A Ferromagnetic Pyroelectric Phase Prepared by Topochemical

Apr 9, 2013 - Clarendon Laboratory, Department of Physics, University of Oxford, Parks ... 136 Fleming Building, Houston, Texas 77204-5003, United Sta...
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Ba2YFeO5.5: A Ferromagnetic Pyroelectric Phase Prepared by Topochemical Oxidation. Kun Luo,† Roger D. Johnson,‡,§ Thao T. Tran,⊥ P. Shiv Halasyamani,⊥ Paolo G. Radaelli,‡ and Michael A. Hayward†,* †

Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, United Kingdom ‡ Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom § ISIS Facility, STFC-Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom ⊥ Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003, United States S Supporting Information *

ABSTRACT: Reaction of the anion-deficient, cation-ordered perovskite phase Ba2YFeO5 with 80 atm of oxygen pressure at 410 °C results in the formation of the Fe4+ phase Ba2YFeO5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba2YFeO5 (space group P21/n), to yield a noncentrosymmetric (NCS) phase Ba2YFeO5.5 (space group Pb21m (No. 26), a = 12.1320(2) Å, b = 6.0606(1) Å, c = 8.0956(1) Å, V = 595.257(2) Å3) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba2YFeO5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation process is discussed on the basis of induced lattice strain and an electronic instability and represents a new strategy for the preparation of NCS materials that readily incorporate paramagnetic transition metal centers. KEYWORDS: topochemical oxidation, cation order, polar materials, ferromagnetic materials, pyroelectric materials

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INTRODUCTION Compounds that adopt noncentrosymmetric (NCS) crystal structures have been the subject of intense and continued study because a lack of structural inversion symmetry is a precondition for materials which exhibit ferroelectricity, piezoelectricity, or second-harmonic generation (SHG).1 Despite the interest in NCS materials, the discovery and preparation of new phases that adopt acentric crystal structures remains a challenge. The main obstacles that need to be overcome in order to prepare NCS materials can be seen by considering simple extended metal oxide phases. These phases tend to adopt structures that can be rationalized as the simple packing of spherical cations and anions to maximize unlike-charge attractions, while minimizing like-charge repulsions. This combination of “symmetric” components (ions) held together by simple nondirectional forces strongly favors the formation of highly symmetric crystal structures. Furthermore, although it is clear that many transition metal oxide systems exhibit significant covalency and directional bonding in addition to simple Coulombic interactions, these directional bonding interactions also have tendency to form highly symmetric metal-anion local coordination polyhedra, which also favor © 2013 American Chemical Society

highly symmetric packing arrangements, again strongly disfavoring NCS structures. Thus considering the bonding forces present, it is hardly surprising that the vast majority of simple inorganic solids crystallize with centrosymmetric structures.2 The most commonly adopted strategy used to overcome the inherent favorability of centrosymmetric structures, utilizes electronically driven structural distortions to break the local symmetry of “lattice components” (i.e., metal-anion coordination polyhedra). The presence of these acentric units not only breaks the local symmetry but can also disrupt the long-range packing in the solid phase, increasing the favorability of NCS structures compared to centric structures. Typically one of two classes of metal cation are used to achieve this: i) d0 transition metal cations e.g. Ti4+, Nb5+, W6+. These cations are susceptible to a second-order Jahn−Teller (SOJT) distortion in which the metal d-orbitals hybridize with the oxygen 2p orbitals, driving a spontaneous symmetry breaking distortion which leads to the ‘off-centering’ of metals within their anion coordination Received: January 29, 2013 Revised: April 5, 2013 Published: April 9, 2013 1800

