Crystal structure and properties of novel oxide Sm0.9Ca1.1Fe0.7Co0.3O4-d 159 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 4. 01 Galayda A. P., Volkova N. E., Dyagileva A. I., Gavrilova L. Ya., Cherepanov V. A., Battle P. D. Chimica Techno Acta. 2018. Vol. 5, No. 4. P. 159–165. ISSN 2409–5613 A. P. Galaydaa*, N. E. Volkovaa, A. I. Dyagilevaa, L. Ya. Gavrilovaa, V. A. Cherepanova, P. D. Battleb a Institute of Natural Sciences and Mathematics, Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russian Federation b Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK *E-mail: anastasia.galaida@urfu.ru Crystal structure and properties of novel oxide Sm0.9Ca1.1Fe0.7Co0.3O4–δ Sm0.9Ca1.1Fe0.7Co0.3O4–δ oxide with the K2NiF4-type structure was prepared us- ing a glycerin-nitrate technique. The XRD pattern of Sm0.9Ca1.1Fe0.7Co0.3O4–δ was refined by the Rietveld method within an orthorhombic structure (space group Bmab). The electrical conductivity, Seebeck coefficient, and thermal expansion of Sm0.9Ca1.1Fe0.7Co0.3O4–δ were measured depending on temperature in air. The change of oxygen nonstoichiometry determined by TGA in air does not exceed 0.01. The oxygen content in Sm0.9Ca1.1Fe0.7Co0.3O4–δ determined by the reduc- tion in a hydrogen flux is equal to 3.96 ± 0.01. The positive value of Seebeck coefficient indicates that the predominant charge carriers in the oxide studied are electron holes. Keywords: complex oxide; Ruddlesden-Popper phase; crystal structure; oxygen nonstoichi- ometry, electroconductivity; thermal expansion. Received: 13.11.2018. Accepted: 05.12.2018. Published: 31.12.2018. © Galayda A. P., Volkova N. E., Dyagileva A. I., Gavrilova L. Ya., Cherepanov V. A., Battle P. D., 2018 Introduction C omplex oxides with the K2NiF4-type structure based on rare earth, alkaline earth and 3d-transition metals are known as materials with high mixed electronic-ionic conductivity and oxygen mobility, and also thermodynamic stabil- ity at high temperature under an oxidiz- ing atmosphere [1–4]. For this reason, K2NiF4-type oxides have attracted much attention as  promising SOFC cathodes [1–3], oxygen-separation membranes [5] and catalysts [6]. The crystal structure of  a  K2NiF4-type oxide is built up by  al- ternating the perovskite layer (ABO3) and rock salt layer (AO) [7]. Depending on the nature of the metals in the A and B sublattices, the crystal structure of  ox- ides with overall composition A2BO4 can be described using a  tetragonal (sp. gr. I4 / mmm) or orthorhombic (sp. gr. Bmab) unit cell [3–13]. Compared with Sr-sub- stituted phases, very little is known about Ca-doped analogues with the K2NiF4-type structure. It has been reported previously that metastable lanthanum calcium ferrite CaLaFeO4–δ decomposes at 1100 °C to lan- thanum ferrite LaFeO3–δ and calcium oxide CaO [14]. One can expect that variation of the Ln / Ca ratio and partial substitution of  more electronegative Co for  Fe ions 160 in  the B-site position will stabilize the K2NiF4-type structure. The present study has focused on the structure and properties of the novel Sm0.9Ca1.1Fe0.7Co0.3O4–δ oxide with the K2NiF4-type structure. Experimental The complex oxide was prepared us- ing a glycerin nitrate technique. Samari- um oxide Sm2O3 (99.99 % purity), calcium carbonate CaCO3 (“pure for  analysis” grade), metallic cobalt Co, iron oxalate FeC2O4·2H2O (“pure for analysis” grade), nitric acid (“special purity” grade) and glycerin were used as the starting materials. Metallic cobalt was obtained by reducing cobalt oxide Co3O4 (“special purity” grade) in the hydrogen flow at 400–600 °C for 6 h. The appropriate stoichiometric