Influence of Topotactic Reduction on the Structure and Magnetism of

Oct 2, 2009 - Andrew J. Overton, James L. Best, Ian Saratovsky, and Michael A. Hayward*. Department of Chemistry, Inorganic Chemistry Laboratory, ...
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4940 Chem. Mater. 2009, 21, 4940–4948 DOI:10.1021/cm9021276

Influence of Topotactic Reduction on the Structure and Magnetism of the Multiferroic YMnO3 Andrew J. Overton, James L. Best, Ian Saratovsky, and Michael A. Hayward* Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, OX1 3QR, United Kingdom Received July 14, 2009. Revised Manuscript Received September 14, 2009

The topotactic reduction of the acentric phase YMnO3 with CaH2 in the temperature range 225 °C < T < 350 °C yields phases of composition YMnO3-x (x = 0 < x < 0.2). Oxide ions are deintercalated from the equatorial anion sites of the material, which induces increasing levels of structural disorder into the resulting phases, ultimately leading to the adoption of a centrosymmetric structure by x = 0.2. Variable-temperature X-ray diffraction measurements indicate a lowering of the “ferroelectric” transition temperature with increasing anion vacancy concentration, consistent with the structural data. Lowtemperature neutron diffraction data show YMnO2.95 and YMnO2.85 adopt antiferromagnetically ordered magnetic structures with P63 and P63cm symmetries, respectively, consistent with the increased level of structural disorder in the latter phase. Introduction There has been a resurgence in the interest in multiferroic materials recently because of the wide array of technological uses a material with coupled ferroelectric and magnetic behavior would have. These go beyond the obvious data storage applications to include roles in transducers and magnetic and electric field sensors, as detailed in a number of good reviews in this area.1,2 The hexagonal perovskite REMnO3 phases (RE = Y, Sc, lanthanide) have been of particular interest in the study of multiferroic behavior because the acentric structures these materials exhibit are not due to an electronically driven distortion, unlike many of the major classes of ferroelectric, e.g., the d0 transition metal oxides (e.g., BaTiO3) or the post-transition metal oxides with stereo active lone pairs (e.g., BiFeO3),3 but due to geometric packing effects. At high temperature, hexagonal REMnO3 phases adopt centrosymmetric structures consisting of MnO5 trigonal bipyramids that share corners to form layers with three-fold rotational symmetry. These layers are stacked in an ABAB manner with RE3þ ions located in the interlayer region (Figure 1). On cooling, the MnO5 units undergo a cooperative tilting distortion which buckles the trigonal planes. This leads to an expansion of the crystallographic unit cell and a lowering of the space group symmetry from P63/mmc to P63cm and ultimately ferroelectric behavior. On further cooling *To whom correspondence should be addressed. E-mail:michael. [email protected]. Tel. þ44 1865 272623. Fax: þ44 1865 272690.

(1) (2) (3) (4)

Eerenstein, W.; Mathur, N. D.; Scott, J. F. Nature 2006, 442, 759. Fiebig, M. J. Phys. D: Appl. Phys. 2005, 38, R123. Hill, N. A. J. Phys. Chem. B 2000, 104, 6694. Munoz, A.; Alonso, J. A.; Martinez-Lope, M. J.; Casais, M. T.; Martinez, J. L.; Fernandez-Diaz, M. T. Phys. Rev. B 2000, 62, 9498.

