Oxygen content and defect structure of the perovskite La0.5Ba0.5CoO3 117 D O I: 10 .1 58 26 /c hi m te ch .2 01 8. 5. 4. 04 Malyshkin D. A., Novikov A. Yu., Tsvetkov D. S. Chimica Techno Acta. 2018. Vol. 5, No. 2. P. 117–124. ISSN 2409–5613 D. A. Malyshkin, A. Yu. Novikov, D. S. Tsvetkov Institute of Natural Sciences and Mathematics, Ural Federal University, 19 Mira St., 620002, Ekaterinburg, Russian Federation e-mail: Dmitry.Malyshkin@urfu.ru Oxygen content and defect structure of the perovskite La 0.5 Ba 0.5 CoO 3–δ Perovskite-type complex oxide La 0.5 Ba 0.5 CoO 3–δ , promising cathode mate- rial for solid oxide fuel cells and precursor for synthesis of double perovskite LaBaCo 2 O 6–δ , was prepared as a single-phase material. Its oxygen content was measured by two independent techniques in the temperature range 1000– 1100 °C and at oxygen partial pressures corresponding to the stability field of cubic phase. The defect chemistry of this material was studied using the measured δ = f(pO 2 , T) dependences. The defect structure model based on the localized nature of the electronic defects was proposed and successfully verified. Keywords: oxygen nonstoichiometry, perovskite, defect structure, coulometric titration. Received: 28.06.2018. Accepted: 25.07.2018. Published: 30.07.2018. © Malyshkin D. A., Novikov A. Yu., Tsvetkov D. S., 2018 Introduction The perovskite-type oxide La0.5Ba0.5CoO3–δ was first prepared in 1979 [1, 2]. By a number of techniques it was shown to have cubic crystal structure with Pm3m space group (S. G.), at least when synthesized in air atmosphere with oxygen partial pressure, pO2, ≥ 0.21 atm [3–17]. Depending on the synthesis method, the final annealing temperature, T, repor- ted in the literature, varies within the fairly wide limits – from 750 [18] to 1300 °C [2]. However, the samples of La0.5Ba0.5CoO3–δ obtained at  T ≤ 850  °C are usually found to contain some impurities [8, 18, 19]. Hence, regardless of  the synthesis method, a  single-phase cubic perovskite La0.5Ba0.5CoO3–δ can be prepared only at temperatures above 850 °C in air [16]. According to the results of in situ high- temperature X-ray diffraction studies [14], the crystal structure of La0.5Ba0.5CoO3–δ re- mains cubic (S. G. Pm3m) in the tempera- ture range 300–800 °C and under different gas atmospheres (O2, air and N2). On the other hand, annealing of La0.5Ba0.5CoO3–δ at  900–1200  °C under reducing conditions led to the formation of the double perovskite LaBaCo2O6–δ[3, 5, 10–15, 20–23]. Thus, according to the results of  studying the ordering-disor- dering phenomenon in  LaBaCo2O6–δ  – La0.5Ba0.5CoO3–δ system [23], the stability field of cubic perovskite at 1000–1100 °C corresponds to the pO2 range –1.3 ≤ log (pO2 / atm) ≤ –0.68. 118 T h e o x y g e n c o n t e n t i n   t h e La0.5Ba0.5CoO3–δ was measured as a func- tion of  pO2 and temperature using cou- lometric titration [24] and thermogravi- metric technique [6, 7, 11, 12, 14, 17]. However, the results obtained are discre- pant, as  evidenced by  the comparison in Table 1 and in Fig. 1. Moreover, some- times even the data reported by the same scientific group [6, 7] turn out to be incon- sistent with each other, as seen in Fig. 1. Unfortunately, in most cases it is difficult to understand the origin of these discre- pancies because of  the incomplete and often only superficial description of  the specific experimental conditions (heat- ing / cooling rate, equilibration time etc.) used by different authors. The only available attempt to describe the defect structure of La0.5Ba0.5CoO3–δ re- ported in [17] should also be considered as unsuccessful due to obvious errors in the charge neutrality and mass balance condi- tions given in [17]. Table 1 Comparison of the available literature data on the absolute oxygen content in La0.5Ba0.5CoO3–δ at room temperature The oxygen content in sample at room