8La0.2FeO3 were collected with the powders loaded in 6 mm diameter thin-walled vanadium cans. The diffraction data were analyzed by the Rietveld method using either the GSAS27 or FULLPROF28 programs. The Pb pressure marker was modeled as a second phase and the pressure estimated from the extracted cell constant.29 In the final stages of the refinements, variation of the atomic displacement parameters (ADPs) of the Bi0.9La0.1FeO3 phase led, in some cases, to unphysical negative values due to the pressure cell limiting the available low d-spacing data. The ADPs are therefore omitted from the structural models presented in Table 1.

a

0 R3c 6 5.5786(2) 13.809(1) 372.18(4) 6a 0 0 0.0393(1)

6a 0 0 0.263(2) 18b 0.431(1) 0.018(2) 0

7.90 11.50 14.6 3.86(4)a 6.2

4.20 Pbam 8 5.5099(9) 11.161(2) 7.7681(9) 477.7(1) 4g 0.686(4) 0.125(2) 0 4h 0.745(3) 0.132(2) 0.5 8i 0.235(2) 0.134(1) 0.258(4) 4g 0.290(5) 0.156(2) 0 4h 0.286(4) 0.076(2) 0.5 8i 0.017(4) 0.259(2) 0.302(3) 4f 0 0.5 0.191(4) 4e 0 0 0.201(3) 2.00 6.36 5.92 μy = 3.15(4) 11.2

7.65 Ibmm 4 5.422(1) 5.5574(9) 7.740(2) 233.2(1) 4e 0.036(5) 0 0.25

4b 0.5 0 0 4e 0.098(4) 0.5 0.25 8g 0.75 0.25 0.0449(10)

1.14 8.41 8.11 μx = 3.59(6) 22.3

Cycloidal spin structure with k-vector = 0.0036(5), 0.0036(5), and 0.

satisfactorily with a Pbam (PbZrO3-type) structural model, with a √2ap × 2√2ap × 2ap superstructure similar to that previously reported for lanthanide substituted phases.9,30 At P > 4 GPa a further structural transition becomes evident due to the decay of the superlattice reflections. The diffraction data are now consistent with a smaller, √2ap × √2ap × 2ap unit cell and good agreement to the patterns recorded at 6.40 and 7.65 GPa was obtained using a structural model with Imma symmetry adapted from that recently observed for Bi0.9Sm0.1Fe0.7Mn0.3O3 at ambient conditions.20 To aid in comparison with the cell parameters of the Pbam model the nonstandard, Ibmm setting was used to describe the structure.

3. RESULTS The neutron diffraction patterns from Bi0.9La0.1FeO3 recorded at selected pressures are shown in Figure 1. From the diffraction data three distinct crystal structures are apparent. At ambient pressure the phase is R3c. At the lowest applied pressure of 1.3 GPa a change in the structure is apparent with splitting of reflections at d ≈ 4 and 2.4 Å, and growth of new reflections at d ≈ 5.0, 3.1, and 2.7 Å. This new phase was fitted 1181



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reversal of the peak intensities at the second phase transition consistent with a realignment of the direction of the moment. From analysis of the data collected at ambient conditions from a larger Bi 0.9 La 0.1FeO3 sample (data not shown), the incommensurate AFM order propagation vector, k = (δ,δ,0), was refined to give δ = 0.0036(5), and a moment of 3.86(4) μB was obtained. Both the high pressure phases display two magnetic peaks in this region, indicating k = 0 magnetic structures, and we have used representation analysis in an attempt to restrict the number of possible solutions. Of the eight possible magnetic representations for the Fe (8i) site of the Pbam structure, only the representations with moments aligned along a, with moments along b, and with moments along c gave reasonable intensities. Indeed, for each of these three representations, although components along a, b, and c are permitted, it was found that allowing moments along directions other than the ones just presented for each representation lead to large discrepancies between the calculated and observed diffraction patterns. The ratio of the first peak (101) to the second (021) seems to favor moments aligned along the b-axis. The refined value of the moment was 3.15(4) μB, and the final Rmag = 11.2%. The situation for the higher pressure Ibmm phase is less complex with eight onedimensional irreducible representations but only four appearing in the decomposition of the magnetic representation for the Fe (4b) site. They correspond to antiferromagnetic (AF) alignment along c, AF along b/ferromagnetic (F) along a, F along b/ AF along a and F along c, respectively. The magnetic structure factors calculated with the moments coupled AF and aligned along the a-axis describe the data better than the other alternatives. The refined moment is 3.59(6) μ B (no ferromagnetic b-axis component is observed within the limit of our experiment performed on a small sample in a pressure cell) and the final Rmag = 22.3%. For the analysis of the ambient Bi0.8La0.2FeO3 pattern, a Pbam model was used, and the best agreement to the magnetic intensity was found with a G-type structure and moments aligned along the c-axis. See the Supporting Information for Rietveld fit (Figure S1) and derived structural and magnetic parameters (Tab1e S1) for this phase. A representative Rietveld fit to the data from Bi0.9La0.1FeO3 at 4.2 GPa is shown in Figure 3, and the crystal and magnetic