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phases adopt antiferromagnetic order (e.g., YMnO3 TN=70 K4), which in the case of YMnO3 has been shown to be coupled to the ferroelectric behavior of the phase.5 As the symmetry breaking distortion is driven by geometric factors (much like the analogous octahedral tilting distortions of cubic perovskites) and not electronically driven, there is some freedom to tune the magnetic behavior of the manganese ions in REMnO3 phases by electronic doping, without losing the acentric structure of the host phase. There have been a number of doping studies examining the influence of the valence electron count on the magnetic and ferroelectric behavior of REMnO3 phases. These have been hampered by the low solubility of dopants in REMnO3 materials and the prevalence of low levels of highly magnetic impurity phases (most notably Mn3O4), which can mask any subtle changes to the magnetic behavior on doping.6-9 Here we present a study of the topotactic reduction of YMnO3. Removal of oxide ions from the hexagonal REMnO3 host lattice would be expected to disrupt the cooperative distortion, which leads to the loss of structural inversion symmetry, thus influencing the ferroelectric behavior of materials while simultaneously electron doping the materials, modifying their magnetic behavior. (5) Huang, Z. J.; Cao, Y.; Sun, Y. Y.; Zue, Y. Y.; Chu, C. W. Phys. Rev. B 1997, 56, 2623. (6) Van Aken, B. B.; Bos, J.-W. G.; de Groot, R. A.; Palstra, T. T. M. Phys. Rev. B 2001, 63, 125127. (7) Park, J.; Kamg, M.; Kim, J.; Lee, S.; Jang, K.; Pirogov, A.; Park, J.-G. Phys. Rev. B 2009, 79, 64417. (8) Katsufuji, T.; Masaki, M.; Machida, A.; Moritomo, M.; Kato, K.; Nishibori, E.; Takata, M.; Sakata, M.; Ohoyama, K.; Kitazawa, K.; Takagi, H. Phys. Rev. B 2002, 66, 134434. (9) Adem, U.; Nugroho, A. A.; Meetsma, A.; Palstra, T. T. M. Phys. Rev. B 2007, 75, 14108.

Published on Web 10/02/2009

r 2009 American Chemical Society

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Figure 1. (a) High-temperature centrosymmetric (P63/mmc) and (b) lowtemperature acentric (P63cm) structures of YMnO3.

Experimental Section Preparation of YMnO3. Samples of YMnO3 were prepared by a ceramic route described previously.4 Suitable quantities of Y2O3 (99.999%, Alfa Aesar, dried at 950°C) and MnO2 (99.999%, Alfa Aesar) were thoroughly mixed in an agate pestle and mortar, pressed into 13 mm pellets at 5 tonnes pressure, and then heated in air for two periods of two days at 1300 °C with intermediate regrinding. X-ray powder diffraction data collected from this material revealed the sample was a single phase with lattice parameters in good agreement with previously reported values.4 The oxygen stoichiometry of samples was confirmed to be YMnO3.001(5) by iodometric titration. Reduction of YMnO3. Small samples (∼300 mg) of YMnO3 were ground together with double molar quantity of CaH2 in an argon filled glove box (O2 and H2O levels < 1ppm). These mixtures were then sealed under vacuum in silica ampules and heated at temperatures between 100°C and 600°C. Due to the hazards associated with the production of hydrogen gas when using calcium hydride as a reducing agent, large samples suitable for neutron powder diffraction studies were prepared by means of a spring loaded venting apparatus to prevent the build up of gas pressure, as described previously.10 Approximately 5 g of YMnO3 was mixed with a double stoichiometry of CaH2 and heated for two periods of 2 days with intermediate regrinding. After reaction, all samples were washed under nitrogen with 4  100 mL of a 0.25M solution of NH4Cl in methanol to remove the calcium containing phases (CaO, CaH2). Samples were then washed with 4  80 mL of clean methanol to remove any remaining NH4Cl before being pumped to dryness. Characterization. Room-temperature X-ray powder diffraction data were collected from samples contained within homemade air sensitive sample holders utilizing a Panalytical X’pert diffractometer incorporating an X’celerator position sensitive detector (monochromatic Cu KR1 radiation). Neutron powder diffraction data were collected from samples contained in vanadium cans, which had been sealed under argon with an indium washer. Data were collected using the POLARIS instrument at the ISIS neutron source (Rutherford Appleton Laboratory, UK) at 2 and 298 K. Additional data sets were collected using the D2b diffractometer at the ILL neutron source (Grenoble, France) (λ = 1.59 A˚) at 298 and 5 K. X-ray powder diffraction data were collected from samples in the temperature range 500 K < T < 1200 K using a Siemens D5005 diffractometer fitted with a Goebel mirror and an Anton Paar HTK1200 furnace. Samples were contained within an alumina holder and maintained under a flowing helium atmosphere during the measurement. The furnace temperature was (10) O’Malley, M.; Lockett, M. A.; Hayward, M. A. J. Solid State Chem. 2007, 180, 2851–2858.

