Study and optimization of the synthesis routine of the single phase YBaCo2O6-δ double perovskite 183 D O I: 1 0. 15 82 6/ ch im te ch /2 01 7. 4. 3. 03 Sednev A. L., Tsvetkov D. S. Chimica Techno Acta. 2017. Vol. 4, No. 3. P. 183–190. ISSN 2409–5613 A.L. Sednev, D.S. Tsvetkov Institute of Natural Sciences and Mathematics, Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russia e-mail: Anton.Sednev@urfu.ru Study and optimization of the synthesis routine of the single phase YBaCo 2 O 6–δ double perovskite The chemical interaction of YCoO 3–δ and BaCoO 3-δ with formation of double perovskite was studied depending on temperature and oxygen partial pressure. The stability of YCoO 3 was shown to have а crucial influence on the kinetics and mechanism of YBaCo 2 O 6-δ formation. It was found that at 1000 °C in air, i.e. under conditions when YCoO 3 is unstable, the double perovskite YBaCo 2 O 6-δ is formed much slower compared to the pure oxygen atmosphere where YCoO 3 is stable at the same temperature. Thus controlling YCoO 3 stability was shown to be the factor of key importance for optimal preparation of the YBaCo 2 O 6-δ single phase. Keywords: YBaCo 2 O 6 synthesis, YCoO 3 instability, pO 2 acceleration, double perovskite synthe- sis, YBaCo 2 O 5 , YCoO 3 , BaCoO 3 . Received: 07.09.2017; accepted: 25.09.2017; published: 20.10.2017. © Sednev A.L., Tsvetkov D.S., 2017 Introduction Complex oxide YBaCo2O6–δ with double perovskite structure has been extensively investigated in recent years as a promising material for oxygen mem- branes [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 elec- trolytes [3]. However, YBaCo2O6–δ is un- stable in air at temperatures between 800 and 850 °C [13] and decomposes to mix- ture of the simple perovskites YCoO3–δ and BaCoO3–δ, which are more thermody- namically stable under these conditions. This significantly impedes obtaining a single phase material. Moreover, a syn- thesis routine, which could be provided the single phase YBaCo2O6–δ obtaining, has not been discussed in literature so far. The lack of the appropriate data also inhibits a commercial application of the YBaCo2O6–δ based materials. Therefore the main aim of the cur- rent work was to study a formation of the YBaCo2O6–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 YBaCo2O6–δ proceeds through the forma- tion of intermediate phases of YCoO3–δ and BaCoO3–δ like other double perovs- 184 kites LnBaCo2O6–δ [14] as well as that a synthesis routine for these intermediate phases has been already described in lite- rature [15, 16] we selected YCoO3–δ and BaCoO3–δ as starting reagents for prepara- tion of the YBaCo2O6–δ double perovskite. Powder samples of YCoO3–δ, BaCoO3–δ were synthesizes by means of glyserol- nitrate technique, using Co, Y2O3 and BaCO3 as starting materials. All the ma- terials used had a purity of 99.99 %. Me- tallic Co was obtained by reduction of Co3O4 (purity 99.99 %) in H2 atmosphere at 600 °C. Y2O3 and BaCO3 were prelimi- nary calcined at 1100  °C and 600  °C, re- spectively, in air for two hours in order to remove adsorbed H2O and CO2. Stoi- chiometric mixture of starting materials was dissolved in concentrated nitric acid. Then the required quantity of glycerol as a complexing and reducing agent was ad- ded 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 BaCoO3–δ and at 900  °C for YCoO3–δ. Phase composition of the as-prepared powder samples was confirmed by X-ray diffraction using Shi- madzu XRD-7000 diffractometer (CuKα radiation, 20≤2θ, °≤90). X-ray diffraction patterns of the as-synthesized YCoO3–δ and BaCoO3–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 diffrac- tion pattern of BaCoO3 was interpreted as a mixture of two compounds: BaCoO3 and BaCoO2.61 (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 YBaCo2O6-δ was stu- died by annealing equimolar mixture of YCoO3–δ and BaCoO3–δ for 72h (6 steps with duration of 12 h at each step) at temperatures 900 and 1000  °C in atmos- pheres with oxygen partial pressure (pO2) Fig. 2. X-ray diffraction pattern and its