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polyhedra.3−7 (ii) “Lone-pair” cations with ns2 electronic configurations, e.g., Pb2+, Bi3+. In this case, a hybridization of metal s- and p-orbitals with oxygen 2p orbitals drives a structural distortion, often interpreted as a “lone-pair” at the metal center, which breaks the symmetry of the metal-anion coordination polyhedra.8−13 Thus by utilizing either of these classes of metals within extended solids, materials with NCS structures can be prepared. For example the inversion symmetry of the ABO3 cubic perovskite lattice can be lifted by either introducing d0 transition metals onto the B-site of the lattice (e.g., BaTiO3) or “lone-pair” cations onto the A-site of the lattice (e.g., BiFeO3). It should be noted, however, that the presence of “distortion centers” within materials is not a guarantee of an NCS structure as, in the absence of structural features that prevent this, dipolar couplings between distorted metal centers have a tendency to antialign any distortion present, recovering the inversion symmetry of the extended lattice. This is a particularly important consideration when taking an alternative “bottom up”, crystal engineering approach to the design and synthesis of NCS materials. In this approach, rather than starting with an extended solid lattice and then breaking the inversion symmetry via local distortions as described above, specific polar basic building units (BBUs) are designed and then crystallized to construct an NCS phase. Common BBUs include early transition metal oxide-fluoride anions such as NbOF52− or MO2F42− (M = Mo, W)14−20 or pyramidal anions such as SeO3− or IO3−,21−26 which can be seen as respectively analogous to class i and class ii cations described above. In common with the preparation of NCS extended oxide phases, the key challenge to be addressed in the “bottom up” synthesis of NCS materials is directing the crystallization of polar BBUs to form NCS structures, rather than high symmetry centrosymmetric arrangements. Extensive studies have revealed that the crystallization of BBUs can be directed and influenced by a number of factors including lattice strain, the extended bond network connecting units, cation−cation repulsions, and a range of electronic effects. Thus for example by exploiting a combination of the bonding preferences of the NbOF52‑ anion and lattice strain, replacement of the large Cs+ cation with the smaller K + cation can disrupt the simple packing in centrosymmetric CsNaNbOF5 and drive a change to an NCS structure for KNaNbOF5.27 Alternatively the extended bond network can be used to direct the formation of NCS materials by incorporating asymmetric cations which disrupt the simple high-symmetry packing of polar BBUs as observed for “bent” cations such as the Cu2+ centers in CuVOF4(H2O)7 or MoO22+ cations in RbMoO3(IO3).28,29 Recently, we have sought to utilize some of the interactions employed to direct the NCS crystallization of polar BBUs (the extended bond network and lattice strain) to prepare lowsymmetry, cation-ordered oxide phases. By locating cations with differing sizes and preferred coordination numbers within anion deficient perovskite lattices, high-symmetry structures can be disfavored as simple packing arrangements cannot conserve the required bond connectivity of the B-cation network while maintaining reasonable lattice strain. Thus the anion deficient perovskite phase Ba4CaFe3O9.5 adopts a complex cation-ordered structure driven by the need to maintain an octahedral coordination around Ca2+ while minimizing the lattice strain arising from the differing ionic radii of the calcium and iron centers.30 In fact, the resulting low-symmetry arrangement of CaO6, FeO5 and FeO4 centers

on the B-cation sites of the perovskite lattice, breaks the structural inversion symmetry of the material and leads to SHG activity, demonstrating that NCS materials can be prepared without the use of local “distortion centers” by following this strategy. Furthermore, by removing the need to include local “distortion centers” within NCS materials, the incorporation of paramagnetic cations within acentric lattices is facilitated potentially offering a route for the preparation of multiferroic materials. In this report, we describe the preparation of a new extended oxide NCS phase, Ba2YFeO5.5, which is prepared via a novel two-step synthesis route. Initially the centrosymmetric complex cation-ordered perovskite phase Ba2YFeO5 is prepared by exploiting the difference in size and coordination number of the YO6 and FeO4 B-site cations.31 Then the material is topochemically oxidized to break the inversion symmetry at some of the iron centers resulting in an NCS material.

