Article Cite This: Chem. Mater. 2018, 30, 1045−1054
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YCrWO6: Polar and Magnetic Oxide with CaTa2O6‑Related Structure Sun Woo Kim,† Thomas J. Emge,† Zheng Deng,‡ Ritesh Uppuluri,§ Liam Collins,∥ Saul H. Lapidus,⊥ Carlo U. Segre,# Mark Croft,∇ Changqing Jin,‡ Venkatraman Gopalan,○ Sergei V. Kalinin,∥ and Martha Greenblatt*,† †
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States ∥ Center for Nanophase Material Science & Institute for Functional Imaging Materials, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States # Department of Physics, and Center for Synchrotron Radiation Research and Instrumentation, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, Illinois 60616, United States ∇ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghusen Road, Piscataway, New Jersey 08854, United States ○ Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: A new polar and magnetic oxide, YCrWO6, was successfully synthesized and characterized. YCrWO6 crystallizes in polar orthorhombic space group Pna21 (no. 33) of edgesharing dimers of CrO6 and WO6 octahedra, which are connected by corner-sharing to form a three-dimensional framework structure with Y3+ cations located in the channels. The structure of YCrWO6 is related to that of CaTa2O6; however, the ordering of Cr3+ and W6+ in the octahedral sites breaks the inversion symmetry of the parent CaTa2O 6 structure. X-ray absorption near edge spectroscopy of YCrWO6 confirmed the oxidation state of Cr3+ and W6+. Temperature-dependent optical second harmonic generation measurements on YCrWO6 confirmed the noncentrosymmetric character and evidenced a noncentrosymmetric-tocentrosymmetric phase transition above 800 °C. Piezoresponse force microscopy measurements on YCrWO6 at room temperature show strong piezoelectric domains. Magnetic measurements of YCrWO6 indicate antiferromagnetic order at TN of ∼22 K with Weiss temperature of −34.66 K.
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INTRODUCTION
oxygen octahedral rotation in perovskites or layered perovskites.21−23 However, it is still a challenge to design and to synthesize new polar and magnetic oxide materials with optimal characteristics. Our earlier study on PbSb2O6-related materials (ABB′O6) demonstrated that by ordering or rearrangement of the B/B′ cations the inversion symmetry of the parent compound could be broken (e.g., P3̅1m, no. 162 → P312, no. 149, or P6̅2m, no. 189).24−26 Similarly, in our search for new multiferroic materials we have considered new structural phases where
Polar and magnetic oxide materials are investigated intensely, because of their fundamentally interesting and useful physical properties such as ferroelectricity, pyroelectricity, piezoelectricity, multiferroicity, and magnetoelectricity,1−5 all important for potential applications in advanced devices.6−9 Several strategies along with first-principles calculations have been proposed and investigated to search for new polar and magnetic oxides including the following: (1) the combination of stereo active lone pair (6s2) cations (Tl+, Pb2+, Bi3+) and/or secondorder Jahn−Teller (SOJT) d0 cations (Ti4+, Nb5+, Mo6+, W6+, etc.) with magnetic cations,10−13 (2) the substitution of magnetic cations in parent polar structures (e.g., LiNbO3-type or Ni3TeO6-type, etc.),14−20 and (3) octahedral tilting or © 2018 American Chemical Society
Received: November 25, 2017 Revised: January 2, 2018 Published: January 5, 2018 1045
DOI: 10.1021/acs.chemmater.7b04941 Chem. Mater. 2018, 30, 1045−1054
Article
Chemistry of Materials
should be good candidates for designing new polar magnetic oxide materials, if substitution of magnetic cations (3d−5d transition metals) into the crystal structure are possible. Previously, LnMWO6 (M = V, Cr, Fe) were proposed to exhibit an ordered arrangement of M3+ and W6+ with polar space group, Pna21 (no. 33); however, this study did not provide any detailed structure information and physical properties. 34 Recently, Ghara et al. reported ordered aeschynite-type polar magnets, RFeWO6 (R = Dy, Eu, Tb, and Y), with type-II multiferroic behavior.35 Based on this observation, we focused our efforts to find new polar and magnetic oxide materials with CaTa2O6-related structures through exploratory synthesis. Here we report the successful synthesis, crystal growth, and characterization of the CaTa2O6-related new polar and magnetic oxide, YCrWO6, and the investigation of its crystal structure−properties relationships. Cation ordering of Cr3+ and W6+ was expected, because of the charge difference of Cr3+/W6+ as well as the SOJT character of W6+ in an octahedral environment.
