Oxygen Stoichiometry in the Geometrically Frustrated KagomÐ¹ System

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Oxygen Stoichiometry in the Geometrically Frustrated Kagomé System YBaCo4O7+δ: Impact on Phase Behavior and Magnetism S. Avci,*,†,‡ O. Chmaissem,†,§ H. Zheng,† A. Huq,⊥ P. Manuel,# and J. F. Mitchell† †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Materials Science and Engineering, Afyon Kocatepe University, Afyon 03200, Turkey § Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, United States ⊥ Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States # ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11-0QX, United Kingdom ‡

ABSTRACT: Ternary Co oxides of the general formula RBaCo4O7+δ (where R = rare earth or Y) represent a family of materials that demonstrate impressive oxygen storage capabilitiesas much as 3.25% by weightat a relatively low temperature of ∼350 °C. This behavior results from structural features amenable to formation of oxygen interstitials and the availability of wide channels offered by the structure for the facile diffusion and storage of oxygen. Remarkably, this material is also a model system for exploring geometrically frustrated magnetism due to the presence of interleaving Kagomé and triangular cobalt sublattices. Unraveling the interplay of structure, oxygen stoichiometry, and phase behavior is important in optimizing the properties of this class of oxygen storage candidates and for broadening understanding of unusual magnetism. In this paper, we use thermogravimetric analysis, synchrotron X-ray diffraction, and neutron diffraction to explore how oxygen stoichiometry can be varied systematically and the impact of oxygen variability on phase behavior. In particular, we report the existence of a miscibility gap in lightly oxygenated YBaCo4O7+δ materials (0.0 < δ < 0.08) and the effects of the additional interstitial oxygen atoms on the material’s nuclear structure and magnetism. Structural phase transitions that were previously observed in the pristine stoichiometric parent YBaCo4O7.0 material are suppressed by extra oxygen, leading to suppression of long-range magnetic order in favor of short-range correlations. KEYWORDS: complex cobalt oxide, oxygen storage, frustrated magnetism, miscibility gap

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also depend essentially on the oxygen stoichiometry,5,22 making a comprehensive understanding of how oxygen stoichiometry impacts phase behavior and structure a priority. During the past several years, we have systematically studied the magnetic properties of a number of 114 compounds and their relationship to structural phase transitions and oxygen stoichiometry.6,7,23−28 Specifically, we have focused on the nonmagnetic trivalent Y analog as a model system. At 313 K, YBaCo4O7.0 exhibits a first-order structural phase transition from paramagnetic trigonal P31c to orthorhombic Pbn21, which is characterized by short-range magnetic correlations.28 Due to the loss of trigonal symmetry (and hence rigorous geometric frustration), below ∼115 K long-range magnetic order appears,28 concomitant with a monoclinic structural distortion (P21).7 This long-range magnetic structure is described as a 3D network of corner sharing triangles and trigonal bipyramidal

INTRODUCTION The RBaCo4O7 family “R-114” (where R = rare earth, Y, or Ca) represents a remarkable model system, as it offers the opportunity to explore complex magnetism under the constraint of geometric frustration in relation to the structural chemistry,1−10 thermodynamics, and kinetics of solid-state oxygen uptake in a material with demonstrated potential for oxygen storage.11−19 The mixed Co oxidation states (2+/3+) inherent to the 114 system can be modified by adjusting the oxygen stoichiometry reversibly at low temperature, 200−400 °C, yielding a nominal formula RBaCo4O7+δ, with δ ranging from zero to as high as 1.5.11−20 Indeed, it is this property that has raised the possibility that 114 materials may be suitable for application, a fact corroborated by the high O storage capacity, up to ∼3.25 wt %.21 At the same time, geometric frustration of magnetism arises from a Co sublattice built from twodimensional Kagomé layers interleaving with triangular layers along the c-axis,3−10 which can be considered as a topological variant of the pyrochlore, a canonical geometrically frustrated lattice.6,7 Importantly, the magnetic behavior of 114 materials © 2013 American Chemical Society

Received: May 28, 2013 Revised: September 16, 2013 Published: September 17, 2013 4188