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EXPERIMENTAL SECTION

Synthesis. Ba2YFeO5.5 was prepared via the low-temperature oxidation of the cation ordered perovskite Ba2YFeO5.31 Ba2YFeO5 was synthesized via a high-temperature ceramic route, as previously reported. Suitable stoichiometric ratios of BaCO3 (99.997%), Y2O3 (99.998%, dried at 900 °C) and Fe2O3 (99.99%) were ground together in an agate pestle and mortar and then heated in air at 1000 °C to decompose the carbonate. The resulting sample was then reground, pressed into 13 mm diameter pellets and heated at 1425 °C for 2 periods of 40 h under flowing argon, to prepare a phase-pure sample of Ba2YFeO5, confirmed by X-ray powder diffraction. The as-prepared Ba2YFeO5 sample was then placed into a high pressure vessel and heated at 410 °C for 24 h under 80 atm of oxygen pressure to form Ba2YFeO5.5. Characterization. X-ray powder diffraction data were collected using a PANalytical X’pert diffractometer incorporating an X’celerator position sensitive detector (monochromatic Cu Kα1 radiation). Neutron powder diffraction data were collected using the D2B diffractometer (λ=1.59 Å) at the ILL neutron source in Grenoble, France. Rietveld profile refinements were performed using the GSAS suite of programs.32 Average iron oxidation states were determined by iodometric titration. Samples were dissolved in HCl solution containing an excess of KI and the liberated I2 was titrated with Na2S2O3 solution. Magnetization data were collected using a Quantum Design MPMS SQUID magnetometer. Powder second harmonic generation (SHG) measurements were performed using a modified Kurtz-NLO system33,34 incorporating a pulsed Nd:YAG laser with a wavelength of 1064 nm. The equipment and methodology has been described in detail previously.35 As the powder SHG efficiency has been shown to depend strongly on particle size,33 the reported materials were ground and sieved into distinct particle size ranges (90 μm). Relevant comparisons with known SHG materials were made by grinding and sieving crystalline α-SiO2 and LiNbO3 into the same particle size ranges. No index matching fluid was used in any of the experiments. A 0.75 mm thick polycrystalline pellet of Ba2YFeO5.5 was prepared be heating a sintered pellet of Ba2YFeO5 at 410 °C under 80 atm of oxygen pressure for 24 h. Silver paint electrodes of area 4 mm2 were then mounted on sections cut from the resulting pellet. Variable temperature dielectric constant measurements were performed using an Andeen Hagerling 2700A high precision capacitance bridge and an Oxford Instruments helium flow cryostat with a custom probe. In addition, the pyrocurrent was measured on warming at 1 K/min once the electrodes had fully discharged at 10 K, having cooled the sample in an electric field of both ±230 V/mm from room temperature. This measurement was repeated under an external magnetic field of 2 T, applied perpendicular to the electric field in a Quantum Design PPMS. Ferroelectric switching was investigated at 78 K by employing the PUND method.36 1801

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Figure 1. Structures of Ba2YFeO5 and Ba2YFeO5.5. (a, b) View down the b-axis of Ba2YFeO5 and Ba2YFeO5.5 respectively. (c, d) View down the caxis showing polyhedra at z ≈ 0.75 for Ba2YFeO5 and z ≈ 0 for Ba2YFeO5.5 respectively. (e, f) View down the c-axis showing polyhedra at z ≈ 0.25 for Ba2YFeO5 and z ≈ 0.5 for Ba2YFeO5.5 respectively. Solid lines indicate the respective unit cells. Gray octahedra represent YO6 units, orange tetrahedra FeO4 units, blue square-based pyramids FeO5 units, gray spheres Ba2+ cations.

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RESULTS

constructed in spacegroup Pb21m. The atomic positions of all the atoms and the fractional occupancies of the anion sites within the model were refined against neutron powder diffraction data collected at room temperature. This led to the refinement of a structural model strongly reminiscent of the Ba2YFeO5 parent structure, but in which additional oxide ions are inserted into alternate layers in the xy-plane, to yield an anion vacancy ordered structure of composition Ba2YFeO5.5 containing both FeO4 and FeO5 coordination sites as shown in Figure 1. Refinement of the iron and yttrium occupancies revealed no evidence of antisite disorder, indicating the cation order of the Ba2YFeO5 parent phase is faithfully retained on oxidation. The refinement converged rapidly to give a good visual and statistical fit (χ2 = 5.886). Full details of the refined

Structural Characterization of Ba2YFeO5.5. Iodometric titrations performed on the oxidized sample indicated an average iron oxidation state of Fe4+, consistent with the stated composition. X-ray and neutron powder diffraction data collected from the Ba2YFeO5.5 could be readily indexed using an orthorhombic unit cell (a = 12.1320(2) Å, b = 6.0606(1) Å, c = 8.0956(1) Å, V = 595.257(2) Å3) with extinction conditions consistent with Pb21m (No. 26) spacegroup symmetry. As Ba2YFeO5.5 was prepared via the low-temperature oxidation of the cation ordered perovskite Ba2YFeO5,31 the starting model for the structural refinement of Ba2YFeO5.5 was an oxygen stoichiometric cubic perovskite phase of composition Ba2YFeO6, with the B-cation ordering pattern of Ba2YFeO5, 1802

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structure of Ba2YFeO5.5 are given in Table 1, with selected bond lengths given in the Supporting Information (Table S1). Table 1. Refined Structure of Ba2YFeO5.5 at Room Temperaturea atom

site

x

y

z

Uiso (Å2)