cation ordering/rearrangement might lead to breaking of the center of symmetry. The crystal structure of aeschynite-type materials (general formula, LnMM′O6) exhibits a CaTa2O6related structure (space group, SG: Pnma, no. 62)27 consisting of edge-sharing dimers of [(M,M′)O6] octahedra connected by corner-sharing to form a three-dimensional framework structure with the Ln3+ ions located in the channels (Figure 1).28,29 Aeschynite-type materials, Ln(Ti4+M5+)O6 (Ln = La,
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Figure 1. Ball-and-stick structure of CaTa2O6-related materials: (a) CaTa2O627 in the ab-plane (left) and (b) Ln(Ti4+M5+)O6 (Ln = La to Dy; M = Nb or Ta)28−33 in the ab-plane (right).
EXPERIMENTAL SECTION
Reagents. Y2O3 (Alfa Aesar, 99.99%), Cr2O3 (Alfa Aesar, 99.97%), and WO3 (Alfa Aesar, 99.8%) were used without any further purification. Y2O3 was preheated at 950 °C for overnight before use. Synthesis. Polycrystalline YCrWO6 was prepared by a conventional solid state reaction. Stoichiometric amounts of Y2O3 (0.3387 g, 1.5 mmol), Cr2O3 (0.2280 g, 1.5 mmol), and WO3 (0.6955 g, 3.0 mmol) were thoroughly ground and pressed into a pellet. The pellet was placed on Pt foil in an alumina boat and treated at 1150 °C for 12 h in air, then reground into fine powder, pressed into a pellet again, and heated to 1150 °C for 12 h, and then cooled to room temperature (the heating and cooling rate was 200 °C/h, respectively). Brown-
Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy; M = Nb or Ta) have been investigated due to their microwave dielectric properties as well as for phosphor applications. 30−33 In aeschynite-type LnMM′O6, if M and M′ are ordered, their crystal structure will adopt lower symmetry polar space group, Pna21 (no. 33), due to symmetry constraints. Hence these types of materials Table 1. Crystal Data and Structure Refinement for YCrWO6 formula weight temperature wavelength crystal system space group unit cell dimensions
420.76 293(2) K 0.71073 Å orthorhombic Pna21 (no. 33) a = 10.8675(8) Å, α = 90° b = 5.1633(4) Å, β = 90° c = 7.3072(5) Å, γ = 90° 410.02(5) Å3 4 6.816 Mg/m3 44.559 mm−1 740 0.140 × 0.070 × 0.020 mm3 3.750−32.603° −16 ≤ h ≤ 15, −7 ≤ k ≤ 7, −11 ≤ l ≤ 11 5424 1483 [R(int) = 0.0329] 100.0% numerical 0.660 13 and 0.018 37 full-matrix least-squares on F2 1483/13/84 1.114 R1 = 0.0200, wR2 = 0.0417 R1 = 0.0211, wR2 = 0.0420 0.43(2) 0.0021(2) 1.634 and −2.216 e.Å−3
volume Z density (calculated) absorption coefficient F(000) crystal size θ range for data collection index ranges reflections collected independent reflections completeness to θ = 25.242° absorption correction max and min transmission refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) absolute structure parameter extinction coefficient largest difference peak and hole 1046
DOI: 10.1021/acs.chemmater.7b04941 Chem. Mater. 2018, 30, 1045−1054
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Chemistry of Materials Table 2. Atomic Coordinates and Displacement Parameter for YCrWO6 atom
Wyckoff position
x
y
z
Ueq (Å2)*
Y(1) Cr(1) W(1) O(1) O(2) O(3) O(4) O(5) O(6)
4a 4a 4a 4a 4a 4a 4a 4a 4a
0.543 23(7) 0.367 42(10) 0.353 14(2) 0.3805(5) 0.2138(5) 0.2910(5) 0.5256(5) 0.3574(5) 0.4779(5)
0.954 56(13) 0.5466(2) 0.556 52(4) 0.6705(10) 0.3840(11) 0.8743(11) 0.7301(11) 0.4400(10) 0.2541(11)
0.5213(2) 0.7727(8) 0.277 12(2) 0.5187(9) 0.3354(8) 0.2063(8) 0.8141(7) 0.0289(10) 0.7313(7)
0.005 32(14) 0.0042(2) 0.004 37(8) 0.0055(9) 0.0052(10) 0.0056(11) 0.0050(11) 0.0065(11) 0.0049(11)