matching refinement plot of BaCoO3-δ: observed X-ray diffraction intensity - points and calculated curve (χ2 = 1.87) – line. The bottom curve is the difference of patterns, yobs − ycal, and the small bars indicate the angular positions of the allowed Bragg reflections for BaCoO3 (blue lines) and BaCoO2.61 (red lines) Fig. 1. X-ray diffraction pattern and its matching refinement plot of YCoO3-δ: observed X-ray diffraction intensity – points and calculated curve (χ2 = 1.62) – line. The bottom curve is the difference of patterns, yobs − ycal, and the small bars indicate the angular positions of the allowed Bragg reflections 185 0.21 and 1 atm with intermediate mixture regrinding in agate mortar. Phase compo- sition of the samples after each step of an- nealing was controlled by XRD. Results and discussion Fig. 3 shows XRD patterns of the YCoO3–δ + BaCoO3–δ equimolar mixtures annealed at 900 °C in air (pO2 = 0.21 atm) and pure oxygen (pO2 = 1 atm) for 72 h. As seen annealing neither in air nor in oxy- gen atmosphere leads to formation of the single phase YBaCo2O6–δ at least for this time of annealing. Moreover XRD pattern of the mixture annealed at 900 °С in pure oxygen atmos- phere does not show any indication of the chemical interaction between the reagents and formation of YBaCo2O6–δ double pe- rovskite 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 rela- ted to the instability of YBaCo2O6–δ oxide under oxidizing conditions at tempera- tures lower than some threshold value [11–13]. Figs. 4 and 5 show XRD patterns of the YCoO3–δ and BaCoO3–δ equimolar mix- tures annealed at 1000  °C in air (pO2 = 0.21 atm) and pure oxygen (pO2 = 1 atm) for 72 h. As seen annealing in air also did not lead to the formation of the single phase double perovskite. Y2O3, BaCoO3 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 YCoO3, which decomposes in air at T ≥ 900  °C with formation of Y2O3 and CoO [11–13, 22, 23]. Similar behavior is well-known for the perovskite-type cobaltites with small rare-earth elements [24, 25]. Therefore formation of YBaCo2O6–δ at 1000 °C in air seems to proceed according Table 1 Crystallographic parameters of synthesized cobaltites in comparison with literature data Compound Space group a*, Å b*, Å c*, Å Reference YCoO3–δ Pbnm 5.139 5.137 5.132 5.419 5.420 5.411 7.365 7.364 7.360 this work [17] [18] BaCoO3 P-6m2 5.683 5.645 5.652 5.683 5.645 5.6525 4.552 4.752 4.763 this work [19] [20] BaCoO2.63 P63/mmc 5.666 5.665 5.671 5.666 5.665 5.671 28.494 28.493 28.545 this work [16] [21] * uncertainty ±0.001 Å. Fig. 3. XRD patterns of YCoO3–δ and BaCoO3–δ equimolar mixtures after annealing in air (a) and pure oxygen (b) at 900 °C for 72 h 186 to the two-stage process. First YCoO3 de- composes into Y2O3 and CoO upon heat- ing of the equimolar mixture of YCoO3–δ and BaCoO3-δ up to 1000 °C in air YCoO3 = ½Y2O3 + CoO + ¼O2. (1) Then a mixture of Y2O3, BaCoO3 and CoO slowly reacts at 1000 °C with forma- tion of the required double perovskite ½Y2O3 + BaCoO3–δ + CoO = = YBaCo2O6–δ + ¼O2. (2) At the same time annealing the YCoO3–δ and BaCoO3–δ equimolar mix- ture at 1000 °C in oxygen for 72 h leads to formation of the single phase YBaCo2O6–δ as seen in Fig. 5 where appropriate XRD pattern is shown. This pattern was refined as a mixture of two phases having 3×2×2 and 1×2×2 superstructures. The former has tetragonal structure (s. g. P4/mmm) with cell parameters a = b = 11.596(4) Å and c = 7.509(7) Å whereas the latter has orthorhombic structure (s. g. Pmma) with cell parameters a = 3.821(4) Å, b = 7.846(2) Å, c = 7.515(8) Å in full agree- ment with available structural data [5, 7, 12, 26, 27]. Detailed step-by-step investigation of the YBaCo2O6–δ synthesis in oxygen at this temperature revealed that the result- ant mixture at each step except last one contained BaCoO3–δ, Y2O3, CoO, YCoO3–δ and the product YBaCo2O6–δ. This result can be understood, first of all, based on the analysis of the thermodynamics of re- action Eq. (1). Although for this particu- lar 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 