Article pubs.acs.org/cm
Phase Stability and Oxygen Nonstoichiometry of Highly OxygenDeficient Perovskite-Type Oxides: A Case Study of (Ba,Sr)(Co,Fe)O3−δ David N. Mueller,*,† Roger A. De Souza,*,† Han-Ill Yoo,‡ and Manfred Martin†,‡ †
Institute of Physical Chemistry, RWTH Aachen University, Aachen, Germany WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
‡
ABSTRACT: Highly oxygen-deficient perovskite-type oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF5582), are excellent mixed ionic and electronic conductors, with high concentrations of mobile oxygen vacancies and electronic charge carriers. Literature data for the oxygen nonstoichiometry δ of BSCF5582 at a given temperature and oxygen partial pressure show considerable scatter. In addition, the decomposition of cubic BSCF5582 (into hexagonal and cubic phases of differing compositions) at temperatures below 1073 K was not taken into account. In this study we investigated the phase stability and the oxygen nonstoichiometry of BSCF5582 using thermogravimetry (TG) and coulometric titration (CT). TG experiments, based on rapid temperature jumps, permitted the relative oxygen nonstoichiometry Δδ of cubic BSCF5582 to be extracted, free from the effects of the aforementioned slow decomposition. The CT results suggest the existence, at low oxygen partial pressures, of a stoichiometric phase with composition Ba0.5Sr0.5Co0.8Fe0.2O2 in which all the cations are in the lowest oxidation states possible in a solid compound. It is proposed to use this point as a reference to calibrate both CT and TG data, yielding absolute values for δ for the cubic BSCF5582 without any uncertainties posed by the decomposition. The TG experiments also yielded the decomposition temperature for cubic BSCF as a function of oxygen partial pressure. KEYWORDS: BSCF, decomposition, nonstoichiometry, coulometric titration, perovskite, mixed ionic electronic conductor
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observed to date for a cubic ABO3 perovskite-type oxide.2 In addition, it does not form a ABO2.5 brownmillerite-type phase, in contrast to the related material SrCo0.8Fe0.2O3−δ (SCF);3 that is, it also evidently passes through δ = 0.5 without the oxygen vacancies ordering into a superstructure. Although this absence of ordering at δ = 0.5 is advantageous with regard to the oxygen transport properties, it does mean that also at low oxygen activities there is no convenient reference point. In the absence of an inflection point, one may resort to determining the absolute oxygen stoichiometry by total reduction of the compound in a thermobalance. The term “total reduction” in itself is misleading, as it implies a transformation of the compound in question to a mixture of all the metals in their elemental forms. Given that many perovskites have alkaline earth metals on the A site, the oxygen partial pressures that are required to reduce these oxides to the elemental metals are extremely low. For example, the threshold oxygen partial pressure to reduce BaO to Ba at T = 1273 K is pO2 < 8 × 10−31 Pa (calculated with data from ref 4); this is extremely difficult to achieve in a standard thermogravimetric analysis (TGA) setup, even with reducing gas buffer mixtures, such as H2/H2O or CO/CO2. Usually, therefore, the alkaline
INTRODUCTION The deviation of oxygen content from the stoichiometric composition of an oxide, or oxygen nonstoichiometry δ, may govern the structural, magnetic, electrical, and electrochemical properties of an oxide material. Many methods have been devised to measure δ as a function of temperature T and oxygen partial pressure pO2. Most of these methods yield relative changes in oxygen nonstoichiometry Δδ simply and precisely, but they suffer from difficulties in determining absolute values of δ (=δref + Δδ) because they require a reference point of known oxygen nonstoichiometry, δref. A convenient reference point that can be obtained from experimental measurements of the oxygen nonstoichiometry is δ = 0: in an isothermal plot of Δδ versus pO2 a point of inflection is expected at δ = 0.1 All subsequent measurements can be calibrated against this point. Some materials, however, do not exhibit such a convenient calibration point: they may display an oxygen deficit under the most oxidizing conditions that are experimentally accessible, or conversely, they may display an oxygen excess up to the point of decomposition under reducing conditions. One example of the former is the compound Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF5582) with reported values of δ exceeding 10% in air at any temperature high enough for the oxygen sublattice to be equilibrated. This material has an extraordinary ability to accommodate oxygen vacancies, showing the highest oxygen nonstoichiometry © 2011 American Chemical Society