dx.doi.org/10.1021/cm401710b | Chem. Mater. 2013, 25, 4188−4196

Chemistry of Materials

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clusters.7,28 At ∼60 K another magnetic response appears, corresponding to a spin reorientation process but without any obvious magneto-elastic correlation with the nuclear structure.7 Huq et al. reported that oxygen stoichiometric YbBaCo4O7 exhibits a similar first-order P31c to Pbn21 structural transition at 175 K, which is associated with significantly underbonded Ba2+ ions.23 The group also reported that oxygen rich YbBaCo4O7.2 is biphasic. The majority phase is similar to the O7.0 stoichiometric phase, whereas the excess oxygen loads into a minority phase, speculated to be separated by a miscibility gap. Interestingly, the minority oxygen-rich phase remained trigonal (P31c) down to 10 K.23 The authors suggested that Ba−O bonding is enhanced in the minority phase by the excess oxygen which results in suppressing both the P31c to Pbn21 phase transition and the associated long-range magnetic ordering.23 This effect is remarkable considering that only ∼3% interstitial oxygen is apparently capable of suppressing the structural phase transition. It is now firmly established that RBaCo4O7 compounds exhibit exceptional oxygen absorption/desorption capabilities11−19 with physical properties governed by the oxygen stoichiometry.5,17,22 For example, Hao et al. demonstrated that RBaCo4O7+δ (R = Y, Dy−Lu, In) display two distinct oxidation processes in the temperature range 20−1100 °C.14 The first reversible process takes place at relatively low temperatures between 200 and 400 °C, whereas the second irreversible process occurs between 660 and 1050 °C and leads to decomposition.14 In the CaBaCo4O7+δ case, the δ = 0.50 sample is hexagonal at room temperature in contrast with the orthorhombic δ = 0 sample.29 Pralong et al. reported that the δ = 0.50 sample is ferrimagnetic, like the δ = 0 sample with the same TC but with an unusual magnetic hysteresis attributed to presence of defects due to oxidation.29 In the YBaCo4O7 case, Karppinen et al.11 and Räsänen et al.12 successfully added significant extra oxygen to the parent compound to produce a heavily oxidized YBaCo4O8.5 sample (i.e., δ = 1.5) suggesting the successful achievement of an average Co valence of 3+. Chmaissem et al.24 showed that the additional oxygen in YBaCo4O8.1 order in a doubled superstructure unit cell with respect to the parent orthorhombic Pbn21, causing significant displacements of several of the oxygen atoms from their positions in the stoichiometric phase with concomitant conversion of a fraction of CoO4 tetrahedra into CoO6 octahedra. This dramatic oxygen uptake is fully reversible, arguing for the potential of 114 systems as oxygen storage materials. Although elaborate oxygenation processes have been well -documented, details of their mechanism, thermodynamics, kinetics, and structural consequences are not well-understood. Such an understanding can impact any evaluation of applications potential as well as suggest routes to optimize functionality. Toward this end, we combine in this paper quantitative thermogravimetric analysis (TGA) and in situ Xray diffraction (XRD) data to characterize the observed oxygen uptake/release phenomenon in YBaCo4O7+δ in the range 0 ≤ δ ≤ 1.11. We demonstrate that several oxygen-loaded states can be accessed in samples treated in 20% O2/N2 at ∼350 °C. For samples heated slightly above this temperature (i.e., 350−370 °C), the materials reach an equilibrium state with average oxygen content 0 < δ < 0.1 and display biphasic behavior for nominal O concentrations between δ = 0.0 and δ ≈ 0.08 when quenched to room temperature. On the other hand, materials heated at temperatures slightly below 350 °C absorb

significantly more oxygen (δ = 1−1.1) and show the orthorhombic Pbc21 symmetry.24 Using high-resolution synchrotron X-ray diffraction, we confirm the existence of a miscibility gap at room temperature that separates the δ = 0 and δ = 0.08 phases, and leads to the observed biphasic structural properties in the lightly oxygen doped R-114 materials.6,23 Neutron diffraction experiments on single phase δ = 0.11 and 0.15 samples reveal that these samples maintain their trigonal P31c symmetry down to 6 K. As a result, the long-range magnetic ordered phase is suppressed and replaced by shortrange spin correlations below ∼100 K.

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EXPERIMENTAL METHODS

YBaCo4O7+δ samples were prepared by standard solid state synthesis techniques. Stoichiometric ratios of Y2O3 (Alfa, 99.999%), BaCO3 (Alfa, 99.999%) and Co3O4 (Alfa, 99.999%) were thoroughly mixed and ground. Repeated sintering and grinding in air was performed with a final firing at 1150 °C followed by furnace cooling in air. This process resulted in a single phase YBaCo4O7+δ sample as judged by laboratory powder X-ray diffraction. Additional materials with specific oxygen contents in the range 0 < δ < 0.1 were carefully prepared by annealing the samples in a vertical quenching furnace in a 20% O2/N2 atmosphere. Optimum annealing temperatures were determined in separate TGA experiments that also determined δ(T). Samples were quenched into liquid nitrogen to preserve the high T oxygen content, and δ was then verified by reduction in a TGA balance (MettlerToledo model TGA/DTA1) at 900 °C in pure nitrogen, conditions which yield a fixed composition reference point with δ = 0.0. Sample oxygen contents were thus determined for the quenched samples as δ = 0.00, 0.018, 0.030, 0.034, 0.056, 0.058, 0.085 and 0.099, 0.11, and 0.15. We emphasize that throughout this report, δ refers to the nominal O content in the sample. For single phase samples this is identically the oxygen content of the phase. However, for biphasic samples discussed later, δ does not represent the oxygen content of either individual phase, but rather the weighted average of the two. Precision of the TGA measurements was established by triplicate analysis of select specimens; the uncertainty range was 0.004 oxygen/ formula unit. Heat capacity data were collected on a Quantum Design PPMS using crystals grown by the floating zone technique. Specimens were grown in a 20% O2/Ar mixture and confirmed to be single crystal by X-ray diffraction. The stoichiometry of the as-grown crystal was verified to be 7.00 by thermogravimetric analysis. To make an oxygenrich sample, a crystal of approximately 100 mg was cut from the boule and treated at 370 °C in 20%O2/N2 on a TGA balance until a constant weight was achieved. The relative weight gain was identical to that found for a polycrystalline sample of identical starting composition, giving us confidence that the entire crystalline specimen was fully oxidized. X-ray powder diffraction data were collected on a PANalytical X’Pert Pro MPD system with an X’celerator linear high-speed detector and a Co tube (λ = 1.789 Å). For ex situ room temperature measurements, powder was lightly dusted onto a zero-background sample holder. Variable temperature X-ray diffraction patterns were measured in an Anton-Parr TTK-1200N heating chamber purged with either 100% N2 or a mixture of 20% O2/N2 at 300 sccm. Loose powder was placed in a shallow Al2O3 sample holder at the furnace center. The furnace is uncalibrated under conditions of flowing gas. Accordingly we report here the temperature for the in situ X-ray studies based on comparison to the calibrated TGA. Data were typically collected in 0.02° 2θ steps using several samples during several separate runs spanning a six month period. As shown below, consistency of results among the samples demonstrate reproducibility of sample preparation and data collection conditions. Full data scans (20−120° 2θ) were registered in 100% N2 at 370 °C, 40 and 25 °C following an initial heating treatment to 450 °C in N2. This treatment has been shown to give a stoichiometric oxygen content, δ = 0.0.6,23 Full data scans were also collected after switching the gas from 100% N2 to 20% O2/N2 at the end of isothermal runs at 370 and 340 °C. 4189



