Structure, Stability, and Thermomechanical Properties ... - Springer Link

ISSN 1063-7834, Physics of the Solid State, 2017, Vol. 59, No. 4, pp. 694–702. © Pleiades Publishing, Ltd., 2017. Original Russian Text © E.Yu. Pikalova, D.A. Medvedev, A.F. Khasanov, 2017, published in Fizika Tverdogo Tela, 2017, Vol. 59, No. 4, pp. 679–687.

SEMICONDUCTORS

Structure, Stability, and Thermomechanical Properties of Ca-Substituted Pr2NiO4 + δ E. Yu. Pikalovaa, b, D. A. Medvedeva, b, *, and A. F. Khasanova, b a

Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, ul. Akademicheskaya 20, Yekaterinburg, 620137 Russia b Ural Federal University, ul. Mira 19, Yekaterinburg, 620002 Russia *e-mail: [email protected] Received May 30, 2016; in final form, July 30, 2016

Abstract—Ca-substituted layered nickelates with a general Pr2 – xCaxNiO4 + δ composition (x = 0–0.7, Δx = 0.1) were prepared in the present work and their structural and physic-chemical properties were investigated in order to select the most optimal materials, which can be used as cathodes for solid oxide fuel cells. With an increase in Ca content in Pr2 – xCaxNiO4 + δ the following tendencies were observed: (i) a decrease in the concentration of nonstoichiometric oxygen (δ), (ii) a decrease in the unit cell parameters and volume, (iii) stabilization of the tetragonal structure, (iv) a decrease of the thermal expansion coefficients, and (v) enchancement of thermodynamic stability and compatibility with selected oxygen- and proton-conducting electrolytes. The Pr1.9Ca0.1NiO4 + δ material, having highest δ value, departs from the general “properties– composition” dependences ascertained. This indicates that oxygen non-stoichiometry has determining influence on the functional properties of layered nickelates. DOI: 10.1134/S1063783417040187

1. INTRODUCTION In recent years, layered nickelites of the Ruddlesden–Popper homological series Ln2NiO4 + δ (Ln = La, Pr, Nd) have been objects of intense attention due to the potential possibility of their application as an oxygen electrode in intermediate-temperature solidoxide fuel cells (SOFCs) and solid oxide electrolysis cell [1]. When operating temperatures decrease, the activation polarization of an oxygen electrode makes the main contribution to total polarization losses during operation of SOFCs. In the case of the electrode with a mixed oxygen-ionic and electronic conductivity, the polarization is related to the following stages [2, 3]: the adsorption of oxygen from the gaseous phase and the diffusion of atomic oxygen at the electrode surface, reduction of oxygen atoms with formation of O2– ions, the diffusion of oxygen ions through the particles of the electrode with mixed conductivity, and the transport of the ions through the electrolyte/mixed conductor interface. Thus, it is preferable to use oxygen electrodes made of materials with high mixed (oxygen-ionic and electronic) conductivity, a high diffusion coefficient, and a high surface exchange constant. Another important factor that determines the longterm stability is also the consistency of the thermal expansion coefficients of the electrode and electrolyte

materials, along with the absence of chemical interaction between them. At room temperature, the Ln2NiO4 + δ compounds crystallize in the K2NiF4 structural type with a space group Fmmm or Bmab [1, 2]. The orthorhombic distortions of the unit cell increase as the rare-earth element radius decreases; in this case, the content of the superstoichimetric oxygen (δ) increases, compensating stresses in the structure [4]. The superstoichiometric oxygen ions in these compounds possess high mobility and, as theoretical calculations show, the migration of these ions determines the oxygen transport in the nickelate layered structures [5]. The maximum oxygen diffusion coefficients are attained in Pr2NiO4 + δ (for example, 8 × 10–8 cm2 s–1 at 600°C [6]). According to the available data, the cathodes based on Pr2NiO4 + δ exhibit the lowest polarization resistances among layered nickelates [7–9]. In addition, there is indirect evidence of the existence mixed H+/O2–/e– transport in Pr2NiO4 + δ [10], which opens the prospect of using it in SOFCs based not only on oxygen-ionic [11], but also proton-conducting electrolytes [12]. However, the problem of the stability of electrodes based on layered praseodymium nickelate remains unsolved. This is known that Pr2NiO4 + δ decomposes at temperatures higher than 850°C with the formation of Pr-containing impurities [13]. It