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calibrated using a thermocouple prior to the diffraction measurements. Electron diffraction data were collected from finely ground samples supported on lacy carbon grids (deposited from suspension in methanol) using a JEOL 2000FX microscope operating at 200 kV. Rietveld structural refinements were performed against X-ray or neutron powder diffraction data utilizing the GSAS suit of programs.11 Thermogravimetric reoxidation measurements were performed under flowing oxygen on powder samples using a Netzsch STA 409PC balance. Zero-field-cooled and fieldcooled magnetization data were collected in the temperature range 5 K < T < 300 K from powdered samples using a Quantum Design MPMS SQUID magnetometer.

Results Chemical Reactivity of YMnO3. X-ray powder diffraction data collected from the products of the reaction between YMnO3 and CaH2 showed that at temperatures below 250°C there was no significant reaction. Reaction temperatures above 500°C lead to decomposition of the ternary oxide phase and the formation of simple binary oxides (MnO, Y2O3). X-ray powder diffraction data collected from samples prepared in the temperature range 250 °C e T e 500 °C, however, were consistent with the topotactic reduction of YMnO3 to YMnO3-x. Figure 2a shows the X-ray powder diffraction patterns and corresponding thermogravimetric reoxidation data collected from samples prepared in the temperature range 225 °C < T < 350 °C in comparison with those from YMnO3.00. Thermogravimetric reoxidation of these samples back to YMnO3.00 with oxygen revealed the phases prepared had compositions in the range YMnO3-x, 0 < x < 0.2, as shown in Figure 2b and Table 1. Complete reoxidation to YMnO3 was confirmed by X-ray powder diffraction and further confirming the topotactic nature of the reduction reaction reactions. The diffraction data collected from YMnO2.95 and YMnO2.85 can be readily indexed with unit cells analogous to that of the low temperature, acentric, P63cm form of YMnO3 (Table 1). Close inspection of the data collected from YMnO2.80 however reveal the diffraction reflection observed at 2θ ≈ 23° in the less reduced samples is absent (Figure 2a). This peak indexes as the (102) reflection of the P63cm cell. The absence of this and other reflections in the diffraction pattern collected from YMnO2.80 suggest these data can be indexed on a smaller hexagonal cell corresponding to the high-temperature centrosymmetric form of YMnO3 (Table 1). To unambiguously determine the unit cell of YMnO2.80, we collected electron diffraction data. Figure 3 shows electron diffraction data collected from YMnO2.80, which can be indexed as the [121] zone axis using the small centrosymmetric cell of YMnO3. In addition the data can also be indexed as the [111] zone of the larger non-centrosymmetric structure of YMnO3 (peak indices in brackets). However in the latter case the 2,2,0 and 1,1,0 diffraction (11) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System,; Los Alamos National Laboratory Report LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 2000.

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Figure 3. Electron diffraction data collected YMnO2.80. Unbracketed indices are based on the [121] zone axis of a unit cell based on the hightemperature centrosymmetric form of YMnO3. Bracketed indices are based on the [111] zone axis of an expanded unit cell based on the lowtemperature non-centrosymmetric form of YMnO3. Arrows mark the expected positions of the 110 and 220 reflections in the expended cell. Table 2. Structural Parameters Refined against Neutron Powder Diffraction Data Collected from YMnO2.95 at 298 Ka x y z fraction Uiso (A˚3) Y(1) Y(2) Mn(1) O(1) O(2) O(3) O(4)

0 1 /3 0.3302(5) 0.3086(1) 0.6420(1) 0 1 /3

0 /3 0 0 0 0 2 /3 2

0.2695(2) 0.2332(2) 0 0.1614(1) 0.3346(2) 0.4839(1) 0.0150(1)

1 1 1 1 1 0.95(1) 0.951(6)

0.00326(7) 0.00326(7) 0.0042(1) 0.00418(7) 0.00418(7) 0.0065(1) 0.0065(1)

a YMnO2.950(7) space group: P63cm, a = 6.1728(2) A˚, c = 11.4509(4) A˚. χ2 = 2.21, wRp = 3.05%, Rp = 5.44%.