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During the gas switch isothermal period, partial patterns in the range 32−45° 2θ and/or 62−72° 2θ were collected to monitor the phase evolution and to determine the lattice constant variation with time. Partial patterns were also collected in the 20% O2/N2 mixture during slow-cooling over a period of 2.6 h from either 370 or 340 °C (see the Quantitative TGA analysis section below). A full pattern was collected at 25 °C at the end of each cooling. To test reversibility of the oxygen uptake-release process, the slow-cooled sample was reheated at a rate of 20 °C/min to 370 °C in 20% O2/N2 and held isothermally in this atmosphere; partial patterns were recorded during this isothermal process until no visual change was detected in the data. Finally, this reequilibrated sample was rapidly cooled in the 20% O2/N2 atmosphere to 40 °C and then to 25 °C where full patterns were collected. X-ray powder diffraction data were analyzed with TOPAS 3.030 using a fundamental parameters approach to the line shape. Modelindependent Pawley fitting to the data (either full patterns or pattern segments, vide supra) was used to determine lattice constants under various temperature and atmospheric conditions. For these patterns, the lattice constants, a four-term Chebyshev polynomial background function, a sample displacement parameter, and a Lorentzian line broadening parameter phenomenologically representing crystal size effects were refined. Rietveld analysis of several full patterns was also undertaken. In these cases, in addition to the terms refined in the Pawley models, surface-roughness correction, atomic positions, and thermal parameters of the metal ions were refined simultaneously. Site occupancies of the metal atoms refined to ∼1.0 within the estimated standard deviation and were thus fixed in the final refinement cycles. In the case of oxygen, a fixed thermal factor Beq = 1.0 Å2 was assigned with full site occupancy. For TGA measurements of oxidation behavior, all gases were passed over drying columns and/or oxygen gettering columns (for N2). In a typical measurement, the sample (∼50−100 mg) was heated in N2 to 900 °C followed by cooling in N2 to the reference temperature. This procedure assures a reproducible starting value of δ = 0.00 for subsequent processes. The gas was switched to 20% O2/N2 mixture and held isothermally for several hours before cooling at 1 °C/min to 25 °C. The sample was then rapidly (20 °C/min) heated back to the reference temperature, held for several hours and then rapidly cooled back to 25 °C. Experiments were run in duplicate or triplicate to ensure reproducibility. Synchrotron X-ray data were collected at the Advanced Photon Source high-resolution instrument (11-BM-B) at Argonne National Laboratory at 323, 298, and 100 K for the quenched samples with 0.0 ≤ δ ≤ 0.099 using a wavelength of 0.45890 Å. Neutron diffraction experiments were performed at ISIS (WISH)31 at the Rutherford Appleton Laboratory for specimens with δ = 0.11 and 0.15 at temperatures between 300 and 6 K. Structural determinations were performed using the General Structure Analysis System package (GSAS)32 and the graphical user interface (EXPGUI).33 In the final refinement cycle, all the variables were refined, including background and peak shape parameters, scale factor, absorption, atomic coordinates, and thermal factors.

Figure 1. Thermogravimetric data for YBaCo4O7+δ as a function of time, temperature, and atmospheric conditions. The upper panel shows the temperature profiles corresponding to the data in the lower panel. The dotted vertical line marks the switch from 100% N2 to a 20% O2/N2 gas mixture. Roman numerals and colored regions refer to processing segments described in the text. For both panels, solid curve: isotherm at T = 340 °C; dashed curve: isotherm at T = 370 °C. Values of oxygen content are referenced to 7.00 in 100% N2 at 500 °C and refer to the nominal, or average oxygen content in the sample.

Yb,23 Y,6,12 and Tm26 samples contain excess oxygen δ up to ∼0.2 per formula unit when they are either furnace cooled in the air or removed rapidly from the furnace into the air. When the sample is held at 370 °C and the gas is switched from N2 to 20% O2/N2 (red-dashed curve in Region II in Figure 1), an immediate uptake of O occurs, rapidly equilibrating at an oxygen content of δ = 0.112. Cooling the sample to room temperature (red-dashed curve in Region III) yields a spectacular and rapid oxygen uptake to δ = 1.062. Similar oxygen content excursions in the neighborhood of 350 °C have been noted by Karppinen,11 Hao,14 and Kadota.15 Upon reheating rapidly to 370 °C (red dashed curve in Region IV), all of the oxygen gained on cooling is quickly lost to the atmosphere and an equilibrium state with δ = 0.114 results, the same as that found before cooling. Below we show that the lattice constants for these two states are identical within the experimental uncertainties. Taken together, these two observations demonstrate that the oxidation/reduction process is fully reversible above 350 °C on a laboratory time scale, indicating that the system is in chemical equilibrium with the 20% O2/N2 atmosphere in this temperature regime. In an attempt to quench this equilibrium state to room temperature, the sample was cooled as quickly as possible in the TGA furnace (red-dashed curve in Region V). It is clear from the data, however, that even this rapid cooling through ∼350 °C leads to a nontrivial oxygen uptake, δ = 0.333 at room temperature. Later we discuss samples quenched into liquid nitrogen, a process that does preserve the high T stoichiometry. In a separate experiment, we demonstrate that lowering the oxidation temperature by as little as 30 °C leads to a drastically different behavior. Holding the sample at 340 °C in N2 with the gas switched to 20% N2/O2 (solid blue curve in Region II) leads to a rapid O pickup with far more oxygen absorbed by the sample than at 370 °C. Indeed, δ exceeds 1.0 over the duration of the isotherm, although saturation is apparently not achieved over this six hour period. During cooling to room temperature, the sample absorbs an additional 0.1 oxygen per formula unit to δ = 1.111 (solid blue curve in Region III). Unlike the case of samples treated at 370 °C, the sample treated at 340 °C does not equilibrate with the atmosphere over the time scale of the experiment. Evidence for this is seen by comparing the O