Figure 1. Neutron diffraction patterns from Bi0.9La0.1FeO3 on pressurization at 0.0, 4.2, and 7.65 GPa (from top to bottom). The position of reflections originating from the Pb pressure marker is marked for the pattern recorded at 0.0 GPa. Reflections consistent with Pbam symmetry are indicated by arrows for the 4.2 GPa pattern. The scans have been rescaled to aid comparison. Data taken from detector bank 3 of WISH.

The impact of the evolving crystal structure on the purely magnetic intensities centered at ∼4.5 Å is also clear from the high resolution data recorded from the sample in the pressure cell (Figure 2). The broad nature of the magnetic intensity initially reflects the presence of four peaks associated with the spin cycloid for the R3c structure, and these converge as the spin structure becomes collinear. The data also indicates a

Figure 3. Rietveld fit achieved to data from detector bank 3 of WISH for Bi0.9La0.1FeO3 at 4.20 GPa using a Pbam structural model. Crosses are observed data points, the upper line is the calculated diffraction profile, and the lower line is the difference between observed and calculated intensities. The lower set of tick marks indicates the simulated contribution from Bi0.9La0.1FeO3 (nuclear and magnetic) and upper set from the Pb pressure marker.

Figure 2. Evolution of the most intense region of magnetic intensity as a function of pressure. The collapse of the spin cycloid satellite peaks linked to the transition from R3c to Pbam symmetry at approximately 1 GPa, and the intensity reversal that occurs at higher pressures are apparent. The patterns have been rescaled to aid in comparison. 1182



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Figure 4. Crystal and magnetic structures of the three phases of Bi0.9La0.1FeO3: (a) R3c stable for 0 < P < 1 GPa, (b) Pbam (1−5 GPa), and (c) Ibmm (P > 5.4 GPa).

structures are depicted in Figure 4. Refined structural parameters for the three distinct pressure regimes are given in Table 1 and derived interatomic distances for the three structures are given in Table 2. Extracted structural trends, i.e., cell parameters and cell volume as a function of pressure and lanthanum concentration are shown in Figures 5 and 6, respectively.

4. DISCUSSION The diffraction results show that the application of hydrostatic pressure drives two phase transitions in the pressure region 0− 8 GPa for Bi0.9La0.1FeO3. The first occurs at relatively low pressure ∼1 GPa and is linked to the formation of an Table 2. Interatomic Distances for the Rhombohedral and Orthorhombic Phases of Bi0.9La0.1FeO3 R3c 0 GPa Bi(1)−O(1) Bi(1)−O(1) Bi(1)−O(1) Bi(1)−O(1) Bi(1)−O(1) Bi(1)−O(2) Bi(1)−O(2) Bi(1)−O(3) Bi(1)−O(3) Bi(1)−O(4) Bi(1)−O(5) average Bi(1)−O Bi(2)−O(2) Bi(2)−O(2) Bi(2)−O(2) Bi(2)−O(2) Bi(2)−O(3) Bi(2)−O(3) Bi(2)−O(4) Bi(2)−O(5) average Bi(2)−O Fe(1)−O(1) Fe(1)−O(2) Fe(1)−O(3) Fe(1)−O(3) Fe(1)−O(4) Fe(1)−O(5) average Fe(1)−O

2.367(1) × 3 2.4180(1) × 3 3.2705(1) × 3 3.3407(2) × 3

Pbam 4.20 GPa 2.154(23) 2.189(25) 3.490(27) 3.529(23)

× × × ×

1 1 1 1

Figure 5. Pressure dependence of the lattice parameters of Bi0.9La0.1FeO3 during compression up to 7.65 GPa. Error bars associated with the data points are smaller than the symbol size used.

