Oxygen Content and Thermodynamic Stability of YBaCo 2 O 6 − δ Double Perovskite

*e thermodynamic stability of the double perovskite YBaCo2O6−δ was studied using the coulometric titration technique and verified by measurements of the overall conductivity depending on oxygen partial pressure at a given temperature. As a result, the stability diagram of YBaCo2O6−δ was plotted. YBaCo2O6−δ was found to be thermodynamically stable in air at 850°C and higher temperatures, whereas its thermodynamic stability at 900°C is limited by the range of oxygen partial pressures −3.56≤ log(pO2/atm)≤−0.14. Oxygen content in YBaCo2O6−δ slightly decreases at 900°C from 5.035 at log(pO2/atm)�−0.14 to 4.989 in the atmosphere with log(pO2/atm)�−3.565 indicating a crucial role which variation of Co/Co ratio plays in its stability. YBaCo2O6−δ decomposes into the mixture of YCoO3 and BaCoO3−z at the high pO2 stability limit, whereas YBaCo4O7, BaCo1−xYxO3−c, and Y2O3 were identified as the products of its decomposition at the low pO2 one.


Introduction
Cobaltites REBaCo 2 O 6−δ , where REis a rare earth metal, with the double perovskite structure have attracted great attention in the past decade due to their unique properties such as high oxide ion and electronic conductivity as well as very promising activity as cathodes in IT SOFCs [1][2][3][4][5].Despite the fascinating properties of these materials, their successful commercial application as cathodes is restricted by large thermal expansion coefficient (CTE), which significantly exceeds CTE of the stateof-the-art electrolyte materials [4].Nevertheless, there is a trend of lowering CTE with decreasing size of RE [4].In this respect, YBaCo 2 O 6−δ has advantage as compared to other double perovskites since it has the lowest CTE among all REBaCo 2 O 6−δ oxides, which is close to that of doped ceria and zirconia, the state-of-the-art SOFC electrolytes [1][2][3][4][5][6]. is double perovskite was found to have the total conductivity high enough for using it as a cathode for IT SOFCs [6][7][8][9][10][11][12][13][14][15].As a result, materials on the basis of YBaCo 2 O 6−δ showed high performance as cathodes of IT SOFCs [6,9,10,12,15] and oxygen permeable membranes [16,17].It is generally recognized that knowledge of the thermodynamic stability limits of oxides materials is of key importance for understanding their properties and, therefore, their successful application in electrochemical devices for energy conversion and storage.However, the thermodynamic stability of YBaCo 2 O 6−δ is poorly understood, and it remains a controversial topic so far.e authors [14,15] by means of X-ray diffraction (XRD) found secondary phases in samples of YBaCo 2 O 6−δ annealed at temperatures between 800 °C and 850 °C depending on ambient atmosphere and concluded that this oxide is unstable under aforementioned conditions.It is worth noting that, according to Kim et al. [15], YBaCo 2 O 6−δ is unstable at 800 °C in air while it was found [14] that this oxide is quite stable in air even at 850 °C.On the contrary, YBaCo 2 O 6−δ was found to be unstable at 850 °C in atmosphere of nitrogen [14], whereas there is no evidence of this oxide decomposition at 800 °C in the same ambient atmosphere [15].Moreover, if YBaCo 2 O 6−δ is unstable at certain temperature, then one could expect a singularity of temperature dependences of its oxygen content, electrical conductivity, and CTE at this temperature.However, there is no evidence of such singularity in literature except the work of Xue et al. [6] where the authors observed something like that for CTE of YBaCo 2 O 6−δ at 800 °C and 850 °C.However, Xue et al. [6] did not comment this singularity, which was not confirmed in other works.us, the priority purpose of the present work was to obtain reliable data on the thermodynamic stability of YBaCo 2 O 6−δ by means of three independent methods such as coulometric titration, conductivity measurements, and homogenizing annealing of samples in atmosphere with controlled oxygen partial pressure.e novelty of the work consists in the constructing of the thermodynamic stability diagram for interesting and promising double perovskite YBaCo 2 O 6−δ for the first time.Stoichiometric mixture of starting materials was dissolved in concentrated nitric acid (99.99% purity), and required volume of glycerol (99% purity) was added as a complexing agent and a fuel.Glycerol quantity was calculated according to full reduction of corresponding nitrates to molecular nitrogen N 2 .e as-prepared solutions were heated continuously at 100 °C until complete water evaporation and pyrolysis of the dried precursor had occurred.e resulting ash was subsequently calcined for 10 hours at 1100 °C for BaCoO 3 as well as YBaCo 2 O 6−δ and 900 °C for YCoO 3 to get the desired oxide powder.