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RESULTS AND DISCUSSION Quantitative TGA Analysis. The overall oxidation/ reduction behavior of the Y-114 system in the temperature range 25−400 °C is summarized by the thermogravimetric data shown in Figure 1. Two separate experimental runs are shown in the figure, with the temperature profiles in the upper panel and the nominal O content (referenced to 7.00 under 100% N2 at 500 °C) in the lower panel. The as-made powder (Region I in Figure 1) is found to have oxygen excess δ = 0.17 which is subsequently removed in the nitrogen atmosphere with heating to 500 °C thus leading to a stoichiometric δ = 0.0 material. This behavior is in agreement with the anticipated δ = 0.2 content discussed above for the Yb-114 member23 of this family, suggesting this may be a general characteristic of the structure, as has been shown by others12 as well. Indeed, as-made R = 4190



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content in regions II and IV of Figure 1. Rapidly heating the sample from room temperature to 340 °C leads to a small but measurable oxygen loss to δ = 1.096 (solid blue curve in Region IV), significantly higher than the value obtained during the first isotherm at 340 °C (Region II). Subsequently rapid- cooling the sample to room temperature recovers the state with δ = 1.111 (Solid blue curve in Region V). This is the same value as found during the slow-cooling in Region III, indicating a special stability of this particular composition independent of cooling protocol. To summarize the TGA data, several different oxygen loading states can be accessed in 20% O2/N2 in the vicinity of 350 °C. Samples heated slightly above this temperature (i.e., at ∼370 °C) quickly reach thermodynamic equilibrium with the atmosphere with δ ≈ 0.1; those cooled below 340 °C pick up considerably more oxygen, δ ≈ 1−1.1 but remain under kinetic control that hinders the material from achieving its true equilibrium thermodynamic state with higher δ content. Rapid cooling from 370 °C to room temperature leads to considerable additional oxygen pickup (from 7.114 to 7.333 or an increase of ∼0.22 O per formula unit), while those samples treated at 340 °C only gain very small amounts of additional oxygen (from 8.096 to 8.111 or ∼0.015 O per formula unit) in this quenching process. This behavior demonstrates progressively sluggish kinetics of O uptake below ∼350 °C. We note that the reproducible oxygen loadings at 340 and 370 °C are simple fractions: 0.0, 0.11 = 1/9, 0.17 = 1/6, 0.33 = 1/3, 1, 1.11 = 10/9, and 1.5 = 3/2 (the 1.1 and 1.5 compositions are not in this study, but have been reported by Chmaissem,24 Karppinen,11 and others12,13,34). The existence and sheer number of these simple ratios observed in our study opens the speculation of minima in the free energy surface and hence of stable or metastable “line” phases that represent ordered O interstitial configurations rather than a continuous solution. Below we establish a miscibility gap in the phase diagram at room temperature, a necessary but not sufficient condition for proving this conjecture. Detailed study of an extended T−δ range as well as prima facie evidence of O ordering from diffraction (as we have shown for the δ = 1.1 case24) or other structural probes will be required to rigorously establish if this speculation is indeed verifiable, with concomitant implications for evaluating the performance potential for YBaCo4O7+δ and other members of the family for O membranes.24 As mentioned above, a similar biphasic behavior was observed in YbBaCo4O7+δ by Huq et al.,23 indicating this behavior may be widespread if not universal in the RBaCo4O7 family of materials. In Situ XRD. We now describe in situ XRD measurements designed to parallel the TGA experiments and hence to explore the phase behavior and evolution of the sample during these oxidation/reduction processes. Single-phase samples equilibrated above 350 °C in 20% O2/ N2 atmosphere allow the incorporation of excess oxygen into the lattice without significant structural consequences. As shown in Figure 2, the in situ data collected at 370 °C can be well-fit to a single-phase P31c model.6 Plots of Rietveld analyses of the XRD data collected at 370 °C in N2 and after 10.6 h in 20% O2/N2 are shown in panels a and b in Figure 2, respectively; structural parameters are summarized in Table 1. Lattice constants, plotted in Figure 3 as a function of time following the gas switch were determined using the partial Xray data scans. The listed error bars should be considered a lower bound due to the small q-range used for their extraction.

Figure 2. Rietveld analysis of YBaCo4O7+δ using in situ X-ray diffraction data (a) at 370 °C in 100% N2 (δ = 0.00), (b) after 10.6 h in a 20% O2/N2 mixture (δ ≈ 0.11), and (c) the same sample after rapidly cooling to 25 °C in a 20% O2/N2 mixture (δ ≈ 0.33). Values of δ are based on TGA data of Figure 1. All samples are refined in space group P31c.