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leads to fast aging of Pr2NiO4 + δ-based electrodes, particularly, in the fuel cell regime [14]. It is shown in some works that a partial substitution of alkali-earth elements for lanthanides in Ln2NiO4 + δ increases the conductivity of the materials, their structural stability, and, in some cases, improves the polarization characteristics of the electrodes based on these materials [15–19]. However, there is a number of studies devoted to the structural features, magnetic, and electrical properties of Ca-substituted Ln2NiO4 + δ [20–23], but there are no available data on the properties of Ca-substituted Pr2NiO4 + δ. Due to this fact, the aim of this work is to study the features of the structural and thermomechanical properties of Pr2 – xCaxNiO4 + δ (0 ≤ x ≤ 0.7, Δx = 0.1). We also studied their thermal stability, which was important from the standpoint of perspectives of applying these materials as oxygen electrodes in intermediate-temperature electrochemical cells. 2. EXPERIMENTAL The synthesis of Pr2 – xCaxNiO4 + δ (PCNO) was carried out by a ceramic technology. The initial Pr6O11, Pr2(CO3)3, CaCO3, and NiO powders with purity not lower than 99.5% were mixed in required proportion in a SAND planetary mill in isopropyl alcohol, and the preliminary synthesis was performed at a temperature of 1150°C for 2 h; then the product prepared was subjected to milling in the mill to activate it. The final synthesis was performed for two stages at temperatures 1250–1270°C for 5 h with subsequent activation of the powder. The phase composition of the materials synthesized was studied by X-ray diffraction (XRD) analysis using a D/MAX-2200 RIGAKU diffractometer in the angular range 20° ≤ 2θ ≤ 90° in the CuKα radiation at room temperature. The phases were identified using a JCPDS card-index and a PeakFindv1.03 program packet. The oxygen nonstoichiometry was determined for the compositions with x = 0, 0.1, 0.3, 0.5, and 0.7 by long-term high-temperature treatment of these samples in the Ar + 10%H2 atmosphere at a temperature of 800°C up to complete reduction. The absolute oxygen content (4 + δ) in the samples was calculated based on the reaction that schematically describes the sample reduction: [H] Pr2 − x Ca x NiO 4 −0.5 x +δ ⎯⎯⎯ →“Pr2 − x Ca x NiO3−0.5 x ” ≡ (1 − 0.5x)Pr2 O 3 + x CaO + Ni.

The thermomechanical properties of the materials were studied on the compacted samples prepared by uniaxial pressing with subsequent sintering at a temperature of 1450°C for 5 h using a NETZSCH DIL 402C high-temperature dilatometer in the temperature range 50–1000°C under heating and cooling PHYSICS OF THE SOLID STATE

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modes (heating rate and cooling rate was 3°C/min). The dilatometric data were used to determine the average thermal expansion coefficient (TEC) and the temperature dependences of TEC. The structure of Pr2 – xCaxNiO4 + δ was studied using the high-temperature XRD method (the Collective Use Center Ural-M at the Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, DRON-2.0) in the range from room temperature to 800°C. At each temperature, the measurements were performed in the angular range 24° ≤ 2θ ≤ 85° in the CuKα radiation with a scan rate of 1 min–1 and a scan step of 0.02°. To study the thermal stability of Pr2 – xCaxNiO4 + δ (x = 0, 0.1, 0.3, 0.5, and 0.7), the materials were calcined at 850°C for 250 h and then XRD analysis was performed to analyze their phase compositions. To study the chemical compatability of the nickelates with the electrolytes, the single-phase powders of Pr2 ‒ xCaxNiO4 + δ (x = 0 and 0.5) and (ZrO2)0.92(Y2O3)0.08O2 – δ, Ce0.8Sm0.2O2 – δ, La0.85Sr15Ga0.85Mg0.15O3 – δ or BaCe0.5Zr0.3Y0.2O3 – δ were mixed in a weight proportion of 1 : 1; the mixtures were calcined at 1200°C for 2 h and then were studied using the XRD method.