Received: August 12, 2011 Revised: December 14, 2011 Published: December 14, 2011 269
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range of oxygen partial pressures. This is not the case (cf. TGA) if one has to use gas mixtures to fix the oxygen partial pressure (O2/N2 mixtures yield pO2’s down to 1 Pa; CO/CO2 or H2/ H2O yield pO2’s up to 10−5 Pa, depending on the temperature). In this study we investigated BSCF5582 as a generic material for highly oxygen-deficient perovskite-type oxides. We focus on determining the oxygen nonstoichiometry and the phase stability of BSCF5582 using CT and TG. In the literature there are extensive experimental data, covering a broad range of temperatures and oxygen partial pressures, on the oxygen nonstoichiometry of BSCF5582.2,10−14 We summarize this data in Figure 1, a plot of oxygen nonstoichiometry δ as a function
earth cations are not reduced at all, and hence their oxides are products of the “total reduction”. The reaction for the “total reduction” of BSCF5582 in H2, for example, may thus be
(2 − δ)H2 + Ba 0.5Sr0.5Co0.8Fe0.2O3 −δ → 0.5BaO + 0.5SrO + 0.8Co + 0.2Fe + (2 − δ)H2O
(1)
whereby the oxides and/or the metals may exhibit mutual solubility. The product mixture should be analyzed by means of X-ray diffraction, to confirm that the reaction has proceeded to completion and therefore that the measured amount of released oxygen is exactly as expected from eq 1. The main source of error in this method is the formation of volatile compounds other than oxygen; this results in an additional mass loss that is incorrectly attributed to loss of oxygen.5 Another approach to obtaining the absolute oxygen content of an oxide compound is the use of wet chemical methods. By determining the oxidation states of the transition metal cations, one can calculate the oxygen content indirectly, assuming that the other ions have their nominal oxidation states. These methods consist mostly of reduction or oxidation titrations, which have to be matched carefully to the cations to be analyzed. For transition metals such as Co and Fe, iodometric and cerimetric titrations are commonly used.6 The major drawback of these wet chemical methods is the need to dissolve the oxide in a solvent. This solvent, especially when it is an oxidant such as aqueous HNO3 or H2SO4, can react with the compound, resulting in oxygen leaving the sample before the titration is performed. Furthermore, the cations can be coordinated by the solvent in a higher oxidation state than the expected one, also leading to an incorrect calculation of the oxygen content. A more direct approach to determining the absolute oxygen content is provided by neutron diffraction (ND). One collects a diffraction pattern in situ at given pO2 and T; and by performing a Rietveld refinement7 one can obtain the site occupancy of the oxygen sublattice, assuming that the cation sublattices are fully occupied. This method does not rely on any reference point and thus in this sense it should be considered superior to all indirect methods. It is not, however, universally applicable because it is only sensitive to large values of oxygen nonstoichiometry. In addition, one has to exercise care in performing the refinement, as the site occupancy and the thermal parameters (Debye−Waller factors) are strongly correlated. Refining both simultaneously may lead, for example, to overestimated thermal parameters at the expense of underestimated occupancies.8 Finally, we mention that coulometric titration (CT) can be used to determine Δδ of an oxide. In this technique, a sample is enclosed in an electrochemical cell, comprising an oxygen-ion conducting electrolyte. By applying a voltage to the cell, one can pump oxygen in or out electrochemically. The amount of oxygen leaving or entering the cell can be calculated very precisely from the pumping current and duration of pumping. A more detailed description can be found elsewhere.9 CT is generally applied to cases for which Δδ is too small for determination by other methods. The disadvantage in the case of BSCF5582 (for which Δδ is huge) is the large amount of oxygen that needs to be pumped out of the sample chamber, leading to pumping times of the order of days or even weeks. On the other hand, CT can access a continuous (and wide)
Figure 1. Comparison of available literature data on the oxygen nonstoichiometry δ as a function of oxygen partial pressure pO2. Different symbols mark different sources; same colors indicate same temperatures (red, 298 K; orange, 873 K; yellow, 973 K; green, 1073 K; cyan, 1123 K; blue, 1173 K; purple, 1223 K; crimson, 1273 K). Reference points for each study are marked with an asterisk.