Materials and Methods
e phase composition of the powder samples prepared accordingly was studied at room temperature by means of X-ray diffraction (XRD) with XRD-7000 diffractometer (Shimadzu, Japan) using Cu Kα radiation.XRD showed no indication of the presence of a second phase for the asprepared oxides.
e chemical composition of all the oxides prepared was checked using an ICP spectrometer ICAP 6500 DUO and an atomic absorption spectrometer Solaar M6 (both supplied by ermo Scientific, USA).All the as-prepared oxides were shown to have the stoichiometric composition with respect to metal cations within the accuracy of 2%.No impurities were found within the same accuracy range as well.
For the measurements of electrical conductivity, single phase powder of YBaCo 2 O 6−δ was axially pressed into rectangular bars of 30 × 4 × 4 mm 3 at 40 MPa and sintered at 1150 °C for 24 h in air.e relative density of the sample bars used for measurements was found to be higher than 80%.
ermodynamic stability limits of YBaCo 2 O 6−δ were determined by three independent methods such as coulometric titration combined with EMF method, electrical conductivity measurements, and homogenizing annealing of samples in atmosphere with controlled oxygen partial pressure.
e coulometric titration was also employed for measurements of the oxygen nonstoichiometry in YBaCo 2 O 6−δ as a function of pO 2 at a given temperature.e original coulometric titration setup and the measurement procedure are described in detail elsewhere [18,19].
Total conductivity of YBaCo 2 O 6−δ oxide as a function of pO 2 at a given temperature was measured using 4-probe dcmethod, and the original setup described in detail elsewhere [20].
Absolute value of δ in YBaCo 2 O 6−δ sample was determined by direct reduction by hydrogen flux in the TG setup (TG/H2).Experimental details for this method are given elsewhere [19].
For homogenizing annealing experiments, samples of two different compositions were employed.e first one was equimolar mixture of YCoO 3 and BaCoO 3 , whereas the second one was single phase powder of YBaCo 2 O 6−δ .In both cases, a sample was heated first to 700 °C in an atmosphere with a given pO 2 and then was equilibrated at this temperature for 72 h followed by quenching to room temperature.After that, the quenched sample was grinded in mortar, and its phase composition was determined by XRD.
en, annealing temperature was increased on 100 °C, and the rest of the experimental procedure was similar to that described above.After annealing at the highest temperature, the gas atmosphere surrounding the sample was changed, and measurement procedure was repeated as described above.Oxygen partial pressure in the gas atmosphere around the sample was adjusted using a YSZ-based electrochemical oxygen pump installed in the outer regulating unit and governed by the automatic controller (Zirconia 318, Russia).ree gas atmospheres with pO 2 � 1, 0.21, and 10 −3 adjusted accordingly with gas flow rate of about 50 ml/min (to avoid oxygen partial pressure gradients along the sample) were used.

Results and Discussion
3.1.Sample Characterization.X-ray diffraction patterns of the as-prepared single phase powder samples of YCoO 3 , YBaCo 2 O 6−δ , and BaCoO 3 are given in Figures 1-3, respectively.It should be mentioned that the pattern of BaCoO 3 was interpreted as a mixture of two compounds BaCoO 3 and BaCoO 2.61 .
e space groups used for the XRD patterns indexing along with the refined cell parameters are given in Table 1 in comparison with those reported in literature.
As seen, the cell parameters found in the present study and those reported earlier are in good agreement with each other.