temperature, 3–δ Cooling rate, °C / h 60 300 unknown pO2, atm 0.21 1 3.00 [7] 2 [6] 2 2.98 [14] 1, [17] 2 2.96 [11] 2 2.94 [17] 2 2.91 [12] 1 method for determining the absolute value of δ: 1 – reduction by hydrogen flux 2 – iodometric titrations Fig. 1. a) The oxygen content in La0.5Ba0.5CoO3–δ as a function of temperature in air; b) The oxygen content in La0.5Ba0.5CoO3–δ vs. pO2 at different temperatures. The curves are shown as guides to the eye only 119 Therefore, the priority purposes of this work were (i) to  prepare single-phase La0.5Ba0.5CoO3–δ oxide, (ii) to get reliable da- ta on its oxygen content as a function of T and pO2 and (iii) to carry out the analysis of the defect structure of La0.5Ba0.5CoO3–δ. Experimental Powder sample of the La0.5Ba0.5CoO3–δ was prepared by means of the glycerol-ni- trate method using La2O3, BaCO3 and Co as starting materials. All of the materials used had a purity of 99.99 %. A stoichio- metric mixture of  the starting materials was dissolved in concentrated nitric acid (99.99 % purity) and the required vol- ume of glycerol (99 % purity) was added as  a  complexing agent and a  fuel. The glycerol quantity was calculated accor- ding to  the amount required for  the full reduction of  the corresponding nitrates to molecular nitrogen N2. The as-prepared solution was heated continuously at 100 °C until complete water evaporation and py- rolysis of the dried precursor had occurred. The resulting ash was subjected to  step- wise calcination at temperatures between 900 and 1100 °C in air with intermediate regrindings. Annealing at the last stage was carried out for 30 hours. The sample was then cooled to room temperature at a rate of 100 °C / h. The phase composition of the powder samples prepared accordingly was studied at  room temperature by  means of  XRD with a Shimadzu XRD 7000S diffractome- ter (Shimadzu, Japan) using Cu Kα radia- tion. The XRD spectra showed no indi- cation for the presence of a second phase in the as-prepared samples. T h e ox y ge n n ons toi ch i om e t r y of La0.5Ba0.5CoO3–δ was measured as a func- tion of  pO2 and temperature by  means of thermogravimetric (TG) and coulomet- ric titration techniques. The absolute value of  δ in  La0.5Ba0.5CoO3–δ was determined by direct reduction of the oxide samples by hydrogen flux in the TG setup. The de- tails can be found elsewhere [25]. Results and discussion The absolute oxygen content in  the sample of La0.5Ba0.5CoO3–δ slowly (100 °C / h) cooled to  room temperature was found to be 2.992±0.005, in agreement with avail- able literature data [6, 7]. The XRD pattern of the as-prepared single-phase perovskite La0.5Ba0.5CoO2.992 is shown in Fig. 2. It was indexed using the cubic Pm3m space group. The cell parameter a = 3.888 (6) Å deter- mined as a result of the Rietveld refinement of the XRD profile (also shown in Fig. 2) is in good agreement with those reported for La0.5Ba0.5CoO3–δ previously [3–5, 9, 26]. The oxygen content in La0.5Ba0.5CoO3–δ measured as a function of pO2 and tem- perature is given in Fig. 3. As seen, the data obtained by coulometric titration and TG are in good agreement with each other. It is worth noting that the presented range of  pO2 corresponds to  the stability field of the cubic phase as determined in [23]. For the sake of comparison, Fig. 3 also shows the results reported in Ref. [17]. It is noteworthy that the values of the oxy- gen content measured in the present study exceed those found in Ref. [17] on about 0.052 under the same conditions, whereas the slope of the pO2 dependences of 3–δ is practically the same in the both data sets. In order to analyze the defect structure of La0.5Ba0.5CoO3–δ using the quasichemi- cal approach, LaCoO3 with fully occupied 120 oxygen sublattice was chosen as  a  refer- ence crystal in the same way as for related compounds [27]. In this case, the regu- lar constituents