of pO2 for different temperatures. The reference point used for each data set is marked with an asterisk. Evidently, there are huge discrepancies in the value of δ at fixed pO2 and T, not only between data determined by different methods but also between data determined by the same method by different groups. This is clear from consideration, for example, of the data obtained by Bucher et al.,13 Wang et al.,11 and Svarcova et al.,12 who used total reduction with H2/Ar or H2/N2 mixtures as monitored by thermogravimetric analysis to determine the respective reference points. The wet chemical methods used to find a reference point by Zeng et al.10 and Kriegel et al. respectively yield different results, showing that the flaws in these techniques as discussed above are not neglible. The most important issue here would be that in order to undertake an experiment like this, the sample has to be quenched to room temperature, from whatever reference temperature was chosen. As BSCF has an extremely high oxygen exchange rate, it is thus highly questionable, if any quenching procedure would result in a true “frozen in” state of the oxygen nonstoichiometry. McIntosh et al.2 employed in situ neutron diffraction, and refined each diffraction pattern independently for every (T, pO2) data point. Although they did not have to rely on determining a reference point to calibrate the nonstoichiometry data, it is striking that this method yields values of δ much higher than those obtained by any other method. A source of error here might be the underestimation of the site occupancies.8 270
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RESULTS A. Coulometric Titration. From the pumping current I and pumping duration t, the amount of oxygen released by the sample, which is pumped out of the cell, can be calculated according to
Besides problems characteristic of the experimental methods (and possibly differences in cation composition between individual groups), there is a critical problem arising from the material itself that has not been taken into account in the literature, namely the decomposition of the cubic BSCF5582 at temperatures below 1173 K into hexagonal and cubic perovskite phases of significantly different cation compositions and oxygen nonstoichiometries:15
nO =
Ba 0.5Sr0.5Co0.8Fe0.2Ocub,educt 3 −δ → 1 2 Ba 0.5 + xSr0.5 − xCoO3hex,prod −δ + 1 2 Ba 0.5 − xSr0.5 + xCo0.6Fe0.4Ocub,prod 3 −δ
⎛ It ⎞ 2V ⎜ − d ΔpO2,gas⎟ ⎝ 2F ⎠ RT
(3)
Vd is the dead volume inside the cell, T the absolute temperature, and ΔpO2,gas the difference of the oxygen partial pressure inside the cell before and after each titration run. In Figure 3a we plot the measured pO2 in the cell against the amount of released oxygen nO. Because of the instability of BSCF5582 with regard to the formation of a hexagonal phase at temperatures below 1173 K,12,15 data were only obtained for two temperatures above this threshold. At small amounts of oxygen pumped on the left part of the graph, one finds a regime, where the logarithm of the measured pO2 varies almost linearly with the pumped amount of oxygen nO. This linear regime extends to pO2 = 10−4 Pa for T = 1273 K and pO2 = 10−5 Pa for T = 1173 K, respectively. For partial pressures below 10−7 Pa (bottom right of Figure 3a), one observes a regime where the oxygen partial pressure remains constant regardless of how much oxygen is pumped out. Such behavior is generally typical of a decomposition,17 and we ascribe it to the decomposition (“total reduction”) of cubic BSCF5582 to the earth alkaline oxides and the transition metals in elemental form. All the oxygen pumped out of the cell stems from the decomposition reaction, which takes place at a