HoCoO3 decomposi- tion starts at 1051  °C in air whereas Er- CoO3 decomposes already at 866 °C in the same atmosphere. YCoO3 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 YCoO3 may be roughly estimated by averaging corres- ponding standard enthalpies and entro- pies for Er- and Ho-containing cobaltites. The thermodynamic quantities of re- action Eq. (1) obtained in this way are also Fig. 4. XRD pattern of YCoO3–δ and BaCoO3–δ equimolar mixtures annealed in air at 1000 °C for 72 h Fig. 5. X-ray diffraction pattern and its matching refinement plot of YBaCo2O6–δ obtained by annealing at 1000 °C in oxygen for 72 h: observed X-ray diffraction intensity – points and calculated curve – line. The bottom curve is the difference of patterns, yobs − ycal, and the small bars indicate the angular positions of the allowed Bragg reflections for YBaCo2O6–δ with 3×2×2 superstructure (blue) and 1×2×2 superstructure (red) 187 shown in Table 2. They allow estimating corresponding equilibrium decomposi- tion temperatures for YCoO3 in air and oxygen. As seen in Table 2 this estima- tion gives 953  °C as the decomposition temperature of YCoO3 in air, which is in line with that reported earlier [11–13, 22, 23]. The value of decomposition tempera- ture in oxygen is around of 1060 °C. Tak- ing into account that this is only a very rough estimation one may expect the real decomposition temperature for YCoO3 in oxygen in the range of 1000–1100  °C, i. e. during annealing of the YCoO3–δ and BaCoO3–δ equimolar mixture at 1000  °C in pure oxygen atmosphere its first com- ponent is in equilibrium with oxides Y2O3 and Co O. Therefore synthesis of the YBaCo2O6–δ double perovskite under these conditions can be described by the following parallel reactions YCoO3 + BaCoO3 = YBaCo2O6 (3) ½Y2O3 + CoO + ¼O2 = YCoO3 (4) The equilibrium of reaction Eq. (4) is shifted to the right due to consumption of YCoO3 as a reagent of reaction Eq. (3). Comparison of the results of synthe- sis 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 influ- ence of high oxygen pressure. Intuitively it seems quite expected that the combi- nation (or interaction) of two ‘simple’ perovskites representing elementary ‘building’ units of the double perovskite structure is a faster process then a com- bination of barium cobaltite with two oxi- des. Significant diffusion difficulties are quite expected in the last case. However the exact reasons and detailed microsco- pic mechanism of an interaction in oxy- gen or air atmosphere should be studied in order to make meaningful conclu- sions. We only would like to emphasize once again the key role, which thermo- dynamic stability of YCoO3 plays in the optimization of synthesis routine for the YBaCo2O6–δ double perovskite. Table 2 Thermodynamics of reaction Eq. (1) for the selected cobaltites at 927 °C ΔH°, kJ·mol–1 ΔS°, J·mol–1·K–1 Tair*, °C TO2**, °C Reference HoCoO3 44.88 30.63 1051 1192 [24, 25] ErCoO3 51.34 41.3 866 970 [24, 25] YCoO3 48 36 953 1060 Estimated in this work * Equilibrium temperature for RCoO3 (R = Y, Ho, Er) decomposition in air ** Equilibrium temperature for RCoO3 (R = Y, Ho, Er) decomposition in oxygen Fig. 6. XRD patterns of YCoO3–δ and BaCoO3–δ equimolar mixture step-by-step annealed at 1000 °C in pure oxygen 188 Conclusions Synthesis of YBaCo2O6–δ from equimo- lar mixture of YCoO3 and BaCoO3–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 obtain- ing the single phase YBaCo2O6–δ. Detailed step-by-step investigation of the synthe- sis was carried out at 1000 °C in pO2 = 1 atm. 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Probing phase coexistence and stabili- zation of the spin-ordered ferrimagnetic state by calcium addition in the Y(Ba1−xCax) Co2O5.5 layered cobaltites using neutron diffraction. Phys Rev B. 2007;76(21):214417. DOI:10.1103/PhysRevB.76.214417. Cite this article as: Sednev AL, Tsvetkov DS. Study and optimization of the synthesis routine of the sin- gle phase YBaCo2O6–δ double perovskite. Chimica Techno Acta. 2017;4(3):183–90. DOI:10.15826/chimtech/2017.4.3.03.