Figure 2. (a) X-ray powder diffraction data collected from the washed products of reaction between YMnO3 and CaH2 as a function of reaction temperature. (b) Thermogravimetric data collected during the reoxidation of YMnO3-x phases. Table 1. Lattice Parameters and Compositions of YMnO3-x Phases As a Function of Reaction Temperature reaction temperature (°C) unreacted 225 250 350

a (A˚)

c (A˚)

6.1542(3) 6.1728(2) 6.1806(1) 3.5765(1)  [6.1946]

11.4031(2) 11.4509(4) 11.4545(4) 11.4994(3)

composition YMnO3.00 YMnO2.95 YMnO2.85 YMnO2.80

peaks are expected to be observed in the positions marked by arrows in Figure 3. Their absence confirms the small centrosymmetric cell listed in Table 1 is correct for YMnO2.80. Samples prepared by reduction in the temperature range 375 °C < T < 500 °C show much larger levels of reduction and large changes in crystal structure, and will be described elsewhere. Structural Characterization of YMnO3-x (x = 0.05, 0.15). Neutron powder diffraction data collected from samples of composition YMnO2.95 and YMnO2.85 could

be readily indexed on the basis of the hexagonal unit cells listed in Table 1 and were consistent with the space group P63cm. Structural models based on the reported structure of YMnO34 were refined against these data. The atomic positions and displacement parameters of all atoms were refined along with the oxide ion site occupancies. It was observed that the occupancies of the O(1) and O(2) “axial” oxide ion sites did not deviate from unity within error so were set to full occupancy. Displacement parameters were constrained by element and site to increase refinement stability. The refinement against the data collected from YMnO2.95 converged readily to yield a structural model with a composition of YMnO2.950(7) consistent with the thermogravimetric data, full structural details are given in Table 2, with selected bond lengths in Table 5. A plot of the observed and calculated diffraction data is given in the Supplementary Information Comparison of the calculated diffraction data with the data collected from YMnO2.85 revealed a relatively poor fit. This was improved by the addition of anisotropic parameters to describe the displacement of the yttrium and oxygen atoms in the structure (χ2 = 4.23). Close inspection however, revealed the diffraction reflections in the YMnO2.85 data set displayed significant hkl dependent broadening. Attempts to fit the data to a two-phase model proved unsatisfactory (the refinement was highly unstable) instead an anisotropic broadening axis parallel

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to (001) was introduced to the model leading to a significant improvement to the fit (χ2 = 3.21). This allowed the refinement of a structural model with a stoichiometry of YMnO2.856(6) consistent with the thermogravimetric data. Full structural details are listed in Table 3, with selected bond lengths in Table 5. A plot of the observed and calculated diffraction data is given in the Supporting Information. High-Temperature Behavior. X-ray powder diffraction data collected from YMnO3-x (x = 0, 0.05, 0.25) samples could be indexed with hexagonal cells broadly consistent with those shown in Table 1. Models based on those refined against room temperature neutron diffraction data for the respective samples (Tables 2 and 3) were used as starting points for refinements against the high-temperature X-ray data to yield fits with high levels of agreement (χ2 < 2.1 for all refinements). A plot of the refined lattice parameters of YMnO3, YMnO2.95, and YMnO2.85, as a function of temperature, is shown in Figure 4. There are clear simultaneous anomalies in the rate of change of both lattice parameters of YMnO3 at 960 K, which is broadly consistent with previous observations by Nenet et al.12 The data from YMnO2.95 and YMnO2.85 also show analogous simultaneous anomalies at 910 and 825 K, respectively, consistent with an analogous phase change in the reduced materials. X-ray data were only collected at temperatures up to 1150 and 950 K from YMnO2.95 and YMnO2.85, respectively. This is because in preliminary measurements additional diffraction reflections consistent with sample