Imma 7.65 GPa 2.10(5) × 1 3.32(5) × 1 2.788(4) × 2

2.626(6) × 4 2.905(7) × 4

2.849(2)

2.00274(7) × 3 2.06983(8) × 3

2.0363(2)

2.759(16) 3.373(18) 2.400(14) 2.488(17) 2.784(2) 2.567(23) 2.680(29) 2.885(29) 2.928(23) 2.351(18) 2.606(18) 3.032(15) 3.317(17) 2.806(3) 2.041(16) 2.062(11) 2.137(16) 2.071(18) 1.893(11) 1.977(13) 2.030(2)

× × × ×

2 2 2 2

× × × × × × × ×

1 1 1 1 2 2 2 2

× × × × × ×

1 1 1 1 1 1

2.76(2)

Figure 6. Evolution of normalized cell volume due as a function of pressure for Bi0.9La0.1FeO3 (circles) and level of La substitution in to Bi1−xLaxFeO3 (squares). Broken lines represent estimated phase transitions induced by pressure and the solid line by chemical modification. Error bars associated with the data points are smaller than the symbol size used.

1.979(10) × 2 1.971(1) × 4

orthorhombic phase with Pbam symmetry. In comparison with BiFeO3,21−23 the presence of 10% La at the A-site reduces the

1.973(2) 1183



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the Pbam and Ibmm phases of Bi0.9La0.1FeO3 but that, under pressure, stabilization of the cation displacements along one unique polar axis becomes less favorable. The FeO6 coordination also changes from two groups of equal distances in R3c to a highly distorted configuration with six nondegenerate Fe−O interactions in Pbam, before reverting to a more symmetric 4 + 2 arrangement for the Ibmm structure. To emphasize the distortion of the Fe−O coordination in the Pbam phase, the internal O−Fe−O bond angles lie in the range 77 to 98° (expected 90° for perfect octahedra) for the structure refined at 4.2 GPa (Table 1). This distorted environment leads to a somewhat unrealistic closest intraoctahedral oxygen to oxygen separation of ∼2.4 Å emphasizing that the structure should be considered as a representation of the average longrange crystal structure. In general, increasing pressure results in shorter, average cation to oxygen ion distances (Table 2), as expected from the global decrease in the normalized unit cell volume (Figure 6). The impact of lanthanum substitution on the normalized perovskite cell volume is also shown in Figure 6. The decrease in cell volume for the Bi1−xLaxFeO3 series with increasing La content obtained here was approximately linear in the region 0 ≤ x ≤ 0.3, and similar in magnitude to that reported in ref 13. The comparison shown in Figure 6 demonstrates that applied mechanical pressure results in a much more dramatic compression of the phase in comparison to the role of chemical pressure. A fit of the data for Bi1−xLaxFeO3 allowed a decrease of 0.029(3) Å3/mol % of La to be estimated, which compares to the pressure induced contraction of 0.47(2) Å3/ GPa observed for Bi0.9La0.1FeO3 and a compression of 0.6 Å3/ GPa for BiFeO3 over the pressure range 0−8.6 GPa estimated from ref 33. This indicates that the bulk modulus is increased by the substitution of La and that the application of 0.1 GPa (1 kbar) on BiFeO3 can be considered as being equivalent to replacing ∼2.0 mol % of Bi3+ ions by La3+ ions. The volume compressibility data for Bi0.9La0.1FeO3 were fitted to a third order Birch−Murnaghan equation of state,36 with the pressure derivative term, B′, fixed equal to 4, to yield a bulk modulus, B0 = 120(4) GPa. Values of 100(5)33 and 111(6) GPa22 are reported for BiFeO3 in the R3c phase (0 < P < 4 GPa). Given the cell volume reduction in Bi1−xLaxFeO3, it is clear that the effective radius of La3+ is smaller than that of Bi3+ reflecting the anisotropic nature of the stereochemically active Bi3+ lone pair. It follows, therefore, that increasing the applied pressure and increasing the lanthanum substitution level, equivalent to a greater chemical pressure, will drive similar structural transitions. In fact, from comparison of the diffraction patterns of Bi0.9La0.1FeO3 in the P range 1−5 GPa, and the ambient pressure phase of Bi0.8La0.2FeO3 shown in Figure 7, the closeness of the crystal structures is clear and the neutron diffraction pattern of Bi0.8La0.2FeO3 is adequately fitted with a Pbam model (Figure S1). Recent findings for the Bi1−xCaxFeO3−δ series (0 ≤ x ≤ 0.14)37 have revealed an identical sequence of phase transitions from ferroelectric R3c to antipolar Pbam to a paraelectric Imma phase as observed here for Bi0.9La0.1FeO3 under pressure. In contrast to the pressure driven behavior of Bi0.9La0.1FeO3 and the isovalent La3+ substitution for the Bi1−xLaxFeO3 series, calcium substitution also introduces oxygen vacancies that may act to further destabilize the initial polar structure.37 In series such as Bi1−xTbxFeO3,12,16,38 with a smaller lanthanide, the samples proceed more directly to the Pnma structure of GdFeO3 that emerges at P > 10 GPa for BiFeO3. The correlation between