Ba(1) Ba(2) Y(1) Y(2) Fe(1) Fe(2) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9)

4c 4c 2b 2a 2a 2b 2b 4c 4c 2b 2b 2a 2a 2a 2a

0.1264(6) 0.3852(6) 0.3743(4) 0.1249(4) 0.3566(4) 0.1548(5) 0.5086(13) 0.3697(5) 0.1089(5) 0.2526(11) 0.7818(8) 0.2553(12) 0.7424(10) 0.9855(10) 0.4836(8)

0.2310(22) 0.7392(22) 0.2439(18) 0.7311(21) 0.2337(18) 0.7203(16) 0.0090(24) 0.2567(14) 0.7202(19) 0.9570(21) 0.9539(18) 0.9913(24) 0.9785(22) 0.9404(22) 0.0375(16)

0.2471(8) 0.2376(7) 1/2 0 0 1/2 1/2 0.2259(8) 0.2890(7) 1/2 1/2 0 0 0 0

0.0150(6) 0.0150(6) 0.0069(6) 0.0069(6) 0.0070(11) 0.0116(12) 0.0190(17) 0.0139(7) 0.0139(7) 0.0139(7) 0.0139(7) 0.0139(7) 0.0139(7) 0.0190(17) 0.0123(21)

a Ba2YFeO5.5: space group Pb21m. a = 12.1320(2) Å, b = 6.0606(1) Å, c = 8.0956(1) Å, V = 595.257(2) Å3, χ2 = 5.886, wRp = 5.34%, Rp = 4.15%.

Observed calculated and difference plots from the refinement are shown in the Supporting Information (Figure S1). A representation of the structure of Ba2YFeO5.5 is shown in Figure 1. The refined structure of Ba2YFeO5.5 detailed above is described in a noncentrosymmetric space group, Pb21m, indicating that oxidative anion insertion breaks the inversion symmetry of the host Ba2YFeO5 lattice (space group P21/n). To confirm this feature of the oxidation reaction, powder SHG measurements were performed. These measurements confirmed Ba2YFeO5.5 exhibits type 1 phase-matchable SHG behavior with an efficiency comparable to that of α-SiO2 (see the Supporting Information, Figure S2), thus confirming the acentric nature of the crystal structure of Ba2YFeO5.5. Magnetic Characterization. Zero-field-cooled and fieldcooled DC magnetization data were collected in an applied field of 100 Oe from Ba2YFeO5.5 in the temperature range 5 < T/K < 300, and are shown in Figure 2a. The data collected in the range 40 < T/K < 300 can be readily fitted to the Curie−Weiss law (χ = C/(T − θ)) to yield values of C = 2.54 cm3 K mol−1 and θ = 14.7 K. At temperatures below 25 K, the zero-fieldcooled and field-cooled data diverge after going through a sharp local maximum at T ≈ 20 K. Magnetization-field isotherms collected at 20 and 5 K (Figure 2b) exhibit hysteresis and indicate moments of 1.27 and 1.45 μB per formula unit, respectively, in an applied field of 5 T. The irregular shape of the low-temperature magnetizationfield isotherms collected from Ba2YFeO5.5 motivated the collection of AC susceptibility measurements to determine the frequency dependence of the magnetic response of this phase and thus detect any spin-glass behavior. AC susceptibility data were therefore collected at frequencies of 1, 10, 100, and 1000 Hz in the temperature range 10 < T/K < 30. As shown in the Supporting Information (Figure S3), the real part of the magnetic response exhibits a sharp maximum at T ≈ 20 K which does not change as a function of measuring frequency, inconsistent with spin-glass behavior.

Figure 2. (a) Zero-field-cooled and field-cooled data collected from Ba2YFeO5.5. Inset shows a fit to the Curie−Weiss law to yield values of C = 2.54 cm3 K mol−1 and θ = 14.7 K. (b) Magnetization-field isotherms collected from Ba2YFeO5.5 at 5, 20, and 300 K, inset shows expanded region around zero applied field.