amounts of starting materials were dissolved in ni- tric acid, and then glycerin was added to the solution. The resulting gel was dried in a porcelain cup, decomposed to the dark powder, then placed in  an alumina cru- cible and calcined in air at 700–1000 °C for 8–10 h. The final annealing was per- formed at  1100  °C in  air for  120 h with intermediate grindings, followed by slow cooling to  room temperature at  a  rate of  ~100  °C / h. The phase composition of the annealed samples was determined by  X-ray diffraction u sing a   S himadzu XRD-7000 (CuKα-radiation, angle range 2Θ = 20–90°, step 0.03°, 5 s / step) in air. The s tructural p arameters w ere r efined by the Rietveld profile method using t he Fullprof-2008 package. Thermogravimet- ric analysis (TGA) was carried out using an STA 409 PC instrument (Netzsch) over the temperature range 25–1100 °С in air in dynamic (heating / cooling rate 2 K / min) mode. The absolute values of oxygen con- tent were determined by a reduction of the samples in a hydrogen flux inside the TGA cell at  1200  °C [15]. Thermal expansion measurements were carried out within the temperature range of  25–1100  °C in  air using a dilatometer DIL 402C (Netzsch) at a heating / cooling rate of 5K / min. The total conductivity and Seebeck coefficients of ceramic samples were measured in air by a 4-probe method with platinum elec- trodes. A bar-shaped sample (3×4×25 mm) for  thermal expansion coefficient (TEC) and conductivity measurements was ob- tained by pressing powder that was mixed with 2–3 drops of ethanol using a manual- ly-operated press. Afterwards, the samples prepared accordingly were slow heated and then sintered at 1200 °C for 14 h in air fol- lowed by slow cooling (the rate of heating and cooling was 50 K / h). The relative den- sity of the sample was evaluated by a com- parison of measured values to those cal- culated from the XRD-data. The relative density was found to be 90 %. Results and discussion In contrast with previously reported SmCaCoO4–δ [10] and LnSrFeO4–δ (Ln=La [16], Nd [17, 18], Gd [19], Sm [13]), we have failed to synthesize samarium-calci- um ferrite with an equimolar Sm / Ca-ratio at 1100 °C in air. It is known that the ho- mogeneity range limits for such solid solu- tions depend significantly on temperature, ionic radius of dopants and oxygen partial pressure. The decrease of temperature from 1500 °C to 1100 °C leads to decomposition of CaLaFeO4–δ to LaFeO3–δ and CaO [14]. T h e c o m p l e x o x i d e Sm0.9Ca1.1Fe0.7Co0.3O4–δ was prepared 161 by  a  standard glycerin-nitrate technique with annealing temperature 1100 °C in air. Diffraction data for Sm0.9Ca1.1Fe0.7Co0.3O4–δ analyzed by  the Rietveld method are shown in  Fig. 1. XRD pattern of Sm0.9Ca1.1Fe0.7Co0.3O4–δ was indexed in the orthorhombic structure (sp. gr. Bmab). The value of  the oxygen content in Sm0.9Ca1.1Fe0.7Co0.3O4–δ at room tempera- ture determined by the TGA reduction was found to be 3.96 ± 0.01, and is consistent with that for SmCaCoO4–δ [10]. The TGA measurements within the temperature range of 25–1100 °С in air revealed a small change in oxygen content, 4-δ, that is less than 0.01 (Fig. 2). T h e t e mp e r at u r e d e p e n d e n c e of  the thermal expansion in  air for Sm0.9Ca1.1Fe0.7Co0.3O4–δ is given in  Fig. 3 in comparison with SmCaCoO4–δ [10]. As can be seen, the shape of the meas- ured dependence is non-linear. Since this phenomenon cannot be explained by a no- ticeable oxygen exchange, we suggest that the non-linearity of the dilatometric plot is mainly associated with redistribution of  electron density between Co and Fe and / or  changes in  Co spin states