certain pO2. However, the amount of oxygen pumped between each data point is not sufficient for a complete decomposition and so there is a mixture between already decomposed products and some not yet decomposed initial material. Turning to the intermediate pO2 between the decomposition and the linear regime, one notes that the behavior is somewhat different for both temperatures. The part of the graph in question is shown enlarged in Figure 3b. At the lower temperature T = 1173, a constant pO2 was observed for nO > 0.7 mmol. After even more oxygen is pumped out, the curve of pO2 versus nO displays an inflection point. This indicates a stability region of an intermediate phase. We postulate that this phase is Ba0.5Sr0.5Co0.8Fe0.2O2 because (i) this composition corresponds to all cations being in their lowest known oxidation states for an oxide and (ii) stable oxides of this nonstoichiometry (δ = 1) are known to exist, namely, SrFeO2 and BaCoO2 (and also CaFeO2). SrFeO2 and CaFeO2 can be obtained by reducing SrFeO3−δ or CaFeO3−δ, respectively, with CaH2.18,19 Such infinite layer structures are closely related to the perovskite structure. BaCoO2 has been synthesized from BaCO3 and Co in vacuum conditions and exhibits a hexagonal structure.20 (iii) It is the only possible explanation of an invariance in the CT diagram, as all other stoichiometric points can be ruled out: ABO3 is obviously impossible, and existence of an ABO2.5 brownmillerite phase has been discounted by Vente et al.21 The phase transition that takes place at the plateau in the nO/ log pO2 graph can be written as follows:
(2)
Although the decomposition occurs on a time scale of hours, the equilibration times and temperatures employed in the aforementioned studies may well have yielded non-negligible phase fractions of the hexagonal phase, with consequent errors in the reported values of δ. In this study we combine TG experiments based on rapid temperature jumps with CT to obtain precise data for the oxygen nonstoichiometry of BSCF5582.
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Article
EXPERIMENTAL SECTION
A. Synthesis. Different samples were prepared for coulometric titration and thermogravimetric experiments. For both samples the BSCF material was synthesized via the ethylenediaminetetraacetate (EDTA)−citric acid sol−gel route, as described elsewhere.16 The raw powders from the synthesis were calcined for 12 h at 1273 K. For the TGA experiments these calcined powders with a mean particle size of several micrometers were used as received. For the coulometric titration the powder was pressed into discs of 10 mm diameter and 2 mm thickness by uniaxial pressing, and the pressed samples were sintered at 1323 K for 8 h; the densities achieved were above 95% of the theoretical density (as measured by Archimedes method). B. Thermogravimetric Analysis. All thermogravimetric experiments were conducted on a SETSYS 16/18 thermobalance (SETARAM, Paris, France). The sample was placed in an alumina crucible of volume 150 μL. The oxygen partial pressures were fixed by diluting 5 N O2 with 6 N N2. The mass changes have been corrected for buoyancy by measurements on an inert Al2O3 sample. C. Coulometric Titration. The general procedure of a coulometric titration experiment and the underlying theory are described in detail elsewhere.9 The sintered sample was held in an alumina crucible, which was sealed against a Pt/YSZ/Pt electrochemical cell, as shown schematically in Figure 2.
Figure 2. Experimental setup for coulometric titrations with constant current supply (1), platinum electrodes and leads (2), 8YSZ electrolyte (3), alumina crucibles (4), glass sealant (5), and BSCF5582 specimen (6).
