Study and optimization of the synthesis routine of the single phase YBaCo 2 O 6 – δ double perovskite

Study and optimization of the synthesis routine of the single phase YBaCo2O6–δ double perovskite The chemical interaction of YCoO3–δ and BaCoO3-δ with formation of double perovskite was studied depending on temperature and oxygen partial pressure. The stability of YCoO3 was shown to have а crucial influence on the kinetics and mechanism of YBaCo2O6-δ formation. It was found that at 1000 °C in air, i.e. under conditions when YCoO3 is unstable, the double perovskite YBaCo2O6-δ is formed much slower compared to the pure oxygen atmosphere where YCoO3 is stable at the same temperature. Thus controlling YCoO3 stability was shown to be the factor of key importance for optimal preparation of the YBaCo2O6-δ single phase.


Introduction
Complex oxide YBaCo 2 O 6-δ with double perovskite structure has been extensively investigated in recent years as a promising material for oxygen membranes [1,2] and solid oxide fuel cells (SOFCs) [3,6,7,9,12] due to high mixed ionic-electronic conductivity [12] and moderate thermal expansion comparable to that of the state-of-the art SOFC electrolytes [3].However, YBaCo 2 O 6-δ is unstable in air at temperatures between 800 and 850 °C [13] and decomposes to mixture of the simple perovskites YCoO 3-δ and BaCoO 3-δ , which are more thermody-namically stable under these conditions.This significantly impedes obtaining a single phase material.Moreover, a synthesis routine, which could be provided the single phase YBaCo 2 O 6-δ obtaining, has not been discussed in literature so far.The lack of the appropriate data also inhibits a commercial application of the YBaCo 2 O 6-δ based materials.
Therefore the main aim of the current work was to study a formation of the YBaCo 2 O 6-δ double perovskite at 900 and 1000 °C in different gas atmospheres in order to optimize its synthesis routine.

Experimental
Taking into account that synthesis of YBaCo 2 O 6-δ proceeds through the forma-tion of intermediate phases of YCoO 3-δ and BaCoO 3-δ like other double perovs-kites LnBaCo 2 O 6-δ [14] as well as that a synthesis routine for these intermediate phases has been already described in literature [15,16] we selected YCoO 3-δ and BaCoO 3-δ as starting reagents for preparation of the YBaCo 2 O 6-δ double perovskite.
Powder samples of YCoO 3-δ , BaCoO 3-δ were synthesizes by means of glyserolnitrate technique, using Co, Y 2 O 3 and BaCO 3 as starting materials.All the materials used had a purity of 99.99 %.Metallic Co was obtained by reduction of Co 3 O 4 (purity 99.99 %) in H 2 atmosphere at 600 °C.Y 2 O 3 and BaCO 3 were preliminary calcined at 1100 °C and 600 °C, respectively, in air for two hours in order to remove adsorbed H 2 O and CO 2 .Stoichiometric mixture of starting materials was dissolved in concentrated nitric acid.Then the required quantity of glycerol as a complexing and reducing agent was added to the obtained solution.Afterwards the solution was evaporated to dryness, and resulted dry powder was pyrolyzed.The product of pyrolysis was put in a cru-cible and calcined in a furnace.The final calcination was carried out at 1100 °C in air for two hours С for BaCoO 3-δ and at 900 °C for YCoO 3-δ .Phase composition of the as-prepared powder samples was confirmed by X-ray diffraction using Shimadzu XRD-7000 diffractometer (CuKα radiation, 20≤2θ, °≤90).X-ray diffraction patterns of the as-synthesized YCoO 3-δ and BaCoO 3-δ are shown in Figs. 1 and 2.
The results of the structureless Le Bail fitting are also shown in Fig. 1 and 2. It should be noted that the X-ray diffraction pattern of BaCoO 3 was interpreted as a mixture of two compounds: BaCoO 3 and BaCoO 2.61 (see Fig. 2).The refined cell parameters of the prepared compounds given in Table 1 are in a good agreement with those reported in literature.
Synthesis of YBaCo 2 O 6-δ was studied by annealing equimolar mixture of YCoO 3-δ and BaCoO 3-δ for 72h (6 steps with duration of 12 h at each step) at temperatures 900 and 1000 °C in atmospheres with oxygen partial pressure (pO 2 )