Nonetheless, the trend in lattice constants mirrors the rapid change in the data, with essentially all changes complete by ∼100 min (Note: the time constant for this process will depend on sample size, particle surface area, etc.); as shown in the inset to Figure 3b, diffraction peaks rapidly shift to higher angle after the gas switch with no obvious change in structural symmetry. It should be noted that in the case of a significant O uptake (δ = 1.1) this line is expected to split.24 The a- and c-axes evolve reciprocally with time, yielding a sharp and substantial decrease in the unit cell volume of ∼0.17%, which then changes little throughout the remainder of the experiment. Lattice constants are listed in Table II. Because the c/a ratio is less sensitive to systematic errors than the lattice constants themselves, we use this parameter to compare results determined at different temperatures, using various instruments, or for data collected at different times. After the gas switch from N2 to 20% O2/N2 at 370 °C, a small but significant increase in this ratio is observed (1.62995(5) to 1.63276(5)), accompanying the ∼0.64 Å3 decrease in cell volume. The size of this lattice change follows from the modest increase in oxygen content at this temperature. As discussed above, TGA data show that Y-114 picks up about 0.1 oxygen atom per formula unit at T ≈ 370 °C. Upon rapidly cooling the trigonal sample (20 °C/min) to 25 °C, no impurity lines are observed and the data can also be well fit to a trigonal structure (Figure 2c). Data in Table II show a comparison of samples at 40 °C with δ = 0.0 and δ = 0.33 (the latter value is determined by TGA under similar conditions, but this value may be sensitive to the cooling protocol). The slightly elevated temperature is required to compare these two samples, as the δ = 0.0 sample transforms at room temperature to a distorted orthorhombic structure (space-group Pbn21, ao = ah, bo = ah√3, co = ch, where the subscripts “o” and “h” refer to the orthorhombic and trigonal lattice, respectively).6,7 Small but statistically significant differences in lattice constants are found, and the c/a ratio decreases from 1.63133(5) to 1.62837(2). 4191



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Table 1. Structural Parameters for YBaCo4O7+δa 370 °C, N2 a (Å) c (Å) V (Å3)

6.31762(3) 10.29736(7) 355.930(4)

x y z B (Å2)

2/3 1/3 0.8750(3) 0.72(16)

x y z B (Å2)

2/3 1/3 0.5 1.75(17)

x y z B (Å2)

0 0 0.426(1) 0.32(8)

x y z B (Å2)

0.181(1) 0.838(1) 0.6780(8) 0.32(8)

x y z B/Å2

0.488(3) 0.433(3) 0.730(1) 1.0

x y z B (Å2)

0 0 0.234(2) 1.0

x y z B (Å2) N p Rexp (%) Rwp (%) χ2

0.112(2) 0.769(2) 0.488(1) 1.0 6283 29 1.854 2.243 1.46

370 °C, 20%O2 25 °C, 20%O2 Rapid Cooling 6.31018(2) 10.30302(7) 355.286(4) Y 2/3 1/3 0.8716(4) 0.19(22) Ba 2/3 1/3 0.5 1.89(25) Co(1) 0 0 0.437(3) 1.03(17) Co(2) 0.174(3) 0.832(3) 0.686(1) 0.22(1) O(1) 0.441(3) 0.473(4) 0.761(2) 1.0 O(2) 0 0 0.252(2) 1.0 O(3) 0.124(3) 0.799(3) 0.495(2) 1.0 6176 30 2.753 2.767 1.01

6.2965(4) 10.487(1) 351.883(6) 2/3 1/3 0.8738(4) 0.5 1/3 1/3 0.5 1.36(14) 0 0 0.428(2) 0.5

Figure 3. Isothermal (370 °C) time dependence of the lattice constants of YBaCo4O7+δ determined by laboratory X-ray diffraction as the oxygen content increases from δ = 0.0 to 0.11 after switching from 100% N2 to 20% O2/N2 gas mixture. Except for the last point, only portions of the pattern from 60 to 70° 2θ were used in the Pawley refinements; the final points on the plots (10.3 h) are obtained from a full pattern refinement. Lines are guide to the eye. In a and b, the point at T = 0 h shows a comparison of the present data (open symbols) with a separate full pattern refinement measured on the same instrument five months earlier (crosses). The inset to b shows a portion of the X-ray data before and after switching from N2 to 20% O2/N2 and holding in isothermal conditions; several curves are superimposed under the 20% O2/N2 gas mixture.

0.170(3) 0.827(3) 0.689(2) 0.5 0.459(4) 0.473(4) 0.734(2) 1.0 0 0 0.243(2) 1.0

this structural phase transition has significant impact on the magnetic properties of the YBaCo4O7+δ system, which will be discussed later in this paper. Miscibility Gap. Laboratory X-ray data (not shown) collected at room temperature on slightly oxygenated samples showed additional diffraction peaks consistent with a phase separation and suggested the likelihood of a miscibility gap in the oxygen variable. To confirm the existence and extent of this miscibility gap, high-resolution synchrotron data were collected on the samples synthesized for this study with finely tuned values of δ as listed in the Experimental Section above (the δ = 0.11 and 0.15 samples were not measured). We will show below that the single phase δ = 0.00 sample behaves as described in the literature,6,7 whereas all the remaining samples with nominal stoichoimetry 0 < δ ≲ 0.08 are biphasic. Singlephase behavior is recovered in samples from δ = 0.099 to 0.15. The results of this study have consequences not only for understanding O storage, but also for the magnetic properties of YBaCo4O7+δ. Figure 4 shows a portion of the synchrotron X-ray diffraction pattern containing the (004) reflection for various nominal δ contents at 323 K (This temperature was chosen to allow comparison among the trigonal samples in the absence of the orthorhombic Pbn21 phase that appears below T ≈ 315 K). Even a small increase in the nominal oxygen content (δ = 0.018) results in the appearance of a new reflection at ∼2.56 Å, which could not be attributed to any superstructure or structural distortion even considering the lowest triclinic symmetry of P1.25 We thus assign this line to the (004) reflection of a second phase with a slightly shorter c-axis, and

0.115(3) 0.794(3) 0.517(2) 1.0 5711 27 2.691 2.700 1.01

a

Space group P31c, N, and p are the number of experimental observations and refined variables, respectively. Rexp, Rwp, χ2 are refinement agreement factors.