Neutron powder diffraction data collected from Ba2YFeO5.5 at 5 K exhibit additional diffraction features compared to analogous data collected at 298 K, indicative of magnetic order (Figure 3). The additional diffraction features fall into two classes: strong sharp features which can be indexed using the crystallographic unit cell and weaker broader features which can be indexed using a 1 × 1 × 2 expansion of the crystallographic unit cell. The intensities of the sharp magnetic diffraction features are best described using a ferromagnetic model in which the magnetic moments of the FeO5 iron centers are aligned paralleled to the a-axis with an ordered moment of 3.9(2) μB as shown in Figure 4. The intensities of the broad magnetic diffraction features are best described using an antiferromagnetic model in which the magnetic moments of the FeO4 iron centers are aligned parallel to the b-axis with an ordered moment of 3.6(4) μB as shown in Figure 4. Thus the ordered magnetic structure of Ba2YFeO5.5 can be seen as a noncollinear combination of ferromagnetically ordered layers FeO5 units interleaved with antiferromagnetically ordered layers FeO4 units. Attempts to introduce the FeO4 1803

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temperature between 10 and 240 K. The data, given in Figure 5a and b, show a large drop in εr coincident with a maximum in

Figure 3. A comparison of neutron diffraction data collected from Ba2YFeO5.5 at 5 and 298 K. Additional diffraction features in the 5 K data set fall into two classes: those which can be indexed using the crystallographic cell (solid arrows) and those indexed using a 1 × 1 × 2 expanded cell (dashed arrows).

Figure 4. Magnetic structures of Ba2YFeO5.5 refined against neutron diffraction data collected at 5 K. FeO5 centers (blue) order ferromagnetically with an ordered moment of 3.9(2) μB, FeO4 centers (orange) order antiferromagnetically with an ordered moment of 3.6(4) μB.

Figure 5. (a, b) Temperature dependence of the real component of the dielectric constant and the loss tangent, measured at 10 kHz. (c) The pyrocurrent measured under opposite poling fields. The lower inset shows detail around the magnetic ordering temperature with and without an applied 2 T magnetic field. Results of a PUND ferroelectric hysteresis measurement at 78 K are given in the upper inset.

centers into the ferromagnetic model, or the FeO5 centers into the antiferromagnetic model resulted in refinement of ordered moments on these centers which were zero, within error. A complete description of the magnetic and crystallographic models refined against neutron diffraction data collected at 5 K is given in the Supporting Information (Tables S2−S4), with the observed and calculated diffraction data (see Figure S4 in the Supporting Information). Examination of neutron diffraction data collected from Ba2YFeO5.5 at 20 K reveals weak magnetic diffraction peaks corresponding to both magnetic models, consistent with the anomaly observed in the magnetization data being associated with the magnetic ordering transition and indicating that both ferromagnetic and antiferromagnetic components of the magnetic model order at this temperature. It should be noted that the ordered ferromagnetic moments obtained from fits to neutron diffraction data collected at 5 and 20 K exceed the values observed in analogous magnetization measurements (Figure 3). It is not clear why this is, but suggests the magnetic structure of Ba2YFeO5.5 has a complex field dependence. Polar Behavior. The real component of the dielectric constant and loss tangent were measured as a function of

tan δ at 210 K. This behavior may be explained by Maxwell− Wagner relaxation, where charges in electrically inhomogenous materials become trapped at interfaces (e.g., grain boundaries) to give rise to Debye-like relaxation processes under an AC measuring voltage. This effect is typical of polycrystalline samples.37,38 In our Ba2YFeO5.5 measurement, the sample behaves intrinsically only below approximately T ≈ 100 K; once the material′s resistivity is sufficiently large to prevent the movement of charge, and hence charge trapping. Greater detail at low temperature is given in the inset of Figure 5a, which shows no evidence for an electric transition upon magnetic ordering at T ≈ 20 K. Figure 5c shows the pyrocurrent measured on warming, having cooled the sample with opposite applied electric fields from room temperature. No changes in the pyrocurrent were observed in the intrinsic region below 100 K, hence we find no evidence for a change in electric polarization or entry into a pyroelectric phase at low temperature. Above 100 K, an extrinsic pyrocurrent evolved due to the thermal relaxation of charge trapped at grain boundaries (as observed in the dielectric measurement described above), the sign of which is determined by the field cooling. The pyrocurrent was also 1804

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measured under an applied magnetic field to investigate any coupling of the polar state to the magnetic structure which orders below 20 K. As shown in the lower inset, the pyrocurrent had no dependence on magnetic field. This result doesn′t rule out ferroelectricity if the phase transition from paraelectric to ferroelectric occurs above 100 K. However, a measurement of ferroelectric hysteresis at 78 K via the PUND method36 gave a negative result, shown in the upper inset of Figure 5c.

