Crystal structure, thermal and electrotransport properties of NdBa1–xSrxFeCo0.5Cu0.5O5+δ (0.02 ≤ x ≤ 0.20) solid solutions Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(3), № 20218301 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.3.01 1 of 5 Crystal structure, thermal and electrotransport properties of NdBa1–xSrxFeCo0.5Cu0.5O5+δ (0.02 ≤ x ≤ 0.20) solid solutions A.I. Klyndyuk * , Ya.Yu. Zhuravleva, N.N. Gundilovich Belarus State Technological University, 13a Sverdlova str., Minsk, 220006, Belarus Republic * Corresponding author: klyndyuk@belstu.by This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract Using solid-state reactions method, the solid solutions of layered ox- ygen-deficient perovskites NdBa1–xSrxFeCo0.5Cu0.5O5+δ (0.02 ≤ x ≤ 0.20) were prepared; their crystal structure, thermal stability, thermal expansion, electrical conductivity and thermopower were studied. It was found that NdBa1–xSrxFeCo0.5Cu0.5O5+δ phases crystallize in tetragonal syngony (space group P4/mmm) and are p-type semiconductors, whose con- ductivity character at high temperatures changed to the metallic one due to evolution from the samples of so-called weakly-bonded oxy- gen. Partial substitution of barium by strontium in NdBaFeCo0.5Cu0.5O5+δ leads to the small decreasing of unit cell parameters, thermal stability and thermopower of NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions, increasing of their electri- cal conductivity values and slightly affects their linear thermal ex- pansion coefficient and activation energy of electrical transport val- ues. Keywords layered perovskites thermal stability thermal expansion electrical conductivity thermopower Received: 02.06.2021 Revised: 01.07.2021 Accepted: 08.07.2021 Available online: 08.07.2021 1. Introduction Layered oxygen-deficient double perovskites of LnBaMe’Me”O5+ (Ln  Y, rare-earth element (REE), Me’, Me”  3d-metal) have a complex of unique properties, in- cluding large values of electrical conductivity and thermopower, and contain in their structure labile oxygen, so they may be used as functional materials for different purposes: high-temperature oxide thermoelectrics, elec- trode materials for solid-oxide fuel cells (SOFC), materials for working elements of semiconducting chemical gas sen- sors, catalysts of hydrocarbons oxidation, etc. [16]. LnBaCo2O5+ phases demonstrate high electrochemical performance in oxygen reduction reaction (ORR) [48], but values of their linear thermal expansion coefficient (TEC, ) are too large (circa (1529)10 6 K 1 [79]) in comparison to the TEC of commonly used in SOFC zirco- nia, ceria, or perovskite-like based solid electrolytes, which are equal to (1011)10 6 K 1 , (1213)10 6 , and (1013)10 6 K 1 respectively [10], which limits the practi- cal implementation of these phases as cathode materials in SOFC. Many studies [8,9,1118] have demonstrated that par- tial substitution in LnBaCo2O5+ of cobalt by other 3d- metal or barium by strontium essentially improves elec- trochemical performance of solid solutions forming at such substitution and reduces their TEC value. So, partial substitution of cobalt by iron in LnBaCo2O5+ (Ln  Pr, Nd) leads to the reducing of TEC and polarization resistance of materials forming at this substitution and also improves their long-term stability at implementation as cathode materials of SOFC [12,13,15]. Doping of barium by stronti- um and of cobalt by copper or iron in YBaCo2O5+ lead to the reducing of TEC of forming solid solutions, improving of their structural stability and electrochemical perfor- mance [9,16]. So, obtaining and studying of solid solutions, including complex substituted ones, on the basis of layered oxygen- deficient double perovskites is an actual task, having sci- entific and practical interest. In this work we studied the effect of partial substitu- tion of barium by strontium in NdBaFeCo0.5Cu0.5O5+ on the crystal structure, thermal and electrotransport properties of NdBa1xSrxFeCo0.5Cu0.5O5+ solid solutions as perspective cathode materials for intermediate-temperature SOFC. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.3.01 mailto:klyndyuk@belstu.by http://creativecommons.org/licenses/by/4.0/ Chimica Techno Acta 2021, vol. 8(3), № 20218301 ARTICLE 2 of 5 2. Experimental Ceramic samples of NdBa1xSrxFeCo0.5Cu0.5O5+ (x = 0.02, 0.05, 0.10, and 0.20) solid solutions were prepared by means of solid-state reactions method from Nd2O3 (NO–L), BaCO3 (pure), SrCO3 (pure), Fe2O3 (super pure 2–4), Co3O4 (pure), and CuO (pure for analysis) in air at temperature of 1173 K within 40 h with consequent sintering during 9– 18 h in air at temperatures of 1223–1273 K according to the methods, described in [19,20]. Identification of the samples and determination of their lattice constants was performed by means of X-ray diffraction analysis (XRD) (X-ray difrractometer Bruker D8 Advance XRD, Cu K-radiation). IR-absorption spectra of powders were recorded in the mixtures with KBr within 300–1500 cm –1 (ThermoNicolet Nexus Fourier-Transform Infrared Spectrometer). Apparent (effective) density of the sintered ceramics (eff) was determined from the mass and dimensions of the samples, and their porosity (П) was calculated using Eq. (1): П = (1 – eff/XRD)100%, (1) where XRD – X-ray density of the samples. Thermal stability of the powdered samples was studied by means of thermoanalytical system of TGA/DSC– 1/1600 HF in air within 300–1100 K temperature interval. Thermal expansion of the sintered ceramics was investi- gated using DIL 402 PC quartz dilatometer in air within temperature interval of 300–1100 K [21]. Electrical conductivity and thermopower of NdBa1xSrxFeCo0.5Cu0.5O5+ solid solutions were studied in air within 300–1100 K according to the methods, de- scribed in detail in [20]. Values of TEC and apparent acti- vation energies of electrical conductivity (EA) and thermopower (ES) were calculated from the linear parts of l/l0 = f(T), ln(T) = f(1/T), and S = f(1/T) dependences, respectively. 3. Results and Discussion After final stage of the synthesis, all the samples of the NdBa1xSrxFeCo0.5Cu0.5O5+ solid solutions were single phase, within XRD accuracy (Fig. 1a), and had a structure of tetragonally distorted double perovskite of YBaCuFeO5 type (a  ap,c  2ap) [1], and their reflections were indexed in the framework of the P4/mmm space group with unit cell parameters of a = 3.903–3.914 Å and c = 7.707–7.715 Å (Table 1). As can be seen from the Table 1, increasing of the sub- stitution degree of larger Ba 2+ ion by smaller Sr 2+ one (for C.N. = 12 R(Ba 2+ ) = 1.60 Å, R(Sr 2+ ) = 1.44 Å [22]) leads to the decreasing of the size of the unit cell of the NdBa1– xSrxFeCo0.5Cu0.5O5+δ phases. Porosity of the sintered ce- ramics enlarged at x increasing (Table 1), which let us conclude that partial substitution of barium by strontium in NdBaFeCo0.5Cu0.5O5+δ slightly reduces sinterability of this perovskite. On the IR-absorption spectra of the samples were de- tected some absorption bands with extrema at 351– 353 cm –1 (1), 467–470 cm –1 (2), 576–582 cm –1 (3), and 660–665 cm –1 (4) (Fig. 1b), which were attributed, ac- cording to [23], to the stretching (3, 4) and bending vi- brations (1) of the (Fe,Co,Cu)–O–(Fe,Co,Cu) bonds in the [(Fe,Co,Cu)O2] basal planes (1, 3), as well as stretching vibrations of apical