Such a behavior is opposite to what we observed above for the δ = 0.1 sample. This may imply that as the excess oxygen content increases beyond 0.1, the structure accommodates this excess O in a different way, perhaps by enhanced bonding with Ba and Co or by occupying a different interstitial site. Rapid cooling of the δ = 0.33 sample to room temperature results only in thermal contraction of the lattice. We emphasize that this oxygen-rich sample remains trigonal at 25 °C while its δ = 0.0 counterpart transforms as described above to an orthorhombic superstructure at the same temperature.6,7 The excess oxygen suppresses this symmetry-lowering phase transition as we have remarked elsewhere23 and similar to what was previously reported for YbBaCo4O8.5 Suppression of 4192



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Table II. Experimental Conditions and Lattice Parameters of YBaCo4O7+δ Determined by in Situ Powder X-ray Diffraction T (°C)

atmosphere

δ

sample state

a (Å)

c (Å)

V (Å3/f.u.)

c/a

370 340 40 370 370 40 25

N2 N2 N2 20%O2/N2 20%O2/N2 20%O2/N2 20%O2/N2

0.0 0.0 0.0 ∼0.11 ∼0.11 ∼0.33 ∼0.33

baseline, see Figure 1 baseline, see Figure 1 slow cooled from 450 °C first heating reheat from 25 °C rapid cooled from 370 °C rapid cooled from 370 °C

6.3176(3) 6.31575(3) 6.29854(4) 6.3102(1) 6.31015(5) 6.29664(4) 6.29635(4)

10.29736(7) 10.29323(7) 10.27248(8) 10.3030(1) 10.3023(1) 10.25325(8) 10.2487(1)

355.930(4) 355.576(4) 352.87(1) 355.29(1) 355.26(1) 352.055(5) 351.867(6)

1.62995(5) 1.62977(1) 1.63133(5) 1.63276(5) 1.63266(3) 1.62837(2) 1.62772(1)

Figure 4. Portion of synchrotron X-ray diffraction data collected at 323 K, showing the (004) Bragg reflections for both δ = 0.08 and δ = 0.0 phases. The δ ≈ 0.08 trigonal phase grows at the expense of the parent δ = 0.0 phase as the oxygen content is increased. Here δ labeling the curves refers to nominal, or average, O stoichiometry in the sample.

hence δ > 0.00 (See Table II). Furthermore, the position of the (004) reflection at 2.57 Å does not shift with the increase in nominal δ. These two observations can be interpreted as a phase separation into stoichiometric δ = 0.0 and offstoichiometric δ > 0 boundary phases at room temperature. The disappearance of the 2.57 Å peak at δ ≈ 0.085 sets an upper bound on the high O content boundary of the miscibility gap, which we show in the Appendix to be close to δ ≈ 0.08. We note that the quenching process is distinctly nonequilibrium, so that discussion of boundary phase compositions and their relationship to phase fractions (via the lever rule) is only approximate. In the present case, δ = 0.0 and δ ≈ 0.08 phases refer to two distinct versions of the trigonal P31c structure with distinct lattice parameters. The δ ≈ 0.08 phase exhibits a smaller unit cell than δ = 0.0 because of its significantly smaller c-axis, as shown in Figure 5c. As the figure shows, the cell parameters remain constant within error by the increase of nominal oxygen content up to δ = 0.085. It is clear from Figure 4 that, as the oxygen content increases, the δ = 0.0 endmember phase is gradually suppressed while the δ ≈ 0.08 endmember phase continues to grow. Finally, single phase behavior is recovered for δ = 0.099. The behavior summarized in Figures 4 and 5 demonstrates the presence of a miscibility gap at and near room temperature in the intermediate range between δ = 0.00 and δ ≈ 0.08. We did not investigate the temperature dependence of this phase separation to determine where the miscibility gap closes because of experimental difficulties associated with maintaining constant O content during heating. As discussed above, a miscibility gap of this type is a necessary condition for the presence of an ordered O interstitial phase. Figure 5e shows synchrotron X-ray diffraction data of the inplane (220) peak at 323 K for various δ values. As the oxygen

Figure 5. Unit-cell parameters: (a) a-axis and (c) c-axis, (b) unit-cell volume, and (d) phase fractions of the δ = 0.0 (open red circles) and δ ≈ 0.08 (solid blue circles) phases as a function of oxygen content at 323 K. (e) Portions of synchrotron X-ray diffraction data showing the two (220) peaks at 323 K for samples with 0.00 < δ < 0.099. In all three panels δ refers to nominal, or average, O stoichiometry in the sample.

content increases, a second peak appears due to the growth of the δ ≈ 0.08 phase described above. However, the peak positions remain unchanged across the sample series, except for δ = 0.099, for which the peak position shifts to lower d-spacing, in concert with the reappearance of single-phase behavior. The data are consistent with the refined cell parameters showing that the a-axis shrinks at δ = 0.099. Figure 6 shows portions of the synchrotron data with best-fit Rietveld refinements for the δ = 0.034 and δ = 0.099 samples. At 323 K, the δ = 0.034 sample is a mixture of the δ = 0.0 (∼55%) and δ ≈ 0.08 (∼45%) phases. As mentioned above, YBaCo4O7.0 (δ = 0.0) undergoes a structural transition from P31c to Pbn21 at ∼312 K. Therefore, at 298 K, most of the δ = 0.0 trigonal phase converts into orthorhombic Pbn21, whereas the δ ∼0.08 phase remains unchanged. The appearance of a new (004) reflection as seen in the left-middle panel of Figure 6 confirms the coexistence of the three phases at this temperature. The weight fractions for this particular sample at 298 K are 12% (δ = 0.0), 48% (δ ∼ 0.08) and 40% (Pbn21). There is no evidence in our 100 K data, where the original δ = 0.0 trigonal phase is completely absent, for the reported low4193



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Figure 7. Neutron diffraction data for (a) δ = 0.11, (b) δ = 0.15 at various temperatures. The arrows in a point to the locations of broad magnetic diffuse scattering. (c) Specific heat data for δ = 0.00 (solid red circles) and δ = 0.12 (open blue circles). The data for δ = 0.00 crystal have been offset by 20 J/(mol K) for clarity.