pressure required to obtain a new, orthorhombic structure by approximately 3 GPa. We note the earlier findings of isothermal and pressure driven transitions to a Pbam structure in Bi1−xLaxFeO3 at higher substitution levels (0.17 ≤ x ≤ 0.19) by Troyanchuk et al.30 that suggested the enhanced structural sensitivity of lanthanum substituted phases. It is likely that this behavior is linked to a dilution of the lone pair activity of the Bi3+ ions at the A-site that destabilize the ferroelectric cation displacements of the R3c structure. Nonetheless, a level of longrange coherence is retained in the Pbam superstructure that is linked to displacements of the cations from centrosymmetric positions similar in nature to those present in PbZrO3.31 The shifts of the cations, e.g., the Bi(1) ion is displaced ∼0.35 Å along a, in relation to the surrounding oxygen cage will produce localized electric dipoles ordered in an antipolar manner. Coupled to the rearrangement of the cation positions is the loss of the antiphase octahedral tilt along one of the primitive cubic perovskite axes, as the a−a−a− tilt scheme (Glazer notation32) of the R3c structure changes to a−a−b0. The present findings for Bi0.9La0.1FeO3 mirror the recent neutron diffraction study of BiFeO3 that reported the stabilization of the Pbam PbZrO3-type structure in the interval 3 ≤ P ≤ 8.5 GPa.33 In contrast to BiFeO3, the Pbam structure is only stable in the range 1−4 GPa for Bi0.9La0.1FeO3, and at higher pressures the Ibmm structure emerges. The main signature of this gradual phase transition is the loss of the antipolar A-site displacements and the two A-site crystallographic positions of the Pbam symmetry are replaced by a single Bi/La site on (x, 0, 0.25), with x ≈ 0.036 (see Table 1). The basic a−a−b0 FeO6 octahedral tilt scheme is retained at this antipolar to paraelectric transition, which suggests that increasing pressure drives a decoupling of the two structural distortions. It is important to note that the structure of BiFeO3 in the pressure range 0−10 GPa remains controversial.2 The antipolar Pbam structure reported by Kozlenko et al.33 based on neutron data differs from synchrotron X-ray studies that support transitions to orthorhombic symmetries but with even larger unit cells under compression.21−23 The reasons for the discrepancy between the neutron and X-ray studies are presently unclear. Given that the structure transitions of BiFeO3 have been shown to be sensitive to nonhydrostatic stress34 this may be due to differences in the pressure transmitting media employed. The pressure dependence of the cell constants of Bi0.9La0.1FeO3 is shown in Figure 5. The contraction is anisotropic with the b and c parameters both showing a tendency to level off. The main impact of the pressure induced structural transitions on the Bi/La−O environment is a significant increase in the number and range of interactions (see Table 2). For the R3c structure, these distances range between 2.36 to 3.34 Å and compare with interatomic distances ranging from 2.15 to 3.53 Å in Pbam at 4.2 GPa. In the Ibmm structure the extreme of one short (2.1 Å) and one long (3.3 Å) Bi/La−O(1) interaction along the a-direction reflects the combined effect of the two antiphase octahedra tilts coupled to the displacement of the cations (primarily the stereochemically active Bi3+) along x that produce these average configurations. According to DFT calculations,35 the ferroelectric displacements within the R3c structure originate from mixing of the Bi3+, 6s2 electrons with the 2s and 2p oxygen states to form a space filling localized lobe that pushes away neighboring atoms and makes the Bi−O bonding anisotropic. The present analysis shows that significant cationic displacements remain for both 1184



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pressure driven phase transitions for Bi0.9La0.1FeO3 occur within the magnetically ordered state, and consistent with predictions,43 the loss of the polar R3c structure results in a collapse of the spin cycloid in favor of commensurate, k = 0, spin structures with the potential for weak (AFM canted) ferromagnetism. Because of the limited statistics inherent to the use of a pressure cell, we are unable to reliably assign a ferromagnetic component from our Rietveld analyses. Magnetisation studies under pressure are therefore suggested as a useful avenue to search for possible enhanced ferromagnetism associated with the structural transitions.