colored YCrWO6 polycrystalline powder was obtained, and the purity of sample was confirmed by powder X-ray diffraction (XRD). Crystal Growth. Crystals of YCrWO6 were grown from Li2CO3/ B2O3 flux. The starting materials (YCrWO6:Li2CO3:B2O3 = 3:5:5) were mixed and placed in a Pt crucible, heated at 1200 °C for 24 h, cooled slowly to 950 °C (rate: 2 °C/h), and then cooled to room temperature (rate: 100 °C/h). Most of the flux evaporated, and the products were washed with distilled water and ethanol aided by sonication and isolated by vacuum filtration. Light brown-colored plate-shaped crystals of YCrWO6 and light green-colored block-shaped crystals of YCrO3 were recovered. Single Crystal Diffraction. A light brown-colored plate-shaped crystal of YCrWO6 (0.140 × 0.070 × 0.020 mm3) was selected for single crystal data collection. The data were collected with a Bruker SMART APEX CCD diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) at 293 K.36 The data were corrected for Lorenz effects and polarization and absorption, the latter by numerical methods. The structure was solved by direct methods (SHELXS-2007), and all atomic positions and thermal parameters were refined with program SHELXL (2014);37,38 the refinement converged for I > 2σ(I). Relevant crystallographic data, atomic coordinates, and selected bond distances and angles for YCrWO6 are given in Tables 1, 2, and 3, respectively. Laboratory and Synchrotron Powder X-ray Diffraction. YCrWO6 was characterized by powder X-ray diffraction (PXRD, Bruker-AXS D8-Advanced diffractometer with Cu Kα, λ = 1.5406 Å, 40 kV, 40 mA) for purity and phase identification. Synchrotron powder X-ray diffraction (SPXD) data were collected at ambient temperature and at 800 °C at the 11-BM beamline of the Advanced Photon Source (APS), Argonne National Laboratory, United States, with X-ray wavelength λ = 0.413 534 Å for ambient temperature and λ = 0.412 620 Å for 800 °C, respectively. Diffraction data analysis and Rietveld refinement for room-temperature and high-temperature phase were performed with the TOPAS39 and GSAS-EXPGUI40 software package based on the single crystal data of YCrWO6. Possible hightemperature structure models of YCrWO6 were predicted by the PSEUDO program.41 No impurities were observed; the calculated and experimental SPXD patterns are in good agreement (see the Supporting Information, SI, Figures S1 and S6). X-ray Absorption Near Edge Spectroscopy (XANES). X-ray absorption near edge spectroscopy (XANES) data of YCrWO6 were collected in both the transmission and fluorescence mode with simultaneous standards on beamline 10-BM (MRCAT) at the Advanced Photon Source (APS) with a 50% detuned, water-cooled, Si(111) double crystal monochromator. W-L3,2,1 measurements were also performed at NSLS-II on the Inner Shell Spectroscopy (ISS) insertion device (damping wiggler) 8-ID beamline with a Si(111) double crystal high-heat load monochromator operated in an extremely rapid continuous energy variation mode. The spectra were fit to linear pre- and postedge backgrounds and normalized to unity absorption edge-step across the edge.15,42−50 Some of the XANES standard spectra were collected in the past on beamline X-19A at the NSLS as described in previous publications.42−44,46−49 Second Harmonic Generation (SHG). Temperature dependence of optical SHG of YCrWO6 was obtained by measurements on pellets in reflection geometry at normal incidence, with an 800 ± 20 nm
Table 3. Selected Bond Distances, Angles, and Bond Valences Sum (BVS) for YCrWO6 cation
anion
bond length (Å)
BVS
Y(1)
O(1) O(2) O(3) O(4) O(4) O(5) O(6) O(6) O(1) O(2) O(3) O(4) O(5) O(6) O(1) O(2) O(3) O(4) O(5) O(6)
2.298(6) 2.445(6) 2.419(6) 2.346(5) 2.441(5) 2.306(5) 2.291(5) 2.389(5) 1.969(9) 2.006(6) 1.998(6) 1.986(6) 1.954(9) 1.953(6) 1.884(6) 1.808(6) 1.848(6) 2.000(5) 1.912(7) 2.107(6)
3.16 (Y3+)
Cr(1)
W(1)
3.03 (Cr3+)
6.12 (W6+)
angle (deg) Cr(1)−O(1)−W(1) Cr(1)−O(2)−W(1) Cr(1)−O(3)−W(1) Cr(1)−O(4)−W(1) Cr(1)−O(5)−W(1) Cr(1)−O(6)−W(1)
140.4(3) 138.0(4) 129.9(3) 101.4(3) 145.2(3) 98.8(2)