with the temperature. Similar behavior was observed in  SmFe1–xCoxO3–δ (x  =  0.2, 0.5, 0.8) [10]. However, additional re- search is needed to clarify this behavior. The dependence of  ΔL / L=f(T) has been described by two linear equations in the temperature ranges of  25–400  °C and 730–1000 °C. Low- and high-temperature TEC values for  Sm0.9Ca1.1Fe0.7Co0.3O4–δ in comparison with those of SmCaCoO4–δ and most common SOFC electrolytes are listed in Table 1. The decrease in the TEC value with the increase of iron content can be explained by  the higher bond energy Fig. 1. Rietveld refined XRD pattern of Sm0.9Ca1.1Fe0.7Co0.3O4–δ. Circles are the experimental XRD data, upper continuous line is the calculated profile, lower continuous line is the difference plot, vertical lines are indicating the Bragg positions Fig. 2. Oxygen content, 4–δ, in Sm0.9Ca1.1Fe0.7Co0.3O4–δ as a function of temperature in air Fig. 3. Thermal expansion of the Sm0.9Ca1.1Fe0.7Co0.3O4–δ and SmCaCoO4–δ [10] ceramics in air 162 for Fe–O (409 kJ / mol) compared to Co–O (368 kJ / mol) [20]. T h e t o t a l c o n d u c t i v i t y of Sm0.9Ca1.1Fe0.7Co0.3O4–δ versus tempera- ture is shown in Fig. 4. T h e c o n d u c t i v i t y of  Sm0.9Ca1.1Fe0.7Co0.3O4–δ monotonously increases with temperature up to 23 S / cm at 1100 °C. In contrast, the Seebeck coeffi- cient decreases with temperature. The posi- tive value of the Seebeck coefficient (see Fig. 5) indicates that electron holes are the predominant charge carriers in the oxide studied. The conductivity activation ener- gies calculated from two linear segments of  Arrhenius plot (see insert in  Fig.  4) are equal to 0.193 eV and 0.283 eV in the temperature ranges 50–250 °C and 300– 1100 °C, respectively. Both values are typi- cal for a hopping conduction mechanism. A comparison of  the temperature dependences of  the total conductivity of Sm0.9Ca1.1Fe0.7Co0.3O4–δ and SmCaCoO4–δ [10] (see Fig. 4) visually shows that they coincide in practical term up to 600 °C and the conductivityhas thermally activated character for both oxides. A strong increase in the conductivity of SmCaCoO4–δ above 600 °C can be explained by the pronounced charge disproportionation process at high- er temperatures: 2Co Co CoCo Co � � � � Co (1) Fig. 5. Seebeck coefficient for Sm0.9Ca1.1Fe0.7Co0.3O4–δ vs. temperature in air Table 1 The average thermal expansion coefficients for Sm0.9Ca1.1Fe0.7Co0.3O4–δ, SmCaCoO4–δ [10] and SOFC electrolytes Zr0.85Y0.15O2–δ [22] and Ce0.8Sm0.2O2–δ [23] in air Composition Temperature range, °C TEC×106, K–1 Sm0.9Ca1.1Fe0.7Co0.3O4–δ 25–400 4.4 730–1000 20.2 SmCaCoO4–δ [10] 25–580 17.7 580–1100 20.2 Zr0.85Y0.15O2–δ [22] 30–1000 10.9 Ce0.8Sm0.2O2–δ [23] 30–1000 12.5 Fig. 4. Total conductivity of Sm0.9Ca1.1Fe0.7Co0.3O4–δ and SmCaCoO4–δ [10] versus temperature in air. The insert shows the Arrhenius plot for Sm0.9Ca1.1Fe0.7Co0.3O4–δ 163 In contrast, simultaneous presence of Co and Fe at the B-sites suppress dis- proportionation of iron and cobalt by the following process: Fe Co Fe CoFe Fe Fe � �� � � � Fe , (2) where iron seems to be a hole trap [21]. Conclusions Single-phase Sm0.9Ca1.1Fe0.7Co0.3O4–δ was successfully synthesized by a glycerin nitrate technique. The structural param- eters of  the oxide prepared were refined by  the Rietveld method. The oxygen content, 4–δ, at  room temperature was found to be 3.96 ± 0.01 and its decrease with temperature does not exceeded 0.01. The lower value of the total conductivity of Sm0.9Ca1.1Fe0.7Co0.3O4–δ compared to that of  SmCaCoO4–δ was explained in  terms of an electronic exchange process. 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