3. RESULTS AND DISCUSSION 3.1. Structural Features of Pr2 – xCaxNiO4 + δ at Room Temperature Table 1 presents the XRD data for the Pr2 ‒ xCaxNiO4 + δ powders synthesized. Pr2NiO4 (PNO) has the orthorhombic structure (space group Fmmm) with the following lattice parameters: a = 5.4681 ± 0.0008 Å, b = 5.3955 ± 0.0008 Å, c = 12.4430 ± 0.0012 Å, and V = 367.1 Å3. La2NiO4 (LNO) synthesized in air also has structure close to the orthorhombic one (space group Fmmm) but with a larger unit cell volume [24]. The decrease in the unit cell volume in the case praseodymium is due to the size factor, since the Pr3+ and La3+ ionic radii in 9-coordinated state are 1.179 and 1.216 Å, respectively. The sizes are given according to Shannon system [25]. The lower cation size leads to an increase in stresses in the structure that is due to different Ln–O and Ni–O bond lengths. It is known that a partial removal of the stresses is possible due to the rotation of NiO6 octahedra, which leads to structural transitions [1]. On the other hand, the stress compensation in the lattice can occur due to the superstoichiometric interstitial oxygen filling the tetrahedral voids. The interstitial oxygen content δ for PNO was found to be equal to 0.255, which is considerably higher than that in LNO (0.150) [24]. However, the obtained values agree well with the available data [15]. As the calcium content increases, the tendency of the decrease in the unit cell parameters and volume up to x = 0.7 was observed. This behavior is in agreement

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Table 1. Structural parameters of Pr2 – xCaxNiO4 + δ materials x 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Denotation PNO PCNO1 PCNO2 PCNO3 PCNO4 PCNO5 PCNO6 PCNO7

Crystal lattice parameters

Symmetry, space group

a, Å

b, Å

c, Å

V, Å3

O, Fmmm O, Fmmm O, Fmmm O, Fmmm T, P42/ncm T, P42/ncm T, P42/ncm T, P42/ncm

5.4681 5.4401 5.4453 5.3902 5.3745 5.3665 5.3554 5.3601

5.3955 5.3965 5.4015 5.3912 – – – –

12.443 12.435 12.398 12.381 12.363 12.349 12.308 12.317

367.1 365.1 364.7 359.8 357.1 355.7 353.0 353.9

ρth, g cm–3

δ

7.32 7.17 6.99 6.91 6.77 6.61 6.47 6.27

0.255 0.372 – 0.091 – 0.013 – 0.091

O is the orthorhombic symmetry, T is the tetragonal symmetry, and ρth is the theoretical density.

with the Vegard law and indicates the formation of a wide series of solid solutions. Such wide range of the existence of Pr2 – xCaxNiO4 + δ solid solution as compared to that of La2 – xCaxNiO4 + δ (the calcium solubility is limited to x = 0.4 [21]) can be related to the favorable ratio of ionic radii of the basic (rPrIX3 + = 1.179 Å) IX and impurity ( rCa 2 + = 1.180 Å) cations. Since the 3+ → Ca2+ substitution does not cause distortions in Pr the A sublattice of oxide A2BO4, the observed decrease in the unit cell parameters and volume can be attributed to the fact that the increase in x in Pr2 ‒ xCaxNiO4 + δ increases the concentration of Ni3+ cations with smaller ionic radius as compared to that VI VI of Ni2+ ( rNi = 0.560 Å). The 2 + = 0.690 Å and r Ni 3 + Pr3+ → Pr4+ transition is improbable, since the ninecoordination state is noncharacteristic for Pr4+. In the range of x = 0–0.5, the interstitial oxygen content δ decreases from 0.255 at x = 0 (PNO) to 0.013 at x = 0.5 (PCNO5), and δ = 0.091 at x = 0.7 (PSNO7). However, in a series with calcium, the sample with x = 0.1 (PCNO1) deviates from the general tendency (δ = 0.372).