fundamental input generated by a Ti-sapphire laser (Spectra-Physics, 80 fs pulses, 2 kHz frequency). The SHG signal was detected with a photomultiplier tube (Hamamatsu H7926). The samples were heated to 800 °C and cooled at a rate of 10.0 °C/min on a home-built heating stage. Piezoresponse Force Microscopy (PFM) and Contact Kelvin Probe Force Microscopy (cKPFM). Piezoresponse force microscopy (PFM) was employed to investigate the existence of ferroelectric domains in YCrWO6 at room temperature. Briefly, PFM is a voltagemodulated atomic force microscopy (AFM) technique which measures the sample deformation resulting from the converse piezoelectric effect. The approach is commonly used to image ferroelectric domains in piezoelectric crystals, ceramics, and thin films.51−55 Here, we implemented band-excitation-PFM (BE-PFM), which allows capture of the full contact resonance peak, hence reducing the influence of inherent artifacts and crosstalk from topography channels that commonly exist in single frequency PFM.56,57 BE-PFM measurements were performed with a Pt/Ir-coated probe (Budget sensors Multi75EG). For BE-PFM imaging a 7 Vpp ac voltage with a drive frequency of the ac bias was centered at the contact resonance (∼350 kHz), and a 1047
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Chemistry of Materials bandwidth of 60 kHz was used to capture the contact resonance as the tip was scanned across the sample surface. In the postprocessing step the contact resonance response was fit to a simple harmonic oscillator model, providing a measure of the cantilever parameters, namely, amplitude, frequency, phase, and quality factor, as described elsewhere.56 In contact Kelvin probe force microscopy (cKPFM), instead of using a typical BE-PFM spectroscopy waveform consisting of linear dc voltage steps with read steps (no bias voltage) in-between, multiple cycles of dc write pulses, with single pulse dc read steps that vary with each cycle, are applied. The dc write pulses induce local changes in the material, which are detected with BE-PFM. The small-amplitude dc voltage steps performed after the write voltage allow exploration of bias dependence of the remnant offsets which can be useful for separation of charge injection vs polarization switching. The ability to distinguish between electromechanical response and electrostatic interactions is particularly important to study materials whose functional properties are unclear.58,59 Magnetic Measurements. The magnetic measurements of YCrWO6 were performed with a commercial Quantum Design SQUID VSM magnetometer. The dc magnetic susceptibility data were collected between 2 ≤ T ≤ 300 K under an applied magnetic field of 10 000 Oe. Isothermal magnetization curves were obtained for magnetic fields: −7T ≤ H ≤ 7T at T = 5 and 300 K.
inversion symmetry to form a polar structure. The origin of such ordering is attributable to the difference of octahedral distortion between Cr3+ and W6+ cations (distortion parameter, Δd × 10−3 = 0.1078 for Cr3+ and 2.7039 for W6+) associated with Coulombic repulsion of Cr3+/W6+ off the centering in the edge-sharing octahedra, as well as the SOJT effect of W6+ cation in an octahedral environment. In YCrWO6 the Cr−O bond distances range between 1.953(6) and 2.006(6) Å; those of the W−O, 1.884(6) and 2.107(6) Å; and those of Y−O, 2.298(6) and 2.445(6) Å. The bond angles of Cr−O−W range between 98.8(2)° and 140.4(3)°. Selected bond distances and angles for YCrWO6 are summarized in Table 3. The local coordination environments are shown in the Supporting Information, Figure S2. Bond valence sum calculations60,61 resulted in values of 3.16, 3.03, and 6.12 for Y3+, Cr3+, and W6+, respectively. (See Table 3.) Cr−K-Edge XANES. The signatures of 3d transition metal valence state variations can be discerned in both the main- and pre-edge regions of their K-edge X-ray absorption spectra. The main-edge features at 3d transition metal K-edges are dominated by 1s to 4p transitions, riding on a