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DISCUSSION Reaction of the cation-ordered, anion-deficient perovskite phase Ba2YFeO5 with 80 atm of oxygen pressure at 410 °C leads to the formation of the Fe4+ phase Ba2YFeO5.5. This lowtemperature oxidation of Ba2YFeO5 can be considered topochemical as the complex cation order and structural topology of the host material is retained in the product phase. However, the insertion of oxide ions into some of the vacant sites within the Ba2YFeO5 framework occurs in an ordered manner which results in a lowering of the crystallographic symmetry of the product relative to the parent phase and the formation of a material with an NCS crystal structure. The lowering of crystallographic symmetry that occurs during the oxidation of Ba2YFeO5 to Ba2YFeO5.5 can be separated into two distinct steps. In the first step oxide ions are inserted into half the vacant anion sites within the cationordered Ba2YFeO5 framework (site O(9)). This changes the Ba2O2−YFeO3−Ba2O2−YFeO3 stacking sequence of Ba2YFeO5 shown in Figure 1a into a Ba2O2−YFeO4−Ba2O2−YFeO3 stacking sequence of a hypothetical centrosymmetric Ba2YFeO5.5 phase. As a result there are now two crystallographically distinct iron centers: a set of FeO4 centers in the YFeO3 layers of the material which are largely unchanged from those present in Ba2YFeO5, and a new set of FeO6 centers that reside in the YFeO4 layers of the material as shown in Figure 6. The ordered manner in which the oxide ions are inserted into the anion deficient Ba2YFeO5 host structure is rather unusual. Typically the partial oxidation of anion deficient perovskite phases proceeds via the disordered insertion of oxide ions across all the vacant anion sites in the host. For example the oxidation of the brownmillerite phase Sr2MnGaO5 to Sr2GaMnO5.5 occurs via the insertion of oxide ions into vacant anion sites of the host lattice which reside within layers of GaO4 tetrahedra. This changes the stacking sequence of the phase from −SrO−MnO2−SrO−GaO−SrO−MnO2−SrO−GaO− to −SrO−MnO2−SrO−GaO1.5−SrO−MnO2−SrO−GaO1.5− and yields a material in which oxide ions have been inserted into all the GaO layers to give an average gallium coordination number of 5.39−41 However, although the oxidation behavior of Sr2MnGaO5 is typical of anion deficient perovskite phases, there are a few counter examples in which the anions are inserted in an ordered manner. For example the related brownmillerite phase Ca2MnGaO5 is oxidized to Ca2MnGaO5.5 via the ordered insertion of oxide ions into alternate layers of GaO4 tetrahedra to convert the −CaO−MnO2−CaO−GaO− CaO−MnO2−CaO−GaO− stacking sequence of the host to −CaO−MnO2−CaO−GaO2−CaO−MnO2−CaO−GaO− in which the gallium cations reside in an ordered array of GaO4 tetrahedra and GaO6 octahedra.42,43 It is thought that the ordered insertion of oxygen into Ca2MnGaO5 is favored over a disordered insertion, as it minimizes lattice strain in the resulting product phase.44 As discussed below, it is likely that a

Figure 6. YFeO3 layers in Ba2YFeO5 (top) and the YFeO4 layers in the hypothetical, undistorted centrosymmetric (middle) and distorted noncentrosymmetric (bottom) structures of Ba2YFeO5.5. Dark gray polyhedra represent YO6, orange and blue polyhedra represent FeOn and gray spheres represent Ba2+.

similar strain driven process is responsible for the ordered oxygen insertion in the Ba2YFeO5−Ba2YFeO5.5 system. An alternative explanation for the ordered insertion of oxygen into Ba2YFeO5 is offered by considering the possible disproportionation of the Fe4+ centers in Ba2YFeO5.5. A large number of perovskite phases containing Fe4+ centers are observed to undergo an internal disproportionation at low temperatures. Thus for example, Mossbauer spectroscopy and diffraction data reveal CaFeO3 consists of an ordered array of Fe3+ and Fe5+ centers at low temperature.45,46 A similar disproportionation of the Fe4+ centers in Ba2YFeO5.5 into Fe5+O5 and Fe3+O4 centers could explain the ordered insertion of oxygen observed on formation of this phase. However analysis of the FeO5 and FeO4 coordination polyhedra using bond valence sums47 (BVS) provide no support for this idea as the calculated BVS values for the two sites are almost identical (FeO5 = Fe+3.73; FeO4 = Fe+3.69) and remain equal even at 5 K. Thus there is no evidence for the disproportionation of Fe4+ in Ba2YFeO5.5 suggesting the observed ordered anion vacancy arrangement occurs to minimize lattice strain in the oxidized product. In the second symmetry breaking step the FeO6 octahedra in the hypothetical centrosymmetric Ba2YFeO5.5 phase undergo a distortion so that the iron centers now reside within FeO5 square-based pyramids, with average Fe−O bond lengths of 1.89 Å (see the Supporting Information, Table S1), with the “sixth” coordinating oxide ion lying 2.67 Å away (Figure 1, Figure 6). It is this distortion that breaks the inversion symmetry of the host phase perpendicular to the crystallographic b-axis resulting in an acentric crystal structure for Ba2YFeO5.5 and the observed SHG behavior of this phase. The distortion of the local iron coordination sphere is driven by two principal factors. The first is the large size mismatch 1805