favored according to classical Heisenberg models.36 Schweika et al. reported such a structure in Y0.5Ca0.5BaCo4O7 from a neutron diffuse scattering study with polarization analysis revealing short-range magnetic correlations and a two-dimensional coplanar ground state.35 The authors observed an intense asymmetric peak at 2q√3 = 1.33 Å −1 and a weak signal at q√3 = 0.67 Å−1 both indicating broken long-range magnetic correlations.35 Stewart et al. attributed the broken long-range magnetic correlations to the random distribution of Ca atoms causing local structural disorder and interfering with the magnetic exchange pathways.37 Manuel et al., on the other hand, reported single crystal magnetic diffuse scattering in stoichiometric YBaCo4O7.0 (a long-range ordered antiferromagnet) at T > TN, where peaks such as these described in the above paragraph can also be attributed to the distinct pattern of shortrange magnetic order derived from the unique topology of linked trigonal bipyramids of Co.28 The data presented in Figure 7 for our δ = 0.11 and 0.15 samples show a broad and intense peak at 4.5 Å and a much weaker reflection (not shown) at 9.05 Å. These diffuse peaks correspond to 2q√3 = 1.39 Å−1 and q√3 = 0.69 Å−1, respectively. Our samples do not contain Ca or other substitution elements at the Y, Ba, or Co sites that could disturb the cationic substructure, but the introduction of interstitial oxygen clearly interferes with both the nuclear and magnetic structures of this material. Samples at (or slightly above) the upper limit of the miscibility gap (i.e., δ = 0.11, 0.15) have both the near room temperature orthorhombic Pbn21 and the low-temperature monoclinic P21 structural transitions suppressed. The location of diffuse magnetic scattering is in agreement with short-range magnetic correlations reported by Manuel et al. above TN.28 The addition of small interstitial amounts of oxygen may impact the magnetic ground state in two ways: (1) by suppressing the structural phase transitions, YBaCo4O7+δ remains geometrically frustrated to lowest temperatures, or (b) by modifying the superexchange links between the triangular and Kagome sublattices. Our data cannot distinguish between these two possibilities, but in either case it is remarkable that the small (∼1.5%) concentration of interstitials can so dramatically impact the structure and magnetism of this compound. More substantial additions of these interstitials result in oxygen and cationic ordering as observed by Chmaissem et al.24 and Karppinen et al.11 in addition to the suppressed long-range magnetic ordering.

Figure 6. Portions of synchrotron powder X-ray data showing the (004) reflections with a best-fit Rietveld refinement for δ = 0.034 (left panel) and δ = 0.099 (right panel) at 323 K, 298 K, and 100 K. For the δ = 0.034 sample, at 323 K two trigonal phases are observed δ ≈ 0.08 (open blue circles) and δ = 0.0 (solid red circles) both with the P31c symmetry. At 298 K two phases are observed with δ ≈ 0.0: trigonal P31c (solid red circles) and orthorhombic Pbn21 (magenta crosses) and a third phase, trigonal P31c with δ ≈ 0.08 (open blue circles) is also found. At 100 K two phases, δ = 0.0 orthorhombic Pbn21 (magenta crosses) and δ ≈ 0.08 trigonal P31c (open blue circles) remain. For the δ = 0.099 sample, only the trigonal δ ≈ 0.08 phase is found. Solid green lines are calculated intensities and the bottom solid blue lines show the difference between the observed and calculated intensities. In the left-hand panel, δ refers to nominal, or average, O stoichiometry in the biphasic sample. See text for details.

temperature Pbn21 to monoclinic P21 magnetoelastic structural transition associated with long-range antiferromagnetic ordering in YBaCo4O7.6,7 It is possible that this temperature (100 K) may be too close to TN for our data to reveal the onset of possible and subtle monoclinic distortions, although we do see slight broadening of the (004) reflection for only the δ = 0.00 sample (data not shown). It is also possible that excess oxygen drives this orthorhombic to monoclinic structural transition to lower temperature or suppresses it entirely, in agreement with the loss of long-range magnetic ordering. High-resolution diffraction data at temperatures lower than 100 K may resolve this issue. Magnetism. To clarify the impact of the suppressed orthorhombic distortion on magnetism for δ ≥ 0.1, we performed neutron diffraction experiments on samples δ = 0.11 and 0.15 at low temperatures. In both samples we observe that the structure remains P31c down to 6 K. In addition, we observe significant short-range magnetic correlations below ∼80 K, panels a and b in Figure 7, rather than the long-range antiferromagnetism reported in stoichiometric YBaCo4O7.0.6,7 Figure 7c shows specific heat measured on single crystals of composition YBaCo4O7.0 (δ = 0.00) and YBaCo4O7.12 (δ = 0.12). An anomaly is seen in the former at the onset of longrange antiferromagnetism. No such feature is seen in the oxidized sample, reflecting the suppression of this magnetically ordered state in favor of short-range correlations. Recently, Schweika et al. described the highly degenerate ground state of antiferromagnetically coupled nearest neighboring spins on the Kagomé lattice as two competing chiral structures that show either uniform or staggered chirality.35 Staggered chirality with ordering wave-vector q = √3 × √3 is 4194