step-feature continuum onset. Despite substantial variations/energy-splitting in the main-edge features, the chemical shift (to higher energy with increasing valence) of the main-edge has been widely used to chronicle the evolution of the transition metal valence state in oxide-based materials.42−44,46−49 In Figure 3a the Cr−K main-edge for YCrWO6 is compared to those for a series of formally Cr3+, Cr4+, and Cr6+-standard compounds. The centrum of the main-edge-step provides a measure of the chemical shift which can be seen to shift systematically to higher energy with increasing valence of the standards. It is worth comparing the Cr3+ standards. The LaCrO3 main-edge crosses the Cr2O3 edge in the center of its split-feature rise, thereby defining the Cr3+ chemical shift. Importantly the YCrWO6 spectrum crosses the centrum of the edge-rises of the Cr3+-standard spectra, thus indicating that it is a basically Cr3+ compound. Figure 3b shows an expanded view of the pre-edge region for the same K-edges shown in the previous figure. The pre-edge features at the K-edges of 3d transition metal compounds are due to quadrupole allowed 1s/3d or dipole allowed 1s/3d-phybridized transitions shifted to the pre-edge region by the 1shole/d-electron Coulomb interaction. The structure and energy shift of the pre-edge features can offer a second method of identifying the transition metal valence changes.42−44,46−49 The structure of the YCrWO6 pre-edge manifests a bimodal a-b structure (with a ∼ 4 eV separation), typical of Cr3+ standards (see Figure 3b for the a-b feature labels). Moreover, the onset energy of the a-feature (see “o” in Figure 3b) occurs at a low energy typical of a Cr3+ material. Thus, both the featurestructure and chemical shift of the pre-edge of YCrWO6 confirm the basically Cr3+ character in this material, as concluded from the main-edge measurements above. W-L3,2,1 Edge XANES. The L2,3 edges of d-row transition metal compounds are dominated by an intense “white line” (WL) feature involving transitions into empty d final states.15,44−46,48,50 Both the chemical shift and the structure of such WL features can be used as a valence/d-occupancy indicator in these materials. It should be noted that the W-L3 edge results presented here were collected at both the APS and NSLS-II.
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RESULTS AND DISCUSSION Structure. The room-temperature single crystal structure of YCrWO6 was determined to be in the polar orthorhombic space group Pna21 (no. 33), with lattice parameters of a = 10.8675(8), b = 5.1633(4), c = 7.3072(5) Å, V = 410.02(5) Å3, and Z = 4. YCrWO6 exhibits a three-dimensional crystal structure consisting of edge-sharing dimers of Cr(1)O6 and W(1)O6 octahedra, which are connected by corner-sharing to form a three-dimensional framework structure with Y3+ cations (YO8 polyhedra) located in the cavities (see Figure 2). Compared to the parent CaTa2O6 structure (Pnma, no. 62), the cation ordering of Cr3+ and W6+ in the octahedral environment induces a lower crystal symmetry and breaks the
Figure 2. Crystal structure of YCrWO6: (a) polyhedral diagram in the ab-plane (top) and (b) ball-and-stick diagram in the ac-plane (bottom). 1048
DOI: 10.1021/acs.chemmater.7b04941 Chem. Mater. 2018, 30, 1045−1054
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Figure 3. (a) Cr−K main-edge spectra for the YCrWO6 along with those for a series of Cr compounds with varying formal valences: Cr3+, LaCrO3, and Cr2O3; Cr4+, CrO2; and Cr6+, K2Cr2O7. Here the Cr−O coordination is octahedral except for the Cr6+ standard where it is tetrahedral (which causes the intense hybridization-induced pre-edge feature). (b) Cr−K pre-edge spectra for the octahedrally coordinated compounds in Figure 3a. The vertical dashed lines underscore the downward energy shift of the YCrWO6 pre-edge features.