oxygen of (Fe,Co,Cu)–O–(Fe,Co,Cu) bonds along c axis (4) in the structure of NdBa1– xSrxFeCo0.5Cu0.5O5+δ phases. At x increasing the 3 and 4 bands shifted to the larger values, Fig. 1 X-ray powder diffractograms (a) and IR-absorption spectra (b) of NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions: x = 0.02 (1); 0.05 (2); 0.10 (3); 0.20 (4) Chimica Techno Acta 2021, vol. 8(3), № 20218301 ARTICLE 3 of 5 Table 1 The unit cell parameters (a, c, c/2a, V), effective density (ρeff) and porosity (П) of NdBa1–xSrxFeCo0.5Cu0.5O5+δ layered perov- skites finally sintered at 1273 K x a, Ǻ c, Ǻ c/2a, Ǻ V, Ǻ 3 ρeff, g/cm 3 П, % 0.02 3.913(1) 7.715(1) 0.9860 118.1(1) 5.54 10 0.05 3.914(1) 7.711(1) 0.9851 118.1(1) 5.62 8 0.10 3.911(2) 7.707(2) 0.9853 117.9(1) 5.54 12 0.20 3.903(1) 7.708(1) 0.9876 117.5(1) 4.84 21 which pointed out to increasing of energy of metal–oxygen interactions in the crystal structure of these phases. Re- sults of IR-absorption spectrocopy correlate with the XRD results, showing that increasing of substitution degree of barium by strontium in NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions leads to the shrinking of their unit cell. According to the results of thermal analysis, near the room temperature all the samples were thermally stable, but, beginning from the temperatures of 623–663 K (Tb) the small mass loss (0.6–0.8%) was detected (Fig. 2a), which took place due to the evolution of the labile (weak- ly-bonded) oxygen from the samples into environment [24]. Values of Tb decreased at x increasing (Fig. 2c, Table 2), which indicated increasing of mobility of weakly- bonded oxygen in the structure of NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions at increasing of substitution degree of barium by strontium. On the temperature dependences of the relative elongation (l/l0) of materials studied an anomaly in a kink near Tk = 640– 680 K accompanied by the increase the TEC value was detected (Fig. 2b, Table 2), which took place due to the rearrangement of oxygen sublattice of the samples with consequent evolution of oxygen from them in air [24]. TEC values of ceramics in high-temperature region (HT, T > Tk) were 15–24% larger than in the low-temperature one (LT, T < Tk) (Table 3). Values of TEC of NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions in both tempera- ture regions slightly varied but temperature of anomaly (Tk) essentially decreased (Fig. 2d) at x increasing. As can be seen from the data given in the Fig. 3a,b, ma- terials studied are p-type (S > 0) semiconductors (/T > 0). Their conductivity character at high tempera- tures (T > Tmax) changed to the metallic one (/T < 0), which was accompanied by the change of the sign of the S/T derivative (S/T < 0 at T < Tmin and S/T > 0 at T >Tmin). Observed anomalies of electrotransport proper- ties of NdBa1–xSrxFeCo0.5Cu0.5O5+δ phases as well as de- scribed earlier anomaly of thermal expansion were due to the evolution of the weakly-bonded oxygen from the sam- ples [24]. Note that temperatures of  and S anomalies, Tmax and Tmin respectively, at x increasing shifted to the smaller temperatures (Fig. 3c,d, Table 2) like Tb and Tk temperatures. It is interesting that values of Tb, Tk, and Tmin temperatures were rather close to each other, but Tmax value was essentially larger (Table 2, Figs. 2c,d, 3c,d). Values of all critical temperatures (Tcr: Tb, Tk, Tmax, and Tmin) decrease at increasing of strontium content in the NdBa1–xSrxFeCo0.5Cu0.5O5+δ solid solutions, hereby depend- ence Tcr = f(x) was almost linear for Tk, underlinear for Tb and Tmin, and overlinear for Tmax increased despite of the Table 2 Values of critical temperatures (Tb, Tk, Tmax, and Tmin) of NdBa1–xSrxFeCo0.5Cu0.5O5+δ ceramics x Tb, K Tk, K Tmax, K Tmin, K 0.02 663 680 734 685 0.05 631 670 733 665 0.10 631 660 730 650 0.20 623 640 709 650 Fig. 2 Temperature dependences of mass loss (a) and relative elongation (b) of NdBa1–xSrxFeCo0.5Cu0.5O5+δ samples: x = 0.02 (1); 0.05 (2); 0.10 (3); 0.20 (4). Dilatometric curves are shifted from each other by 0.1% for the clarity of presentation. Insets shows concentra- tion dependences of temperatures of mass loss onset (c) and kink of the Δl/l0 = f(T) dependences (d) Chimica Techno Acta 2021, vol. 8(3), № 20218301 ARTICLE 4 of 5 Fig. 3 Temperature dependences of electrical conductivity (a) and thermo-EMF coefficient (b) of NdBa1–xSrxFeCo0.5Cu0.5O5+δ sintered ceramics: x = 0.02 (1); 0.05 (2); 0.10 (3); 0.20 (4). Insets shows concentration dependences of temperatures of maximum on the σ = f(T) dependences (c) and minimum of the S = f(T) dependences (d) fact that porosity of the samples enlarged at increasing of strontium content (Table 1). So, our results show that par- tial substitution of barium by strontium in NdBaFeCo0.5Cu0.5O5+δ is an effective way to increase its electrical conductivity. Layered oxygen-deficient double perovskites LnBaMe’Me”O5+ possess polaronic character of charge transfer [1,19,24], and temperature dependences of their electrical conductivity and thermopower obey Eqs. (2–3)  = (A/T)exp(EA/kT), (2) S = (k/e)(–ES/kT + B), (3) where EA = ES + Em and ES – activation energies of electri- cal conductivity and thermopower respectively, ES is also activation energy of charge carriers – polarons, and Em is energy of their transfer [25]. As can be seen from the data given in the Table 3, val- ues of energies of activation of electrical transport, in the whole, slightly varied at variyng strontium content in the samples. Comparing obtained in this work results with the data of [24], where for NdBaFeCo0.5Cu0.5O5+δ layered per- ovskite was found that EA = 0.245 eV, ES = 0.048 eV, and Em = 0.200 eV, we can conclude that partial substitution Table 3 Values of TEC (α) and apparent activation energy of electrical transport (Eσ, ES, Em) for the sintered NdBa1–xSrxFeCo0.5Cu0.5O5+δ ceramics x 10 6 · αLT, K –1 10 6 · αHT, K –1 Eσ, eV (350– 700 K) ES, eV (400– 650 K) Em, eV 0.02 15.8 19.3 0.190 0.047 0.143 0.05 16.3 20.0 0.203 0.044 0.159 0.10 16.6 19.1 0.200 0.054 0.146 0.20 16.4 20.4 0.167 0.038 0.129 of barium by strontium in this parent phase does not af- fect practically the value of activation energy of charge carriers – polarons, but results in essential reducing of transfer energy of charge carriers. 4. Conclusions By means of solid-state reactions method the ceramic samples of NdBa1xSrxFeCo0.5Cu0.5O5+ (x = 0.02, 0.05, 0.10, and 0.20) solid solutions were prepared, and their crystal structure and physico-chemical properties were studied. In was found that obtained materials had tetrago- nal structure, whose unit cell parameters slightly depend on their cationic composition, and are p-type semiconduc- tors, whose conductivity character changes to the metallic one at high temperatures due to the evolution of oxygen from their crystal structure into environment. It was es- tablished that partial substitution of barium by strontium in NdBaFeCo0.5Cu0.5O5+ results in the increasing of electri- cal conductivity, reducing of energy activation of electrical conductivity, thermopower and thermal stability of solid solutions forming at this substitution NdBa1xSrxFeCo0.5Cu0.5O5+. References 1. Klyndyuk AI. Perovskite-like Oxides 0112 Type: Srtucture, Properties and Possible Applications. 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