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CONCLUSIONS In this paper, we report the oxygen uptake/release properties of YBaCo4O7+δ using combined thermogravimetric analysis (TGA), in situ X-ray diffraction (XRD), and synchrotron and neutron diffraction measurements. We show that samples treated in 20% O2/N2 at temperatures near 350 °C can have several oxygen loaded states. For samples heated slightly above 350 °C, the material reaches an equilibrium state with 0 < δ < 0.08 and displays biphasic properties when cooled to room temperature. The materials heated at temperatures slightly below this temperature absorb significantly more oxygen (δ = 1−1.1) and show the orthorhombic Pbc21 symmetry as previously reported by Chmaissem et al.24 High-resolution synchrotron X-ray diffraction experiments unambiguously demonstrate the existence of a miscibility gap at and near room temperature that separates the δ = 0.0 and δ ≈ 0.08 phases and result in the observed biphasic structural properties in the lightly oxygen doped R-114 materials. This miscibility gap, as well as the appearance of a series of O compositions that can be written as the ratio of simple fractions, suggests the possibility of a sequence of ordered O interstitial ‘line’ phases in YBaCo4O7+δ. Understanding the scope of these line phases, their thermodynamic and kinetic stabilities, and their structures could be an important step in the evaluation of YBaCo4O7+δ and other related compounds as candidates for O storage. Additionally, the impact of oxygen nonstoichiometry on low temperature structure and magnetism is profound: the single phase specimens with δ > 0.08 preserve their hexagonal P31c structure down to 6 K, thus retaining a highly geometrically frustrated topology. Accordingly, they exhibit only short-range magnetic correlations below 100 K, in contrast to the 3D antiferromagnetic state found in YBaCo4O7.0, which has undergone a frustration-breaking transition to orthorhombic symmetry at 315 K.6,7 Future work will explore the impact of nonstoichiometry beyond δ = 0.08 on phase behavior and magnetism, including the potential influence of ordered interstitial states at higher O loadings.

Figure A1. Rietveld refined phase fraction (blue circles) and intensity of (400) reflection (red squares) vs nominal oxygen content in quenched samples showing biphasic behavior at room temperature. The solid line is a linear fit to the first five samples. Horizontal error bars are determined from TGA measurements (see text); uncertainty in the phase fraction and reflection intensity are smaller than the symbols. The subscripts 1 and 2 represent the δ = 0.0 and δ ≈ 0.08 phases, respectively.

Division of Materials Science and Engineering under contract no. DE-AC02-06CH11357.

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(1) Valldor, M.; Andersson, M. Solid State Sci. 2002, 4, 923. (2) Valldor, M. Solid State Sci. 2004, 6, 251. (3) Soda, M.; Yasui, Y.; Moyoshi, T.; Sato, M.; Igawa, N.; Kakurai, K. J. Magn. Magn. Mater. 2007, 310, e441. (4) Hollmann, N.; Hu, Z.; Valldor, M.; Maignan, A.; Tanaka, A.; Hsieh, H. H.; Lin, H. −J.; Chen, C. T.; Tjeng, L. H. Phys. Rev. B 2009, 80, 085111. (5) Maignan, A.; Caignaert, V.; Pelloquin, D.; Hebert, S.; Pralong, V.; Hejtmanek, J.; Khomskii, D. Phys. Rev. B 2006, 74, 165110. (6) Chapon, L. C.; Radaelli, P. G.; Zheng, H.; Mitchell, J. F. Phys. Rev. B 2006, 74, 172401. (7) Khalyavin, D. D.; Manuel, P.; Ouladdiaf, B.; Huq, A.; Stephens, P. W.; Zheng, H.; Mitchell, J. F.; Chapon, L. C. Phys. Rev. B 2011, 83, 094412. (8) Caignaert, V.; Maignan, A.; Pralong, V.; Hebert, S.; Pelloquin, D. Solid State Sci. 2006, 8, 1160. (9) Caignaert, V.; Pralong, V.; Hardy, V.; Ritter, C.; Raveau, B. Phys. Rev. B 2010, 81, 094417. (10) Bera, A. K.; Yusuf, S. M.; Banerjee, S. Solid State Sci. 2013, 16, 57. (11) Karppinen, M.; Yamauchi, H.; Otani, S.; Fujita, T.; Motohashi, T.; Huang, Y. −H.; Valkeapäa,̈ M.; Fjellvåg, H. Chem. Mater. 2006, 18, 490. (12) Räsänen, S.; Yamauchi, H.; Karppinen, M. Chem. Lett. 2008, 37, 638. (13) Jia, Y.; Jiang, H.; Valkeapäa,̈ M.; Yamauchi, H.; Karppinen, M.; Kauppinen, E. I. J. Am. Chem. Soc. 2009, 131, 4880. (14) Hao, H.; Cui; Chen, C.; Pan, L.; Hu, J.; Hu, X. Solid State Ionics 2006, 177, 631. (15) Kadota, S.; Karppinen, M.; Motohashi, T.; Yamauchi, H. Chem. Mater. 2008, 20, 6378. (16) Tsipis, E. V.; Khalyavin, D. D.; Shiryaev, S. V.; Redkina, K. S.; Núñez, P. Mater. Chem. Phys. 2005, 92, 33. (17) Valkeapäa,̈ M.; Karppinen, M.; Motohashi, T.; Liu, R.-S.; Chen, J.-M.; Yamauchi, H. Chem. Lett. 2007, 36, 1368. (18) Avdeev, M.; Kharton, V. V.; Tsipis, E. V. J. Solid State Chem. 2010, 183, 2506.

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APPENDIX We determined approximate lower and the upper boundaries of the miscibility gap in YBaCo4O7+δ by extrapolating a linear relationship between the nominal δ and I2/(I1 + I2), where I1 and I2 are the integrated intensities of (004) peaks of specimens showing biphasic behavior. Plotting the refined ratio of phase fractions from full pattern Rietveld analysis, f 2/(f1 + f 2), against nominal δ gives the same result. The results are shown in Figure A1. As noted in the text, there is some uncertainty in the boundary values since the quenching process is distinctly nonequilibrium. Indeed, the specimen with nominal δ = 0.085 shows ∼2.4% of the δ = 0.0 phase as determined by full profile Rietveld analysis, and lies off the predictor line. For this reason, we identify the upper boundary as δ ≈ 0.08.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS TArgonne National Laboratory’s work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, 4195



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Oxygen Stoichiometry in the Geometrically Frustrated KagomÐ¹ System

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