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between the ionic radii of Y3+ and Fe4+, 0.900 and 0.585 Å, respectively.48 This large difference in size, combined with the ordering of the yttrium and iron centers into stripes within each YFeO4 layer, results in a large lattice strain in the hypothetical undistorted centrosymmetric Ba2YFeO5 phase, which cannot be relieved by a simple twisting of the MO6 units (Figure 6). The observed distortion to form FeO5 units relieves this lattice strain by breaking the Fe−O−Fe connectivity within the YFeO4 layers allowing more reasonable Fe−O and Y−O bond lengths to be adopted. The distortion of the iron coordination sphere also has an electronic contribution. Fe4+ centers have a d4 electron count, this means that in an octahedral ligand field they have a degenerate t2g3eg1 electronic configuration and are thus unstable with respect to a primary Jahn−Teller type distortion. Changing the iron coordination geometry from octahedral to square-based pyramidal lifts the orbital degeneracy of the iron centers by stabilizing the dz2 orbital with respect to the dx2− y2 orbital, leading to an overall electronic stabilization of the system. This observation could lead to the suggestion that the Fe4+ cations present in Ba2YFeO5.5 can be considered as ‘distortion centers’ in an analogy to the d0 transition metal cations and ns2 post-transition metal cations present in many NCS phases. However it should be noted there are a number of important differences between the Fe4+ centers in Ba2YFeO5.5 and more conventional distortion centers. The major difference arises from the nature of the electronic instability present. As described above ‘conventional’ distortion centers break local inversion symmetry due to a second-order Jahn−Teller instability, in contrast the octahedral Fe4+ cations in the hypothetical centrosymmetric phase of Ba2YFeO5.5 are unstable with respect to a “primary” Jahn−Teller distortion. Primary Jahn−Teller distortions of d4 octahedral transition metal centers will typically lower the point symmetry of the MO6 unit from Oh to D4h, retaining inversion symmetry, and thus do not naturally drive the formation of NCS structures. The slightly unusual distortion observed in Ba2YFeO5.5, which forms acentric FeO5 units with approximate C4v point symmetry, is attributable to the large bond strain present in the lattice of Ba2YFeO5.5. Thus it can be seen that it is the lattice strain present in centrosymmetric Ba2YFeO5.5 that provides the major driving force for the symmetry lowering distortion and adoption of an NCS structure. The Jahn−Teller instability of the Fe4+ centers only acts in combination with the lattice strain to direct the system to adopt a particular distorted structure from a number of possible strain relieving lattice distortions. The two-step strategy employed to lift the inversion symmetry of Ba2YFeO5.5 is quite different from the strategies generally used to favor the preparation of NCS materials described in the introduction. Rather than forming an acentric material by attempting to control the arrangement of distortion centers within an extended array, Ba2YFeO5.5 is prepared by first synthesizing a centrosymmetric precursor phase, which has a low-symmetry, anisotropic structure, and then topochemically oxidizing this phase to lift the inversion symmetry through the action of the resulting lattice strain and an induced electronic instability. This two-step strategy has a number of attractive features. For example, by carefully designing or selecting the precursor phase, polar displacements in the product material can be directed to align in a parallel NCS manner rather than with antiparallel symmetric disposition. In addition, by dispensing with the need for conventional distortion centers, which are necessarily diamagnetic, this two step approach