ABO3 −δ → ABO2 +ε +
1−δ−ε O2 2
(4)
The invariance of the oxygen partial pressure at this point can be explained by the Gibbs phase rule: When the amount of 271
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because the oxygen stoichiometries of the two phases are equal at this pO2 and T so that a two phase equilibrium (5) ensues:
ABO3 −δ ↔ ABO2 +ε with (3 − δ) = (2 + ε) at a certain pO2
(5)
The differing behavior at the two temperatures investigated is attributed to a form of miscibility (due to the same oxygen stoichiometries) of the two phases at 1273 K, which turns into a miscibility gap at 1173 K. It is striking that the observed inflection points (at the two different temperatures) yield the same nO, and thus the same δ. This provides additional support for the existence of the Ba0.5Sr0.5Co0.8Fe0.2O2 phase. We took this compound as a reference point to calibrate the nonstoichiometry data of both CT and TG experiments. The absolute nonstoichiometry data obtained by CT for cubic BSCF are plotted in Figure 3c. B. Thermogravimetry. To determine the oxygen nonstoichiometry of the cubic BSCF5582 phase at temperatures at which it partly decomposes into a hexagonal phase (and a second cubic perovskite), we employed a nonstandard procedure. Specifically, we utilized the fact that the equilibration of the oxygen sublattice for cubic BSCF5582 powder occurs much faster (with a time constant τ, which is governed by surface incorporation,13 of the order of a few minutes) than the formation of the hexagonal phase (τ of at least several days15). The experiment consisted of first equilibrating a powder sample at T = 1223 K to ensure that cubic BSCF5582 was formed (confirmed by X-ray diffraction), and then rapidly cooling (30 K min−1) to the desired temperature to keep the time for formation of the hexagonal phase as short as possible. After 4 h at the desired temperature, the sample was rapidly heated to T = 1223 K and then held at this temperature to reform cubic BSCF5582. (Essentially this also means that every data point has its own reference point and drift of the balance is ruled out.) The raw data for the experiment at an oxygen partial pressure of 20 kPa is shown in Figure 4a. As mentioned above, the oxygen nonstoichiometry of BSCF5582 adjusts rapidly (within minutes) to the value defined by the new temperature. The decomposition into hexagonal and cubic with differing oxygen nonstoichiometries, δcub,educt > δhex,prod + δcub,prod (see eq 2) is apparent in the slow but steady mass change that occurs over a period of hours. From the kinetic data reported in ref 15 one can estimate that for the longest cooling times of around 15 min the phase fraction of the hexagonal phase is 1% or lower and thus negligible at the point at which equilibrium of the oxygen sublattice of the parent cubic phase is attained. The initial mass change thus refers to changes in the oxygen nonstoichiometry Δδ of cubic BSCF5582. Absolute values of δ were found by calibrating against the data obtained from the coulometric titration: From the CT experiments we know the absolute value of δ for any point in the T−pO2−regime covered in the experiments, so the value for T = 1273 K and pO2 = 103 Pa from the CT has been chosen as the reference point for TGA measurements by assigning δTGA = δCT. All other points from the TGA data have then been determined by using this point as a reference point. The results are shown in Figure 4b, an exert of this data together with the results from the coulometric titration and three sets of literature data in Figure 5. The data sets from both techniques used in this work are in good agreement.
Figure 3. Measured oxygen partial pressure as a function of the amount of oxygen pumped out of the cell (a), closeup of the decomposition region (b), calculated nonstoichiometry as a function of oxygen partial pressure in a double logarithmic graph (c) determined from coulometric titration for temperatures of 1273 K (black) and 1173 K (red). Circles mark the linear domain and triangles the decomposition area.
oxygen pumped out of the system in one titration step is not sufficient to turn all of ABO3−δ to ABO2+ε, that is, ΔnO < (1 − δ − ε)/2, there are three phases coexisting. Considering BSCF as the only component, the degrees of freedom according to the Gibbs phase rule turn to zero, causing this invariance. Turning to the higher temperature (T = 1273 K), one finds that the titration curve smoothly transitions from the cubic stability regime at pO2 > 10−4 Pa to the proposed stability regime of the ABO2 compound. The transition is smooth 272
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The slow but steady mass change after the initial equilibration of the oxygen sublattice is observed only at temperatures below 1113 K and above 823 K. The upper limit corresponds to the temperature above which cubic BSCF5582 is the thermodynamically most stable phase. In contrast, the lower temperature is not a thermodynamic limit but a kinetic limit (XRD analysis indicates that no detectable decomposition has occurred). In other words, the formation of the hexagonal phase is so slow (at this high rate of cooling) that the parent cubic BSCF5582 becomes metastable. At much slower rates of cooling, the decomposition will proceed to a certain extent, before a nonequilibrium mixture of initial and product phases is frozen in at some temperature. We repeated this experimental procedure at other oxygen partial pressures. From the data we estimated the thermodynamic and kinetic limits for the decomposition reaction, that is, the highest and lowest temperatures for which no change in mass was apparent after reaching the particular temperature. The data are shown in Figure 6. When comparing the oxygen
Figure 4. Raw data of a TGA experiment to determine the oxygen nonstoichiometry for different temperatures at a partial pressure of pO2 = 20 kPa. It is clearly visible that between temperatures of 1073 and 823 K there is still a mass change even after reaching the designated temperature. The slow change in mass is attributed to the decomposition of the cubic phase into product hexagonal and cubic phases that exhibit differing oxygen nonstoichiometries from the parent cubic phase (a). Nonstoichiometry data obtained by calibrating the TGA data to the reference point obtained from CT (b).