3.2. Structural Features of Pr2 – xCaxNiO4 + δ Depending on Temperature The crystal structure of Pr2 – xCaxNiO4 + δ as a function of temperature was studied using the high-temperature XRD method. Figure 1 shows some selected data for the PNO and PCNO5. Analyzing the X-ray diffraction data for Pr2NiO4 + δ (Figs. 1a and 1b), the existence of the phase transition from the low-symmetric orthorhombic structure (space group Fmmm) to the tetragonal one (space group P42/ncm) in the temperature range 400–500°C can be found. The transition from the orthorhombic to the tetragonal structure at a temperature of 450°C was also observed by Sadykov et al. [26]. The authors

observed additional possible phase transition related to the reduction of the structure symmetry to the orthorhombic (space group Bmab) with further increasing temperature (to ~600°C). In this work, we did not observe such a transition, which can be explained by fairly high residual content of the interstitial oxygen in our samples at the chosen synthesis and measurement conditions. As mentioned above, the structural transitions in the layered nickelates correlated to the interstitial oxygen content. So, it was shown in [27] that symmetry Fmmm(2) was inherent in the Pr2NiO4 + δ with a high degree of nonstoichiometry (δ = 0.21– 0.22). In the range to δ = 0.16, two isostructural phases coexisted: Fmmm(1) with lattice parameters a = 5.4830 ± 0.0005 Å, b = 5.3891 ± 0.0004 Å, and c = 12.4202 ± 0.0007 Å and Fmmm(2) with lattice parameters a = 5.4587 ± 0.0005 Å, b = 5.4008 ± 0.0004 Å, and c = 12.4387 ± 0.0007 Å. The following region with the coexistence of phases Fmmm(1) and P42/ncm extended to δ = 0.06, and the third region of the existence of tetragonal P42/ncm and orthorhombic Bmab (or Pccn) extended to δ = 0.02; in this case, the tetragonal phase content prevailed at higher δ. Phase Fmmm(2) was the most nonstoichiometric, and it existed at the boundary between the oxide region with Pr4Ni3O10 and PrO1.83. With decreasing the calcium content, the transition temperature from the orthorhombic (Fmmm) to the tetragonal (P42/ncm) structure decreases, which is likely related to the decrease in the interstitial oxygen content in the doped compositions. However, the clear behavior depending on the calcium content was not observed. For example, the transition for PCNO1 was observed at 200°C, while that for PCNO3 took place at 320°C. However, in this case, as the calcium content in Pr2 ‒ xCaxNiO4 + δ increased, the X-ray diffraction patterns demonstrated the clear transition of doublets to singlets, which corresponded to the stabilization of tetragonal P42/ncm at room temperature (Figs. 1c and 1d).

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(c)

(d)

Relative intensity

Relative intensity

(a)

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Fig. 1. (a, c) High-temperature X-ray diffraction patterns and (b, d) temperature dependences of the unit cell parameters for the (a, b) Pr2NiO4 + δ and (c, d) Pr1.5Ca0.5NiO4 + δ compositions.

The temperature dependences of the relative change in the unit cell volume were used to calculate the equilibrium average TECs values of the materials; they were 14.0 × 10–6, 15.0 × 10–6, 13.8 × 10–6, and 13.4 × 10–6 K–1 for PNO, PCNO1, PCNO3, and PCNO5, respectively. 3.3. Termomechanical Properties The study of the thermal properties of Pr2 ‒ xCaxNiO4 + δ in dynamic regimes is important no less than that in equilibrium or nearly equilibrium conditions. For example, the development and testing of SOFCs were accompanied with application of some thermal cycles (co-sintering of functional materials, start, the change in operating temperatures, and shutdown) that were carried out at certain heating and cooling rates. It is likely that, in this case, the properties of the systems will differ from those obtained under conditions of isothermal treatment. To find the specific features of the thermal properties of the ceramic materials Pr2 – xCaxNiO4 + δ, the dilatometric measurements (Fig. 2) at heating/cooling rates of 3°C/min we performed. PHYSICS OF THE SOLID STATE