Figure 4a compares the L3 edge of YCrWO6 to those of a series of standard W compounds. The chemical shift of the center of the WL to higher energy with increasing W-valence can be seen in the W0, W4+O2, W6+O3 sequence of standard spectra. The Sr2MnWO6 standard spectrum nicely illustrates the unresolved A (t2g-related)/B(eg-related) octahedral-ligandfield splitting in the 5d final states typical of such perovskitebased compounds.45,48−50 The centrum of the Sr2MnWO6 standard WL feature also manifests a chemical shift typical of a W6+ compound. The centrum of the A−B split YCrWO6 WL feature also has a chemical shift typical of a W6+ compound. The WL feature-structure indicator of 5d-orbital occupancy is illustrated in Figure 4b for a series of 5d-row compound standards spanning the d0 to d4 configurations. The A (t2g-hole final state) feature can be clearly seen to decrease in intensity with increasing 5d occupancy (decreasing 5d hole count). The B (eg-hole final state) feature maintains a constant relative
Figure 4. (a) W-L3 near edge spectra for the YCrWO6 compound along with those for a series of W compounds with varying formal valences: elemental W0, W4+O2, W6+O3, and the W6+ double perovskite Sr2MnWO6. (b) T-L3 near edge spectra (T = W, Re, and Ir) illustrating the decreasing A-feature intensity with increasing 5dorbital filling. Here the spectra have been displaced in energy to roughly align the A-feature. (c) W-L3 near edge spectra for the YCrWO6 compound along a series of W compounds with varying formal valences: elemental W4+O2, W6+O3, and the W6+ double perovskite Sr2MnWO6.
intensity since it remains formally unoccupied. With this in mind, the similarity of the YCrWO6 A-feature to that of 1049
DOI: 10.1021/acs.chemmater.7b04941 Chem. Mater. 2018, 30, 1045−1054
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Chemistry of Materials Sr2MnWO6 supports the ∼W6+/d0 assignment for the former compound. It is worth noting that the A−B feature splitting in YCrWO6 is noticeably reduced relative to the perovskite standard. Indeed, the root-mean-square distribution of the W− O distances in YCrWO6 is 8-fold larger than that in the perovskite standard45 making this smearing of the ligand field states reasonable. Thus, both the WL feature chemical shift and bipeaked structure indicate the ∼W6+/d0 state for YCrWO6. The L3 WL feature (involving 2p3/2 to 5d5/2,3/2 transitions) is most frequently used to probe valence/d-configuration in 5d transition metal compounds because its large spectral intensity provides the best XAS results with the shortest commitment of scarce synchrotron beam time. With the advent of next generation synchrotron beamlines, like the insertion device ISS8ID at NSLS-II, ∼400 μm beam size XAS measurements can now be made in mere seconds as compared to ∼1 h in the past. This opens the opportunity for a rapid, more complete XAS characterization. The W-L2 near edge, for example, involves 2p1/2 to 5d3/2 transitions and, by virtue of smaller multiplet and spin orbit effects, can be expected to provide a WL-feature A−B structure closer to the d-orbital density of states above the Fermi level.62 This effect is well-illustrated in Figure 4c, where the W-L2 near edge of YCrWO6 is compared to those of the WO2, WO3, and Sr2MnWO6 standards. Specifically, it should be noted that the A(t2g-hole) WL-feature is dramatically more intense than the B(eg-hole) feature, in the W6+-material W-L2 spectra, as compared to the A−B spectral intensities in comparable W-L3 spectra in Figure 4a. Figure S3a shows the W-L1 spectra for WO2, YCrWO6, WO3, and Sr2MnWO6. The W-L1 main-edge features dominated by dipole allowed 2s to p-hole state transitions. The substantially smaller chemical shift of the W4+, WO2 spectrum, relative to the other ∼W6+ compound spectra, is clearly seen in the figure. The much lower-intensity W-L1 pre-edge features, noted in Figure S3a, are due to quadrupole 2s to 5d transitions along with dipole transitions into d-p hybridized final states. Displacement of the W from the center of the O-octahedron allows for greater d-p hybridization and enhanced W-L1 pre-edge feature intensity. The W-L1 pre-edge region is plotted on a greatly expanded scale in Figure S3b (upper left). Although weak, a bimodal A−B feature character can be seen to be present in both the YCrWO6 and Sr2MnWO6 W-L1 pre-edge features. The 5d-hole final state character of these A−B features (and the t2g-eg origin of their energy-splitting) is emphasized by comparison to the W-L3 edges of YCrWO6 and Sr2MnWO6 plotted with the same energy scale factor (and roughly aligned with the A−B features) in Figure S3b (lower right). The intensity enhancement of the W-L1 pre-edge features in YCrWO6 is consistent with its substantially distorted W environment (noted previously) and the concomitant p-d hybridization enhancement of dipole transitions. Interestingly the known strong displacement of the W site in WO3, from the center of its O-octahedra, is consistent with its enhanced W-L1 pre-edge feature, also shown in Figure S3b (upper left).63 Second Harmonic Generation (SHG). We have performed temperature-dependent SHG measurement on YCrWO6 to determine if there is a noncentrosymmetric-tocentrosymmetric (NCS-to-CS) phase transition, and to gain a better understanding of the origin of NCS character of this material. The SHG intensity is shown as a function of temperature for YCrWO6 in Figure 5. A finite SHG signal is observed in the whole measurement temperature range, confirming that YCrWO6 is NCS. Overall, the optical SHG
Figure 5. Temperature dependence of optical SHG intensity for YCrWO6 pellet between 0 and 800 °C.