5. CONCLUSIONS In summary it has been observed that pressure drives the structure of Bi0.9La0.1FeO3 through the following sequence of phase transitions, R3c → Pbam → Imma. The analogy between hydrostatic pressure and chemical pressure for Bi1−xLaxFeO3 was demonstrated to be remarkably strong, with the orthorhombic and antipolar Pbam structure of Bi0.9La0.1FeO3 stabilized in the range 1−5 GPa being akin to the ambient pressure structure of Bi0.8La0.2FeO3. Associated with the first pressure induced transition the spin structure reverts to a k = 0 state as the spin cycloid is destroyed. The present findings demonstrate that lanthanum substitution combined with pressure offers a level of control for the critical pressure required to induce the ferroelectric to antipolar transition for bulk materials and further that the nature of the paraelectric phase can also be influenced. Extending these studies of substituted BiFeO3 materials with epitaxially (2D) strained thin films may allow the interplay between the ferroelectric and antipolar (possibly antiferroelectric) states and thus the magnetic behavior to be tuned through judicious choice of the substrate, dopant ion, and its concentration.

Figure 7. Comparison of the diffraction patterns of Bi0.9La0.1FeO3 at 4.2 GPa and Bi0.8La0.2FeO3 (bottom) at ambient pressure. Both phases have PbZrO3-like structures described with Pbam symmetry. Peaks from the Pb pressure marker are indicated for the Bi0.9La0.1FeO3 sample, and the scans have been rescaled to aid comparison.

chemical pressure and applied pressure can be extended further to consider substitution at the B-site, and in codoped materials of the form Bi0.9Sm0.1Fe1−xMnxO320 and Bi1−xLaxFe1−yMnyO3,39 the Imma structure emerges for Mn levels ≥0.3. Here it seems likely that, in addition to ionic size effects and dilution of the Bi lone pair, modification of the (Fe/Mn)O6 octahedra due to the Jahn−Teller active Mn3+ ion also contribute. Taken together these results show an ability to tune the nature of the paraelectric phase from Pnma to Imma through choice of dopant ion, dopant site, or application of pressure. Finally, we address the impact of pressure on the spin structure of Bi0.9La0.1FeO3. The magnetic structure of BiFeO3 at room temperature and atmospheric pressure is, to a first approximation, a G-type antiferromagnet40 where each Fe3+ moment is antiparallel to its six first neighbors (distance ≈ 3.95°A). A high resolution neutron diffraction study5 slightly modified that picture by revealing the existence of a 620 Å cycloidal spiral modulation where the spins rotate along the [110] direction. Our high resolution data obtained from a sample of Bi0.9La0.1FeO3 outside of the pressure cell is consistent with the spin cycloid model, and through careful fitting, the periodicity of the modulation, λ, could be estimated as 770 Å. A similar lengthening of λ was earlier reported for BiFe1−xMnxO3, 0 ≤ x ≤ 0.2.41 The remarkable feature common to all the magnetic structures of Bi0.9La0.1FeO3 presented in Figure 4, is that the moments tend to lie close to the square base of the octahedra for all the pressures studied. The magnetic structures of the high pressure Pbam and Ibmm phases are G-type with the moments aligned along the b-axis and a-axis, respectively. Our analysis indicates that the subtle reorganizations in the FeO6 bonding that occurs through the Pbam to Imma antipolarparaelectric phase transition result in a reorientation of the predominant spin direction. Similar behavior was recently reported for the Bi1−xCaxFeO3−δ series,37 where the authors were able to use much larger samples for their neutron studies and observed preferential alignment of the magnetic moment along the b and c axes for the Pbam (x = 0.11) and Imma (x = 0.14) phases, respectively. Analogous findings were also reported for Bi0.85Nd0.15FeO3 that shows a spin reorientation at the Pbam to Pbnm transition (alternative setting of the Pnma space group of the GdFeO3 structure) as a function of T and associated marked increased in the magnetization.42 Both the

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ASSOCIATED CONTENT

S Supporting Information *

Rietveld fit obtained for the neutron diffraction data collected for Bi0.8La0.2FeO3 at ambient pressure simulated using a Pbam structural model (Figure S1). The refined structural and magnetic parameters for the material are presented (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(C.S.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council. Dr. Laurent Chapon is thanked for initial discussions in relation to the diffraction experiment. C.S.K. acknowledges support from the Swedish research council (Vetenskapsrådet), grant No. 621-2011-3851.

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REFERENCES

(1) Zhao, T.; Scholl, A.; Zavliche, F.; Lee, K.; Barry, M.; Doran, A.; Cruz, M. P.; Chu, Y. H.; Ederer, C.; Spaldin, N. A.; Das, R. R.; Kim, D. M.; Baek, S. H.; Eimand, C. B.; Ramesh, R. Nat. Mater. 2006, 5, 823.

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High Pressure Crystal and Magnetic Phase Transitions in Multiferroic

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