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It was found that the temperature dependences of the relative change of the linear sizes were nearly linear for most samples in the range 50–1000°C (Figs. 2a– 2c), except for the samples with x = 0 and 0.1, the dependences for which had clear breaks at intermediate temperatures (Figs. 2c and 2d). Such behavior can be related, as was mentioned previously, to the firstorder phase transition, which was accompanied by significant change in the oxide structures and, correspondingly, the size characteristics (unit cell parameters and volume and linear sizes of the ceramics). The existence of clear hysteresis between the curves measured on heating and cooling also confirmed the existence of the first-order phase transition in the materials with low calcium content. Using the data, the average values TECs of the materials were calculated in the linear curve pieces over entire temperature range (Fig. 3, Table 2). The temperature boundary separating the dependence into linear pieces was found by the difference method [31]. For example, the dilatometric data for basic oxide Pr2NiO4 + δ (x = 0) were linear in the ranges of 50–750°C and 750–1000°C during heating and 50– 600°C and 600–1000°C during cooling. The range

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(a)

(b)

(c)

(d)

(e)

Fig. 2. Dilatometric data for Pr2 – xCaxNiO4 + δ ceramic materials measured under heating (a) and cooling (b) modes in air. The dependences are given for compositions with x = (c) 0, (d) 0.1, and (e) 0.7.

boundaries shifted and then disappeared as the calcium concentration in the nickelate increased, as it is seen from the temperature dependences of TEC (Fig. 4). It should be noted that the breaks in the curves took place at higher temperatures under dynamic conditions than those during isothermal

holding. This was likely due to a delayed response of the ceramic materials to the change of in the external medium parameters. Analyzing the average TECs values for Pr2NiO4 + δ (Table 2), a substantial data spread should be noted. For instance, TEC can vary by 2 times depending on


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Cooling

0

0.2

x

0.4

699

age values of TECs independent of the used regimes. This behavior can be also related to the size effects because of a partial substitution of calcium for praseodymium. However, the relative change in TECs was not higher than 15% depending on the composition and 5% depending on the conditions of the measurement of the dilatometric curves. The PCNO1 sample had the highest TEC level as well as oxygen nonstoichiometry. Therefore it can be concluded that there is a direct correlation between δ and TEC.

0.6

3.4. Stability To estimate the stability of the materials the Goldschmidt tolerance factor was calculated:

Fig. 3. Concentration dependences of the average TECs values for the Pr2 – xCaxNiO4 + δ materials measured over the entire temperature range.

t=

the temperature ranges, in which it was determined [29]. Along with this, there are data that indicate fairly low average TEC values of Pr2NiO4 + δ [28].The existing discrepancies can be explained by different oxygen nonstoichiometry of the oxides that is dependent on both the prehistory of preparing the materials (synthesis method, sintering conditions) and the methodological parameters of the study (atmosphere, heating/cooling rate). It should also be noted that our studies [4, 5] gave a higher average TECs in the high-temperature range than those in the low-temperature range. This behavior, as well-known [32], was due to chemical expansion, since the increase in the structure symmetry of Pr2 – xCaxNiO4 + δ must conversely decrease TECs. Thus, the Ca-enriched samples exhibit the thermal stability, namely, the absence of phase transitions and the linearity of the temperature dependence on the change in the ceramics size Pr2 – xCaxNiO4 + δ. Turning back to Fig. 3, we can see that the increase in x in Pr2 – xCaxNiO4 + δ led to the decrease in the aver-

rA + rO , 2(rB + rO )

where rA, rB, and rO are average ionic radii of the A, B, and O ions. Although parameter t is widely used to describe the materials with a perovskite structure [33], it can be also used to analyze other crystal structures [34, 35]. Parameter t of perovskite systems can vary within a wide range (from 0.75 to 1.13) [36, 37]; in this case, its approaching unit favors the formation of more symmetric and close-packed structures, exhibiting, as a rule, an enhanced stability. The values of the tolerance-factor calculated taking into account a transition oxidation level of Ni increased from 0.901 to 0.923 as x increased from 0 to 0.7. Higher values of the tolerance-factor were related to a higher stability of oxides, including its various aspects: phase, microstructural, chemical, and thermal stabilities. Earlier, it was mentioned that the increase in the calcium content in the Pr2 – xCaxNiO4 + δ system led to the increase in the thermal stability of the materials that was in the monotonic change in their relative sizes