Oxygen Nonstoichiometry and ermodynamic Stability of YBaCo 2 O 6−δ .
Oxygen nonstoichiometry in YBaCo 2 O 6−δ as a function of T and pO 2 was measured by means of coulometric titration technique in the ranges 800 ≤ (T, °C) ≤ 1050 and −5 ≤ log(pO 2 /atm) ≤ 0, respectively.Taking into account the absolute oxygen content determined by TG/H 2 (5.016 ± 0.002 at 900 °C in air), the coulometric titration curves measured for YBaCo 2 O 6−δ at different temperatures were recalculated in pO 2 dependences of its oxygen content, 6−δ, given in Figure 4.
As an example, such dependence obtained accordingly at 900 °C is shown in Figure 5.

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Vertical segments of the titration curve (Figure 5) correspond to the sample decomposition and clearly indicate its thermodynamic stability limits with respect to reduction (at low pO 2 ) and oxidation (at high pO 2 ).e curve plot enclosed between vertical segments corresponds to the oxygen content change in YBaCo 2 O 6−δ within the thermodynamic stability region.It is worth noting that the pO 2 dependence of oxygen content measured at this temperature and shown in Figure 5 exhibits in ection when oxygen content of the double perovskite reaches the value of 5.
e same behavior was found earlier [18] for another double perovskite GdBaCo 2 O 6−δ .However, unlike GdBaCo 2 O 6−δ the double perovskite with yttrium possesses really narrow homogeneity range with respect to oxygen since oxygen content changes in the vicinity of 5 and its overall variation is less than 0.83% within the thermodynamic stability region at 900 °C.In other words, such narrow homogeneity region corresponds to a small change of the average oxidation state of cobalt; that is, even slight reduction of Co 3+ or oxidation of Co 2+ may result in YBaCo 2 O 6−δ oxide decomposition.Coulometric measurements carried out at 900 °C were stopped after reaching the highest pO 2 value, 0.72 atm, and the coulometric cell was fast cooled down to room temperature.XRD of the YBaCo 2 O 6−δ sample cooled accordingly showed the presence of the yttrium cobaltite YCoO 3 and the barium cobaltites BaCoO 3 and BaCoO 2.63 as products of YBaCo 2 O 6−δ decomposition.Taking into account possible oxygen nonstoichiometry of the complex oxides, a decomposition reaction can be written as where a value of the oxygen content, 6−δ, depends on temperature and varies from 5.012 at 800 °C up to 5.035 at 900 °C.It is worth noting that reaction (1) is completely in line with the nding of the authors [15] that the perovskites YCoO 3 and BaCoO 3 are products of YBaCo 2 O 6−δ decomposition in air (pO 2 0.21 atm) at temperatures lower than 850 °C.
In order to nd products of YBaCo 2 O 6−δ decomposition at low pO 2 stability limit, its single phase sample was annealed at 1000 °C in gas atmosphere with log(pO 2 /atm) −4 for 12 hours and then quenched to ice at −18 °C.e XRD pattern of the sample prepared accordingly showed the presence of YBaCo 4 O 7 , BaCo 1−x Y x O 3−z , and Y 2 O 3 .
It is worth noting that decomposition of YBaCo 2 O 6−δ is quite di erent from that of another double perovskite, for instance GdBaCo 2 O 6−δ , which decomposes at low pO 2 according to completely di erent reaction [28]: GdBaCo 2 O 4.85 0.5Gd 2 BaCoO 5 + 0.5BaO ( ermodynamic stability limits of YBaCo 2 O 6−δ determined accordingly are summarized in the stability diagram shown in Figure 6 as log(pO 2 ) f( 1