intensity gradually decreases with increasing temperature in the temperature range up to 800 °C, which could indicate a second-order transition from the NCS phase to the CS phase. A NCS-to-CS phase transition could occur above 800 °C. Although differential thermal analysis (DTA) on YCrWO6 (see the SI, Figure S4) did not indicate any phase transition, this is attributed to the difficulty of determining a second-order phase transition in DTA, which needs a smaller amount of energy than a first-order transition. If the phase transition is second order, the CS phase should belong to a supergroup of the polar space group Pna21 (no. 33) structure. Possible supergroup structures were searched within atomic displacements of 2 Å with the PSEUDO program to predict the high-temperature phase of YCrWO6.41 Two possible supergroup structures, Pnna, no. 52, or Pnma, no. 62, were found (see the SI, Figure S5), when reasonable bond distances and coordination environment of each atom were considered. We collected high-temperature (800 °C) SPXD data and attempted to solve the high-temperature structure of YCrWO6; we were able to refine the data with the presence of two simultaneously coexisting phases: a CS HT phase, Pnma (a = 10.926 05(5), b = 7.341 25(2), c = 5.195 68(2) Å, V = 416.750(2) Å3) at ∼43%, and the NCS phase, Pna21 (a = 10.925 90(2), b = 5.195 76(1), c = 7.341 23(1) Å, V = 416.749(1) Å3) at ∼57% (χ2 = 1.582, Rp = 0.0781, Rwp = 0.1028). Refinement without the Pna21 phase was poor. Thus, we conclude that two phases coexist at high temperature, and if the sample were allowed to equilibrate at 800 °C, or higher for a longer time, we might have been able to observe a higher percentage, or a complete conversion of NCS Pna21 to CS Pnma (see the SI, Figure S6). Piezoresponse Force Microscopy (PFM) and Contact Kelvin Probe Force Microscopy (cKPFM). In Figure 6a, the AFM topography of the polished sample embedded in epoxy is provided, showing clear grain structures with some pits, likely a result of the polishing step. The corresponding PFM amplitude image (Figure 6b) shows strong domain contrast with single grains, which suggest that the material has strong piezoelectric response. The domains are correlated with phase changes in Figure 6d which show 180° phase inversion between some domains across the surface. Finally, the frequency maps (Figure 1050
DOI: 10.1021/acs.chemmater.7b04941 Chem. Mater. 2018, 30, 1045−1054
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Figure 6. PFM results on YCrWO6. (a) Topography (scan size is 10 μm). Band-excitation-PFM (BE-PFM) (b) amplitude and (c) resonance frequency and (d) phase images.
6c) show a stable value, with contrast observed only at the pits, resulting from sudden changes in the tip−sample contact, effectively demonstrating that the PFM response is real, and not artifact topography effects. The switching properties of the material were explored with contact Kelvin probe force microscopy (cKPFM). We performed cKPFM on a 40 × 40 grid in a similar region to the previous PFM measurement. The topography before and after the cKPFM measurements are shown in Figure S7a,b, respectively. Clear changes in the sample surface at the locations of the cKPFM measurements can be seen (Figure S7b), which imply that a local electrochemical reaction may have taken place under the tip. The cKPFM curves as a function of read voltage59 (Figure S7c) demonstrate a linear single band structure, which indicate that hysteretic polarization switching has not taken place (e.g., here we would expect a double band structure). Linear fitting was applied to the intercept and the nulling point (i.e., jCPD) from the cKPFM response as a function of read voltage. The jCPD map is shown in Figure S7d that shows strong similarities with the piezoelectric domain response in the previous image. In conclusion, we observe clear piezoelectric domain contrast using PFM, and although the material may in fact be ferroelectric, ferroelectric switching with cKPFM was not observed. It is possible that the high electric fields under the conductive tip resulted in sample modification before polarization switching could occur. Magnetic Behavior. The dc magnetic susceptibility of YCrWO6 was measured under 10 000 Oe in the temperature range 2−300 K and is shown as χ and 1/χ versus T plots in Figure 7a,b, respectively. YCrWO6 exhibits antiferromagnetic behavior with a sharp Néel transition temperature (TN) at ∼22 K, consistent with three-dimensional magnetic behavior.64,65 No significant divergence between ZFC (zero field cooling) and FC (field cooling) magnetization curves is observed. From the 1/χ versus temperature shown in Figure 7b, the susceptibility data were fit to the Curie−Weiss (CW) law, χ = C/(T − θ) for T > 50 K, where C is the Curie constant, and θ is the Weiss constant; C = 1.799 emu K/mol and θ = −34.66 K were extracted from the CW fit of the data. On the basis of the CW fit, the effective magnetic moment, μeff = 3.793 μB/Cr
Figure 7. Magnetic behavior of YCrWO6. (a) Temperature dependence of the magnetic susceptibility of YCrWO6 measured in 10 000 Oe. (b) Inverse magnetic susceptibility of YCrWO6 with a Curie− Weiss fit (solid line).