Figure 6. Stability region of BSCF5582 estimated from TGA temperature jump experiments.
nonstoichiometries of the cubic BSCF5582 at the upper stability limit, it is striking that the values for δ are not equal, which leads to the conclusion that the stability of BSCF5582 is not only a function of the nonstoichiometry; whether this dependence stems from the charge and thus the radius of the Bsite cations as suggested by Arnold et al.22 is subject to future discussion. C.. Comparison of Nonstoichiometry Data. Figure 5 shows the nonstoichiometry data from both used techniques in this work as a function of pO2. A good agreement between the two techniques can be seen; the data obtained in this work lies between the data sets from Kriegel et al.14 and McIntosh et al.2 We also compare in Table 1 the thermodynamic factor calculated from the various data sets:2,13,14
γO =
1 ⎛ ∂ ln pO2 ⎞ ⎜ ⎟ 2 ⎝ ∂ ln(3 − δ) ⎠T
(6)
One finds that the literature data obtained form TGA measurements (Bucher et al.13 and Kriegel et al.14) agree very well in the margin of error. The values obtained in this work agree very well with these literature data for temperatures of 1123 K and highertemperatures for which cubic BSCF5582 is stable. This good agreement is found, despite
Figure 5. Comparison of the oxygen nonstoichiometry δ of cubic BSCF5582 as a function of oxygen partial pressure obtained from TGA (closed circles) and CT (straight line) at 1173 K (blue) and 1273 K (red) to an excerpt of the literature data at similar temperatures. 273
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Table 1. Comparison of the Thermodynamic Factor γO Extracted from the Literature Data Shown in Figure 1 and from the Data Obtained in This Study T/K
method
873
Bucher et al.13 Kriegel et al.14 McIntosh et al.2 this study this study
TGA TGA ND TGA CT
125 ± 2 77 ± 11 89 ± 13
973 138 135 91 90
± ± ± ±
1073 2 3 15 10
differences in the absolute values of δ, because γO is not very sensitive to the absolute value of 3 − δ. At lower temperatures, the thermodynamic factors of Bucher and Kriegel are significantly larger than the ones obtained in this work. We attribute the discrepancy to the decomposition reaction (eq 2) being neglected in the studies of Bucher et al.13 and Kriegel et al.14 As the δ of the hexagonal phase is lower than the one of the cubic, a lower δ is ascribed to the cubic phase, leading to an overestimated thermodynamic factor.