/T).
A nonlinear character of log(pO 2 ) f(1/T) dependence corresponding to low pO 2 stability limit of YBaCo 2 O 6−δ is related probably to a real composition of BaCo 1−x Y x O 3−c as a product of the decomposition reaction (2).Composition of both cation and anion sublattice of this compound is expected to depend signi cantly on temperature and oxygen partial pressure in ambient gas atmosphere.
Overall electrical conductivity of YBaCo 2 O 6−δ was measured as a function of pO 2 at a given temperature in the range 900 ≤ T, °C ≤ 1050 with step of 50 °C.
e overall conductivity measured as a function of pO 2 at di erent temperatures is shown in Figure 7.As seen in Figure 7, the overall conductivity rst slightly decreases with pO 2 descent in ambient atmosphere down to certain threshold value (arround pO 2 10 −2 , 10 −3 , 10 −3.5 , and 10 −4 atm at 1050, 1000, 950, and 900 °C, resp.), indicating in favor of electron holes as    Advances in Materials Science and Engineering dominant charge carriers, and then abruptly drops down on about an order of magnitude; afterwards it remains unchanged in practical term upon further pO 2 decrease.Such conductivity drop observed at low pO 2 at all temperatures studied in the present work is obviously related to the YBaCo 2 O 6−δ decomposition upon reaching the low pO 2 stability limit.
For the sake of comparison, the values of pO 2 at which the conductivity drop was observed are also given in Figure 6 depending on reciprocal temperature.As seen, the datasets on stability obtained by means of di erent techniques are in agreement with each other pretty well.
In order to con rm the high pO 2 stability limit, a single phase sample of YBaCo 2 O 6−δ was annealed at 800 °C for 10 hours in air followed by quenching to room temperature.XRD of the so-prepared sample showed the presence of YCoO 3 and BaCoO 3−z .On the other hand, annealing of equimolar mixture of YCoO 3 and BaCoO 3−z for 72 hours under the same conditions was found not to lead to the formation of the double perovskite whilst it is formed in result of this mixture annealing at 900 °C in air for the same time.ese ndings are in full coincidence with the stability diagram plotted (Figure 6) as well as with the data of the authors [15].It follows from the stability diagram shown in Figure 5 that the double perovskite YBaCo 2 O 6−δ is thermodynamically stable in air only at 850 °C and higher temperatures while it can be formed only as a metastable phase below this temperature.e last conclusion is in full agreement with the results of the paper by Zhang et al. [14] where YBaCo 2 O 6−δ was found to be stable at 850 °C in air.
e results of the thermodynamic stability investigation were further used to optimize the synthesis routine for YBaCo 2 O 6−δ as described elsewhere [29].

Conclusion
e thermodynamic stability and oxygen nonstoichiometry of the double perovskite YBaCo 2 O 6−δ was studied using coulometric titration technique.e stability diagram of YBaCo 2 O 6−δ was plotted.e found limits of its thermodynamic stability were successfully veri ed by measurements of the overall conductivity as a function of oxygen partial pressure at given temperatures.YBaCo 2 O 6−δ was shown to be thermodynamically stable in the range of pO 2 between certain threshold values at a given temperature likewise YBaCo 4 O 7±δ studied earlier [27].For instance, YBaCo 2 O 6−δ was found to be thermodynamically stable in air at 850 °C and higher temperatures, whereas its thermodynamic stability at 900 °C is limited by the range of oxygen partial pressures −3.56 ≤ log(pO 2 /atm) ≤ −0.14.Oxygen content in YBaCo 2 O 6−δ was found to decrease slightly at 900 °C from 5.035 at log(pO 2 /atm) −0.14 to 4.989 in the atmosphere with log(pO 2 /atm) −3.565.Such narrow homogeneity region corresponds to a small change of the average oxidation state of cobalt; that is, even slight reduction of Co 3+ or oxidation of Co 2+ may result in YBaCo 2 O 6−δ oxide decomposition.YBaCo 2 O 6−δ decomposes into the mixture of YCoO 3 and BaCoO 3−z at the high pO 2 stability limit, whereas YBaCo 4 O 7 , BaCo 1−x Y x O 3−c , and Y 2 O 3 were identi ed as the products of its decomposition at the low pO 2 one.Advances in Materials Science and Engineering

Figure 1 :Figure 2 :Figure 3 :
Figure 1: X-ray di raction pattern and its matching re nement plot of YCoO 3 : observed X-ray di raction intensity (points) and calculated curve (line).e bottom curve is the di erence of patterns, y obs − y cal , and the small bars indicate the angular positions of the allowed Bragg re ections.

Figure 4 :
Figure 4: Oxygen content, 6−δ, in YBaCo 2 O 6−δ versus pO 2 at di erent temperatures.Points correspond to experimental data and lines are guide to eye only.

Figure 5 :
Figure 5: Oxygen content, 6−δ, in YBaCo 2 O 6−δ versus pO 2 at 900 °C.Points correspond to experimental data and line is guide to eye only.

Table 1 :
Re ned cell parameters and space groups of the as-prepared oxide compounds.