Results and discussion
Fig. 3 shows XRD patterns of the YCoO 3-δ + BaCoO 3-δ equimolar mixtures annealed at 900 °C in air (pO 2 = 0.21 atm) and pure oxygen (pO 2 = 1 atm) for 72 h.As seen annealing neither in air nor in oxygen atmosphere leads to formation of the single phase YBaCo 2 O 6-δ at least for this time of annealing.
Moreover XRD pattern of the mixture annealed at 900 °С in pure oxygen atmosphere does not show any indication of the chemical interaction between the reagents and formation of YBaCo 2 O 6-δ double perovskite whereas annealing in air leads to formation of significant amount of this double perovskite (see Fig. 3).Possible reason of this difference seems to be related to the instability of YBaCo 2 O 6-δ oxide under oxidizing conditions at temperatures lower than some threshold value [11][12][13].
Figs. 4 and 5 show XRD patterns of the YCoO 3-δ and BaCoO 3-δ equimolar mixtures annealed at 1000 °C in air (pO 2 = 0.21 atm) and pure oxygen (pO 2 = 1 atm) for 72 h.As seen annealing in air also did not lead to the formation of the single phase double perovskite.Y 2 O 3 , BaCoO 3 and CoO can be identified as impurities in the X-ray diffraction pattern shown in Fig. 4. The presence of these impurities is a consequence of instability of the YCoO 3 , which decomposes in air at T ≥ 900 °C with formation of Y 2 O 3 and CoO [11-13, 22, 23].Similar behavior is well-known for the perovskite-type cobaltites with small rare-earth elements [24,25].
Detailed step-by-step investigation of the YBaCo 2 O 6-δ synthesis in oxygen at this temperature revealed that the result-ant mixture at each step except last one contained BaCoO 3-δ , Y 2 O 3 , CoO, YCoO 3-δ and the product YBaCo 2 O 6-δ .This result can be understood, first of all, based on the analysis of the thermodynamics of reaction Eq. ( 1).Although for this particular reaction thermodynamic functions are unknown similar reactions for Ho-and Er-contained cobaltites have already been studied in this respect [24,25].Required thermodynamic data for them are given in Table 2.As seen HoCoO 3 decomposition starts at 1051 °C in air whereas Er-CoO 3 decomposes already at 866 °C in the same atmosphere.YCoO 3 as mentioned above is somewhere between these two compounds since its decomposition in air starts at 900-950 °C [11-13, 22, 23].Therefore standard enthalpy and entropy of reaction Eq. ( 1) for YCoO 3 may be roughly estimated by averaging corresponding standard enthalpies and entropies for Er-and Ho-containing cobaltites.
The thermodynamic quantities of reaction Eq. ( 1) obtained in this way are also shown in Table 2.They allow estimating corresponding equilibrium decomposition temperatures for YCoO 3 in air and oxygen.As seen in Table 2 this estimation gives 953 °C as the decomposition temperature of YCoO 3 in air, which is in line with that reported earlier [11-13, 22, 23].(4) The equilibrium of reaction Eq. ( 4) is shifted to the right due to consumption of YCoO 3 as a reagent of reaction Eq. ( 3).
Comparison of the results of synthesis at 1000 °C in two atmospheres, i. e. air and oxygen, shows that in the second case formation of the double perovskite occurs apparently faster.One may speculate on the reasons of the observed positive influence of high oxygen pressure.Intuitively it seems quite expected that the combination (or interaction) of two 'simple' perovskites representing elementary 'building' units of the double perovskite structure is a faster process then a combination of barium cobaltite with two oxides.Significant diffusion difficulties are quite expected in the last case.However the exact reasons and detailed microscopic mechanism of an interaction in oxygen or air atmosphere should be studied in order to make meaningful conclusions.We only would like to emphasize once again the key role, which thermodynamic stability of YCoO 3 plays in the optimization of synthesis routine for the YBaCo 2 O 6-δ double perovskite.

Conclusions
Synthesis of YBaCo 2 O 6-δ from equimolar mixture of YCoO 3 and BaCoO 3-δ was studied at 900 °C and 1000 °C in air and pure oxygen atmosphere.It was shown that synthesis at 1000 °C in pure oxygen atmosphere is an optimal way of obtaining the single phase YBaCo 2 O 6-δ .Detailed step-by-step investigation of the synthe-sis was carried out at 1000 °C in pO 2 = 1 atm.The mechanism of YBaCo 2 O 6-δ synthesis in different gas atmospheres was proposed based on thermodynamics of YCoO 3 and crucial role of this oxide stability in governing of the synthesis process was revealed.

Fig. 2 .
Fig. 2. X-ray diffraction pattern and its matching refinement plot of BaCoO 3-δ : observed X-ray diffraction intensity -points and calculated curve (χ 2 = 1.87) -line.The bottom curve is the difference of patterns, y obs − y cal , and the small bars indicate the angular positions of the allowed Bragg reflections for BaCoO 3 (blue lines) and BaCoO 2.61 (red lines)

Table 1
Crystallographic parameters of synthesized cobaltites in comparison with literature data