are La CoLa Co × ×, and OO × , whereas the defect species can be defined as Ba V CoLa O Co� � ��, , and CoCo • , where the last two point defects correspond to cobalt in the oxidation state +2 (electron local- ized on cobalt site) and +4 (hole localized on cobalt site), respectively. The following quasichemical reactions were taken into account: oxygen exchange with ambient gas atmosphere with simul- taneous reduction / oxidation of cobalt (1) O Co O V Co V Co O O Co O Co O O Co O � � � � � � � � �� ���� �� 2 212 2 1 0 5 2 2 i ii ii � , . K P ��� ���� ��CoCo i 2 (1) and the charge disproportionation involv- ing the transfer of an electron between ad- jacent sites (2) 2 2 2 Co Co Co Co Co Co Co Co Co Co Co Co � � � � � �� �� ��� �� �� �� � i i , .K (2) Equilibrium constants of the proposed quasichemical reactions along with condi- tions of mass balance and electroneutrality form the following set of nonlinear equa- tions: K P H RT S 1 0 5 2 2 1 0 2� �� ���� �� �� ���� �� � � � � � � � O O Co O Co V Co O Co exp .   11 0 2 2 2 0 2 0 R K H RT S R � � � � � � � ��� ���� �� �� �� � � � � � � Co Co Co exp Co Co Co  �� � � � � � ��� �� � ��� �� � �� �� � �� �� �� �� � � � Co Ba Co V Co C Co La Co O Co  2 oo Co O V Co Co O O �� �� � �� �� � �� �� � � �� �� � � � � �   1 3 � � (3) The analytical solution of  the set (3) yields the model function: P K K K A A O2 0 25 1 2 2 3 12 4 16 1 2 2 . , � � � � � � � � �� � � � � � (4) where Fig. 2. Rietveld refined XRD pattern of the La0.5Ba0.5CoO2.992 sample slowly (100 °C / h) cooled from 1100 °C to room temperature in air: 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 Fig. 3. Oxygen content in La0.5Ba0.5CoO3–δ vs. pO2 at different temperatures. Note that the data of Stingaciu [17] were shifted upward by 0.052 121 A K K K � � � � � �16 32 12 64 82 2 2 2 2� � � �. (5) Note that defect formation enthalpies were treated as constants, since the oxygen nonstoichiometry of La0.5Ba0.5CoO3–δ was measured in  the relatively narrow tem- perature range. This assumption enabled the substitution of  the equilibrium con- stants in the Eq. 4 by their thermodynamic temperature dependences (see Eq. 3) and, as a consequence, simultaneous treatment of the data on oxygen nonstoichiometry obtained at  different temperatures ac- cording to the proposed defect structure model. The result of  the least square fit of the model Eq. 4 to the experimental data on the oxygen content in La0.5Ba0.5CoO3–δ is shown in Fig. 4. As seen, there is a good agreement between the fitted surface and the measured values. The fitted parameters of the model are summarized in Table 2. It is worth noting that the standard en- tropy of charge disproportionation ∆S2 0 , obtained as a result of the least square fit, was close to zero with relatively large error margin. Within this margin fit results were found to  be relatively insensitive to  the particular value of  ∆S2 0 . Furthermore, it is known that the value of  ∆Si 0 corre- sponds to the vibrational entropy change [28] which should be very small for reac- tion (2) since it does not involve the gase- ous species. In addition, the average point defects” coordination environment also does not change significantly in the course of the reaction (2). As a result, the magni- tude of  ∆S2 0 can be expected to be rather low. Therefore, during the final fitting pro- cedure the standard entropy of charge dis- proportionation, ∆S2 0 , was assumed to be zero. Similar reasoning was also presented in Ref. [29]. Conclusions Oxygen nonstoichiometry of the per- ovskite oxide La0.5Ba0.5CoO3–δ was mea- sured as a function of pO2 and tempera- ture in the range 1000–1100 °C by means of  thermogravimetric and coulometric titration techniques. The defect structure Fig. 4. The result of the least square fit of the defect structure model for La0.5Ba0.5CoO3–δ. 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