Table 2. Comparative analysis of the average TEC values for the Pr2NiO4 + δ (x = 0) materials Measurement conditions Air, heating, ∂T/∂τ = 3°C/min

Air, cooling, ∂T/∂τ = 3°C/min

Air, heating Air, heating Air, heating, ∂T/∂τ = 3°C/min Air, heating, ∂T/∂τ = 4.2°C/min PHYSICS OF THE SOLID STATE

Temperature range, °C

αav × 106, K–1

References

50–750 750–1000 50–1000 50–600 600–1000 50-1000 50–800 30–1000 23–700 700–950 25–900 25–900

14.1 18.2 15.5 13.7 15.3 15.2 13.2 13.5 13.1 20.1 13.9 14.3

This work

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The same

[8] [28] [29] [30]

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In this work, we also studied the chemical stability of representatives of the Pr2 – xCaxNiO4 + δ system and some oxygen- and proton-conducting electrolytes: (ZrO2)0.92(Y2O3)0.08O2 – δ (YSZ), Ce0.8Sm0.2O2 – δ (SDC), La0.85Sr0.15Ga0.85Mg0.15O3 – δ (LSGM), and BaCe0.5Zr0.3Y0.2O3 – δ (BCZY) (Fig. 6). It is seen that the basic nickelate PNO had a higher chemical activity to interact with electrolytes of LSGM, SDC, and BCZY than the Ca-containing analog PCNO5. The opposite tendency was observed in the case of YSZ: the degree of chemical interaction in the PCNO5/YSZ pair was higher than that in the PNO/YSZ pair because of formation of the thermodynamically stable CaZrO3-based phase in the former case. It should be noted that the systems based on cerium oxide and barium cerate– zirconate were most preferable electrolytes for PCNO5 due to the absence of significant chemical interaction between them.

αdiff × 106, K−1

(a)

Heating Cooling

αdiff × 106, K−1

αdiff × 106, K−1

(b)

Heating Cooling

4. CONCLUSIONS The materials of the Ruddlesden–Popper homological series Pr2 – xCaxNiO4 + δ (x = 0–0.7) were for the first time synthesized in this work. Their structural and physicochemical properties were studied using a complex of methods (room-temperature and hightemperature X-ray diffraction analysis, thermogravimetry, and dilatometry).

(c)

Heating Cooling

Fig. 4. Temperature dependences of the differential LTEC for the Pr2 – xCaxNiO4 + δ samples with x = (a) 0, (b) 0.1, and (c) 0.2.

because of the absence of phase transitions. In addition, the Ca-enriched nickelates possess some other advantages as compared to the base oxide. For example, the XRD data for the powders held at high temperature for a prolonged time period are shown in Fig. 5. It is seen that the PNO and PCNO1 materials exhibited low thermodynamic stability at these conditions, since the high-temperature treatment led to the formation of impurities based on Pr4Ni3O10, Pr6O11, and NiO, and, thus, the powders ceased to be singlephase (Figs. 5a and 5b). These results agree with the data of [13], where the formation of the abovementioned impurities in the Pr2NiO4 + δ powder subjected to high-temperature treatment was also mentioned. The Ca-enriched materials (both with the orthorhombic and the tetragonal structures) were characterized by higher thermodynamic stability and did not decompose after a prolonged holding at 850°C (Figs. 5c and 5d).

As the calcium content in this system increased, the unit cell parameters and volume decrease up to x = 0.7, which indicated the formation of a wide series of solid solutions. Using the high-temperature XRD method, we detected the existence of the phase transition in Pr2NiO4 + δ from the low-symmetric orthorhombic structure (space group Fmmm) to the tetragonal structure (space group P42/ncm) in the temperature range 400–500°C. Calcium doping led to the decrease in the transition temperature from the lowsymmetric orthorhombic structure (space group Fmmm) to the tetragonal structure (space group P42/ncm). The tetragonal structure of Pr1.6Ca0.4NiO4 + δ was stabilized even at room temperature. We confirmed the existence of the phase transition in the nickelates with low calcium content because of nonlinearity of the relative change in the linear sizes of these samples and the existence of the hysteresis based on the dilatometric data; the dilatometric dependences of the Ca-rich complex oxides were nearly linear. The values of TECs of Pr2 – xCaxNiO4 + δ decreased as x increased. The study of the prolonged heat treatment of individual oxides Pr2 – xCaxNiO4 + δ and their mixtures with the oxygen-ionic and proton-conducting electrolytes showed that the Ca-containing samples were characterized by higher stability to nickelate decomposition or their interaction with electrolyte systems.