facilitates the incorporation of paramagnetic ions into NCS phases offering a route to the preparation of materials that exhibit multiferroic behavior, a much sought but “contraindicated” property.49 Magnetic Behavior. Magnetization data collected from Ba2YFeO5.5 can be fitted to the Curie−Weiss law in the temperature range 40 < T/K < 300 to yield a Curie constant of C = 2.54 cm3 K mol−1 broadly in line with that expected for a paramagnetic S = 2 Fe4+ system (Cexpected = 3 cm3 K mol−1). On cooling below 20 K a magnetically ordered state is adopted in which the FeO5 centers exhibit a ferromagnetic alignment while the FeO4 centers adopt an antiferromagnetic arrangement. The ferromagnetic couplings between FeO5 centers can be understood on the basis of the Goodenough − Kanamori rules.50 The square-based pyramidal coordination geometry of the Fe4+O5 centers would be expected to give rise to a (d xz , dyz)2(dxy)1(dz2)1(dx2−y2)0 electronic configuration, with the dz2 orbital aligned down the 4-fold axis of the FeO5 unit. As a result there would be Fe(dz2)1−O2p−Fe(dx2−y2)0 superexchange between adjacent FeO5 centers resulting in a ferromagnetic coupling, consistent with the refined magnetic structure. The diffraction features due to the antiferromagnetic order of FeO4 centers, observed in the neutron diffraction data collected at 5 K, are considerably broader than the diffraction peaks due to the either crystallographic lattice or the ferromagnetic order of the FeO5 centers. This difference in peak widths indicates that the antiferromagnetic order between FeO4 centers has a shorter ordered length scale than ferromagnetic order of the FeO5 centers, consistent with the absence of an extended Fe− O−Fe bonded network connecting the FeO4 units. Polar Behavior. The space group symmetry of Ba2YFeO5.5 (Pb21m) allows the material to exhibit both SHG behavior and pyroelectricity/ferroelectricity.1,2 Although the former behavior has been clearly demonstrated, we see no evidence for a switchable polarization in the material. This lack of switchability limits the material to pyroelectric behavior only. Attempts to isolate an undistorted centrosymmetric Ba2YFeO5.5 phase at high temperature have been limited by the metastability of the material, as it appears the temperature at which a structural distortion to a centrosymmetric phase occurs, is higher than the decomposition temperature of the phase. In the absence of such a transition, it is not possible to measure the pyroelectric current through a transition to a paraelectric phase, and as a result the size of the pyroelectric polarization in the material is unknown. As described above the NCS crystal structure adopted by Ba2YFeO5.5 is attributable to the large mismatch in ionic radii between Y3+ and Fe4+. This suggests that by reducing this size difference, by substituting Y3+ by another smaller rare earth cation, the temperature of the structural transition between the noncentrosymmetric phase of Ba2REFeO5.5 and a hypothetical centrosymmetric phase, could be lowered below the decomposition temperature of the material. This would not only enable the pyroelectric polarization of the material to be measured but also potentially facilitate the switching of any polarization present. An investigation of this possibility is currently under way.

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CONCLUSION Reaction of the centrosymmetric phase Ba2YFeO5 under oxygen pressure results in the formation of the acentric ferromagnetic pyroelectric material Ba2YFeO5.5. The formation of an acentric phase via the topochemical oxidation of a low1806

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symmetry but centrosymmetric precursor phase represents a new strategy for the preparation of NCS materials. By dispensing with the need to include conventional diamagnetic distortion centers to break the inversion symmetry of materials, this approach offers an opportunity to increase the chemical diversity of NCS materials and allows the incorporation of large numbers of paramagnetic transition metal centers offering a new route to the preparation of multiferroic materials.

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ASSOCIATED CONTENT

S Supporting Information *

Selected bond lengths and bond valence sums from the refined structure of Ba2YFeO5.5 at room temperature. Detailed description of the structural and magnetic refinement of Ba2YFeO5.5 against neutron diffraction data collected at 5 K. Fits to to neutron diffraction data collected from Ba2YFeO5.5 at room temperature and 5 K. Plot of particle-size vs SHG intensity data for Ba2YFeO5.5. AC susceptibility data collected from Ba2YFeO5.5. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +44 1865 272623. Fax: +44 1865 272690. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.L. thanks the Pacific Alliance Group for a scholarship. T.T.T. and P.S.H. thank the Robert A. Welch Foundation (Grant E1457) for support. Work performed at the Clarendon Laboratory was funded by EPSRC Grant EP/J003557/1, entitled “New Concepts in Multiferroics and Magnetoelectrics”. We thank E. Suard for assistance collecting the neutron powder diffraction data.

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