is in good agreement with the theoretical spin only value for Cr3+ (3.87 μB, S = 3/2). The negative Weiss constant indicates AFM interactions, which could arise from super-super-exchange interaction of long-term chains, Cr 3+−O2−−W6+−O2−− Cr3+.66−68 In Figure S8, the isothermal magnetization of YCrWO6 measured at 5 and 300 K as a function of applied field H indicates that some degree of spin reorientations are present below TN (∼22 K).
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CONCLUSION A new polar and magnetic oxide, YCrWO6 with CaTa2O6related structure, was successfully synthesized by a conventional solid state reaction. Because of cation ordering of Cr3+ and W6+ in the octahedral environment, YCrWO6 adopts a lower symmetry polar space group, Pna21 (no. 33), in contrast to its centrosymmetric (Pnma, no. 62) structural analog, CaTa2O6. Temperature-dependent second harmonic generation measurement on YCrWO6 confirms noncentrosymmetric character, and above 800 °C, a noncentrosymmetric-to-centrosymmetric phase transition is suggested. This noncentrosymmetric-tocentrosymmetric phase transition was confirmed by analysis of 1051
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Chemistry of Materials the 800 °C SPXD data, which evidenced partial conversion of the sample from the NCS Pna21 to CS Pnma. PFM measurements show strong piezoelectric domains in YCrWO6; however, no clear polarization switching is observed. YCrWO6 exhibits antiferromagnetic behavior with TN of ∼22 K. Previously, Thorogood et.al. reported the crystal structure of Ln(Ti4+Ta5+)O6 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and showed that their crystal structures changed from aeschynite-type (CaTa2O6-related, Pnma, no. 62) to euxenite-type (CaNb2O6-related, Pbcn, no. 61) structure depending on the size of Ln cation.29 This result suggests that the size of Ln cation also affects the symmetry of LnCrWO6. Further compositional modification studies of LnM(III)WO6 (M = Cr, Fe, Mn) series are in progress to validate symmetry breaking principles to discover new multiferroic/magnetoelectric materials.
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authors wish to thank the NSLS-II scientists Klaus Attenkofer, Eli Stavitski, Sizhan Liu, and Trevor Tyson from NJIT for their prodigious help, without which this work could not have been done.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04941. Rietveld refinement plot from SPXD data for YCrWO6 (room temperature and 800 °C); W-L1 spectra for the YCrWO6; TG/DTA analysis for YCrWO6; contact KPFM (cKPFM) results on YCrWO6; and isothermal magnetization of YCrWO6 at 5 and 300 K as a function of applied field H (PDF) Crystallographic information file (CIF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sun Woo Kim: 0000-0003-1057-2283 Liam Collins: 0000-0003-4946-9195 Carlo U. Segre: 0000-0001-7664-1574 Martha Greenblatt: 0000-0002-1806-2766 Author Contributions
The manuscript was written by contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.W.K. and M.G. gratefully acknowledge support from the NSF-DMR-1507252 grant. S.W.K. thanks Dr. Xiaoyan Tan, Corey Frank, and Prof. David Walker (Columbia University) for preparing a sample for PFM measurement. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. R.U. and V.G. acknowledge support from the National Science Foundation MRSEC Grant DMR-1420620. Part of this research used the ISS, 8-ID beamline at the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The 1052
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