decomposition were observed in data collected above these temperatures. After the high temperature measurements had been completed, room temperature data were collected from each sample to ensure that no decomposition or oxidation had occurred. Structural Characterization of YMnO2.80. Structural refinement of a model based on that of the high temperature form of YMnO3 in space group P63/mmc was performed against the X-ray powder diffraction data collected from YMnO3 with all atom positions and anion site fractional occupancies allowed to refine. In common with the refinements of YMnO2.95 and YMnO2.95, it was observed that the axial anion site refined to full occupancy, within error, and so was set to this value. After a number of refinement cycles, a good statistical fit to the data was achieved. However inspection of the refined model revealed the displacement parameters of the equatorial oxide ions were much larger than any of the other atoms in the model. This suggested a static disordering of the position of this site. To model this disorder the equatorial site was split into two sites (O(2) and O(3)) located either site of the z = 1/4 mirror plane at (0, 0, 1/4 þ z) and (0, 0, 1/4 - z). This improved the statistical quality of the fit and yielded more physically reasonable displacement parameters for the two resulting sites. The fractional occupancies of the equatorial anion sites refined to values consistent with the stoichiometry of YMnO2.80 determined by thermogravimetric reoxidation, without the need for constraints. A very small amount of Y2O3 impurity ( 950 K for YMnO2.85) to a centrosymmetric structure (YMnO2.80) with a very small change in composition suggests there is some critical threshold anion vacancy concentration that triggers the change in the structure, rather than a gradual continuous lowering of the transition temperature at which the centrosymmetric structure is adopted. The removal of oxide ions lowers the average coordination number, introducing some MnO4 units into the layers of corner sharing MnO5 polyhedra. The fraction of 4-coordinate manganese centers in the structure of YMnO3-x phases can be expressed as 3x  100%. Thus YMnO2.85 has 45% MnO4, 55% MnO5, and YMnO2.80 has 60% MnO4 and 40% MnO5. This would suggest a critical concentration of around 50% 4-coordinate manganese leads to sufficient disorder to induce a change to a centrosymmetric structure. Magnetic Behavior. The failure of YMnO3-x to yield physically reasonable value from Curie-Weiss fits of magnetic data is consistent with the observation of strong two-dimensional magnetic correlations in YMnO3.00 above the antiferromagnetic ordering temperature.21 Divergence between the zero-field-cooled and fieldcooled data collected from YMnO2.95 at T ≈ 65K is consistent with the onset of (canted) antiferromagnetic order. Local minima in the zero-field-cooled data collected from YMnO2.85 and YMnO2.80 at T ≈ 20 K are consistent with the suppression of the antiferromagnetic ordering temperature due to structural and oxidation state disorder. Similarly, the smaller ordered moment and higher symmetry arrangement of spins in the magnetic (20) Abrahams, S. C.; Kurtz, S. K.; Jamieson, P. B. Phys. Rev. 1968, 172, 551. (21) Lonkai, T.; Tomuta, D. G.; Hoffmann, J.-U.; Schneider, R.; Hohlwein, D.; Ihringer, J. J. Appl. Phys. 2003, 93, 8181.

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structure of YMnO2.85 compared with YMnO2.95 is also the signature of increasing disorder in the former phase. Acknowledgment. We thank R. Smith for assistance collecting the neutron diffraction data. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council. In addition, we thank E. Suard for assistance

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collecting neutron diffraction data at the ILL, Grenoble, France. Supporting Information Available: Observed, calculated, and difference plots from the structural refinements of YMnO2.95, YMnO2.85, YMnO2.80; full details of the structural and magnetic refinements of YMnO2.95 and YMnO2.85 from low-temperature neutron powder diffraction data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.