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(d)

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Fig. 5. Powder X-ray diffraction patterns of the Pr2 – xCaxNiO4 + δ samples measured before and after prolonged holding at 850°C (250 h); x = (a) 0, (b) 0.1, (c) 0.3, and (d) 0.5. (1) Pr4Ni3O10-based phase, (2) NiO-based phase, and (3) Pr6O11-based phase.

(a)

(b)

(c)

(d)

Fig. 6. X-ray diffraction patterns of burnt powders containing PNO and PCNO5 in the mixture with the (a) YSZ, (b) SDC, (c) LSGM, and (d) BCZY. The baking condition: 900°C for 10 h. PHYSICS OF THE SOLID STATE

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It was found that not only the calcium content (x) in the system but also the oxygen nonstoichiometry of complex oxides (δ) affect the structural and the physicochemical properties. From the standpoint of the thermal characteristics and the stability, the Ca-substituted praseodymium nickelates are more attractive materials for their applying in solid-oxide fuel cells as compared to the base Pr2NiO4 + δ. ACKNOWLEDGMENTS This work was supported by the Government of the Russian Federation (act 211, no. 02.A03.21.0006), the Russian Academy of Sciences (program of fundamental research no. 15-20-03-15), and the Russian Foundation for Basic Research (project no. 14-03-00414). The work was partially performed using the equipment of the Common Use Center “Material Composition of Substances” of the Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences. REFERENCES 1. S. Ya. Istomin and E. V. Antipov, Russ. Chem. Rev. 82, 686 (2013). 2. Y. Chen, W. Zhou, D. Ding, M. Liu, F. Ciucci, M. Tade, and Z. Shao, Adv. Energy Mater. 5, 201500537 (2015). 3. M. M. Kuklja, E. A. Kotomin, R. Merkle, Yu. A. Mastrikov, and J. Maier, Phys. Chem. Chem. Phys. 15, 5443 (2013). 4. E. Boehm, J.-M. Bassat, P. Dordor, F. Mauvy, J. C. Grenier, and Ph. Stevens, Solid State Ionics 176, 2717 (2005). 5. L. Minervini, R. W. Grimes, J. Kilner, and K. E. Sickafus, J. Mater. Chem. 10, 2349 (2000). 6. J.-M. Bassat, M. Burriel, O. Wahyudi, R. Castaing, M. Ceretti, P. Veber, I. Weill, A. Villesuzanne, J.-C. Grenier, W. Paulus, and J. A. Kilner, J. Phys. Chem. C 117, 26466 (2013). 7. P. Batocchi, F. Mauvy, S. Fourcade, and M. Parco, Electrochim. Acta 145, 1 (2014). 8. X. D. Zhou, J. W. Templeton, Z. Nie, H. Chen, J. W. Stevenson, and L. R. Pederson, Electrochim. Acta 71, 44 (2012). 9. C. Ferchaud, J. C. Grenier, Y. Zhang-Steenwinkel, M. M. A. van Tuel, F. P. F. van Berkel, and J. M. Bassat, J. Power Sources 196, 1872 (2011). 10. A. Grimaud, F. Mauvy, J. M. Bassa, S. Fourcade, L. Rocheron, M. Marrony, and J. C. Grenier, J. Electrochem. Soc. B 159, 683 (2012). 11. B. Philippeau, F. Mauvy, C. Mazataud, S. Fourcade, and J.-C. Grenier, Solid State Ionics 249–250, 17 (2013). 12. G. Taillades, J. Dailly, M. Taillades-Jacquin, F. Mauvy, A. Essouhmi, M. Marrony, C. Lalanne, S. Fourcade, D. J. Jones, J.-C. Grenier, and J. Roziére, Fuel Cells 10, 166 (2010).

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Translated by Yu. Ryzhkov Vol. 59

No. 4

2017