1173
111 ± 5 111 ± 8 96 ± 7
1273
95 ± 4
91 ± 7
89 ± 2 102 ± 14 95 ± 8 92 ± 3
82 ± 6 85 ± 2
(6) Jander, G.; Jahr, K. F.; Schulze, G.; Simon, J. Maßanalyse; de Gruyter: Berlin, 2003. (7) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65−71. (8) Satto, C.; Jansen, J.; Lexcellent, C.; Schryvers, D. Solid State Commun. 2000, 116, 273−277. (9) Kang, S. H.; Yoo, H. I. Solid State Ionics 1996, 86−88, 751−755. (10) Zeng, P.; Chen, Z.; Zhou, W.; Gu, H.; Shao, Z.; Liu, S. J. Membr. Sci. 2007, 291, 148−156. (11) Wang, H.; Yang, W.; Tablet, C.; Caro, J. Diffus. Fundam. 2005, 2, 46.1. (12) Svarcová, S.; Wiik, K.; Tolchard, J.; Bouwmeester, H. J. M.; Grande, T. Solid State Ionics 2008, 178, 1787−1791. (13) Bucher, E.; Egger, A.; Ried, P.; Sitte, W.; Holtappels, P. Solid State Ionics 2008, 179, 1032−1035. (14) Kriegel, R.; Kircheisen, R.; Töpfer, J. Solid State Ionics 2009, 181, 64−70. (15) Mueller, D. N.; De Souza, R. A.; Weirich, T. E.; Roehrens, D.; Mayer, J.; Martin, M. Phys. Chem. Chem. Phys. 2010, 12, 10320− 10328. (16) Shao, Z.; Yang, W.; Cong, Y.; Dong, H.; Tong, J.; Xiong, G. J. Membr. Sci. 2000, 172, 177−188. (17) Nakamura, T.; Yashiro, K.; Sato, K.; Mizusaki, J. Solid State Ionics 2009, 180, 368−376. (18) Tsujimoto, Y.; Tassel, C.; Hayashi, N.; Watanabe, T.; Kageyama, H.; Yoshimura, K.; Takano, M.; Ceretti, M.; Ritter, C.; Paulus, W. Nature 2007, 450, 1062−1065. (19) Inoue, S.; Kawai, M.; Ichikawa, N.; Kageyama, H.; Paulus, W.; Shimakawa, Y. Nature Chem. 2010, 2, 213−217. (20) Spitsbergen, U. Acta Crystallogr. 1960, 13, 197−198. (21) Vente, J.; McIntosh, S.; Haije, W.; Bouwmeester, H. J. Solid State Electrochem. 2006, 10, 581−588. (22) Arnold, M.; Gesing, T. M.; Martynczuk, J.; Feldhoff, A. Chem. Mater. 2008, 20, 5851−5858.
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CONCLUSIONS Using BSCF5582 as a generic example for highly oxygendeficient perovskite-type oxides, we have shown that the determination of absolute oxygen stoichiometries is not a trivial task, especially when a clear reference point is missing. In this work, the method of coulometric titration has been used to try to find and access a different reference state, that of a proposed ABO2 compound in which the cations are all in their most reduced form, for the absolute value of δ. Adding to these difficulties is the decomposition of BSCF at temperatures below 850 °C to a hexagonal and cubic phase of different oxygen stoichiometries than the starting material, giving rise to underestimated values for δ for these temperatures. A set of unconventional TG experiments was conducted to circumvent this effect, and utilizing the observance of the decomposition through the mass change, the lower stability limit in temperature of the cubic phase has been determined as a function of oxygen partial pressure.
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1123
115 ± 3
AUTHOR INFORMATION
Corresponding Author
*
[email protected];
[email protected].
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ACKNOWLEDGMENTS The authors thank N.-J. Heo (Seoul National University) for experimental assistance with the coulometric titration experiments and S. Finkeldei (RWTH Aachen University) for performing the thermogravimetric experiments. This work has been funded within the project MEM-OXYCOAL by the Federal Ministry of Economics and Technology, Germany, with the promotion label 0327803A. The responsibility for the content of this work lies with the authors.
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REFERENCES
(1) Wagner, C. Prog. Solid State Chem. 1971, 6, 1−15. (2) McIntosh, S.; Vente, J. F.; Haije, W. G.; Blank, D. H. A.; Bouwmeester, H. J. M. Chem. Mater. 2006, 18, 2187−2193. (3) Kruidhof, H.; Bouwmeester, H. J. M.; Doorn, R. H. E. V.; Burggraaf, A. J. Solid State Ionics 1993, 63−65, 816−822. (4) Kubaschewski, O.; Alcock, C. B. Metallurgical Thermochemistry; Pergamon Press: Oxford, 1979. (5) Karppinen, M.; Matvejeff, M.; Salomaki, K.; Yamauchi, H. J. Mater. Chem. 2002, 12, 1761−1764. 274
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