Crystal Structure and Properties of Gd1-xSrxCo1-yFeyO3-δ Oxides as Promising Materials for Catalytic and SOFC Application

Aksenova, Tatiana V.; Mysik, Darya K.; Cherepanov, Vladimir A.

doi:10.3390/catal12111344

Open AccessArticle

Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application

by

Tatiana V. Aksenova

^*,

Darya K. Mysik

and

Vladimir A. Cherepanov

Institute of Natural Science and Mathematics, Ural Federal University, 620000 Yekaterinburg, Russia

^*

Author to whom correspondence should be addressed.

Catalysts 2022, 12(11), 1344; https://doi.org/10.3390/catal12111344

Submission received: 9 October 2022 / Revised: 26 October 2022 / Accepted: 26 October 2022 / Published: 2 November 2022

(This article belongs to the Special Issue Smart Nanomaterials for Catalytic Environment, Energy, and Sustainability)

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Abstract

:

A series of samples with the overall composition Gd_1-xSr_xCo_1-yFe_yO_3-δ (x = 0.8; 0.9 and 0.1 ≤ y ≤ 0.9), which are promising materials for catalytic and SOFC application, was prepared by a glycerol nitrate technique. X-ray diffraction analysis allowed to describe Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ with 0.1 ≤ y ≤ 0.5 in a tetragonal 2a_p × 2a_p × 4a_p superstructure (SG I4/mmm), while oxides with 0.6 ≤ y ≤ 0.9 exhibit cubic disordered perovskite structure (SG Pm-3m). All Gd_0.1Sr_0.9Fe_1-yCo_yO_3-δ oxides within the composition range 0.1 ≤ y ≤ 0.9 possess the cubic perovskite structure (SG Pm-3m). The structural parameters were refined using the Rietveld full-profile method. The changes of oxygen content in Gd_1-xSr_xCo_1-yFe_yO_3-δ versus temperature were determined by thermogravimetric analysis. The introduction of iron into the cobalt sublattice leads to a gradual increase in the unit cell parameters and unit cell volume, accompanied with increasing oxygen content. The temperature dependency of conductivity for Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ exhibits a maximum (284 S/cm) at ≈600 K in air. The positive value of the Seebeck coefficient indicates predominant p-type conductivity in the Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ complex oxide.

Keywords:

solid solution; X-ray diffraction; crystal structure; thermogravimetric analysis; oxygen nonstoichiometry; conductivity

Graphical Abstract

1. Introduction

The study of Sr-substituted rare earth cobaltites/ferrites is of great interest, since these materials can be used as catalysts [1,2,3,4], cathodes in solid oxide fuel cells (SOFCs) [5,6,7,8,9,10], as magnetic materials [11,12], or gas sensors [13,14,15]. Mixed Fe/Co occupation of B-sites in perovskite structure promotes a good compromise between the catalytic activity and phase stability of complex oxides [16,17,18,19]. The homogeneity ranges, crystal structure, and physicochemical properties of Ln_1-xSr_xCoO_3-δ (Ln = lanthanide ion) were the subject of numerous studies [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. It was shown that both the structure and properties depend on the size of the Ln cations and the concentration of the dopant (Sr). On the other hand, it is widely acknowledged that the variable oxidation state of Co and Fe ions and the concentration of oxygen vacancies play an important role in the catalytic activity of materials that are considered as catalysts for various Red-Ox processes.

The introduction of strontium into the A-sublattice in LnCoO_3-δ, containing a large-size lanthanide (Ln = La, Pr, Nd), leads to the formation of extended ranges of solid solutions of various structures. Cobaltites La_1-xSr_xCoO_3-δ with 0.0 ≤ x ≤ 0.5 crystallized in a rhombohedral structure (SG R-3c) and further Sr substitution 0.6 ≤ x ≤ 0.8 lead to a cubic structure (SG Pm-3m) [20,21,22]. The introduction of Sr into Ln_1-xSr_xCoO_3-δ (Ln = Pr, Nd) within 0.0 ≤ x ≤ 0.5 preserves an orthorhombically distorted perovskite-type structure (SG Pbnm) [23,24,25]. An increase in the Sr-content leads to a change in the structure from orthorhombic to cubic [24].

A significant oxygen deficiency in Sr-enriched Ln_1-xSr_xCoO_3-δ (Ln = La-Nd) solid solutions prepared in oxygen flow and cooled to room temperature leads to the ordering of oxygen vacancies along the c-axis and the formation of a_p × a_p × 2a_p superstructure (SG P4/mmm), where a_p is the unit cell parameter of the primitive perovskite cell [24,26,27].

The increased difference between ionic radii of Sr and Ln for the rare earth elements smaller than Nd (Ln = Sm, Gd) prevents the formation of Ln-enriched Ln_1-xSr_xCoO_3-δ solid solutions with the orthorhombic structure [28,29,30]. At the same time, Sr-enriched oxides form wide ranges of solid solutions with 0.5 ≤ x ≤ 1.0 for Ln = Sm and 0.6 ≤ x ≤ 1.0 for Ln = Gd, which possess a tetragonal 2a_p × 2a_p × 4a_p superstructure (SG I4/mmm) [28,29,30]. It is worth noting that a similar cationic ordering was described by Istomin et al. [31] in terms of the 314-type structure (Sr₃LnCo₄O_12-4δ or Sr_0.75Ln_0.25CoO_3-δ) for Ln = Y, Sm-Tm, with possible substitution of Ln by Sr or vice versa. It was found that the intensity of XRD peaks corresponding to the superstructure in Ln_1-xSr_xCoO_3-δ decreases with an increase of strontium content [27,29,30].

The structural model of a tetragonal 2a_p × 2a_p × 4a_p unit cell for the Ln_1-xSr_xCoO_3-δ complex oxide is shown in Figure 1. It includes three different A-sites, which are successively filled with Ln ions as their total content increases. According to James et al. [27], at the first stage, Ln ions exclusively fill only A1-sites, while A2- and A3-sites remain completely occupied by Sr²⁺ ions. A further increase in the Ln content (x > 0.25) is carried out by replacing strontium with lanthanide in A3-sites, while A2 remain fully occupied by Sr²⁺ ions. Although the general approach used by Istomin et al. [31] for the distribution of Sr and Ln at three different A-sites was the same, they used opposite designations for A2 and A3 sites and the opposite location of Sr and Ln in Ln-rich oxides.

The crystal structure of Gd_1-xSr_xCoO_3-δ oxide is significantly influenced by heat treatment conditions [29,30,32,33,34,35,36]. At temperatures above 1473 K, the oxide with x = 0.8 possesses a cubic perovskite structure with a statistical distribution of strontium and gadolinium ions in the A-sublattice. Slow cooling of the sample to 1363 K leads to the ordering of Gd/Sr cations in the A-sublattice and changes the structure to tetragonal [32,33]. The transition temperature from a tetragonal ordered to a cubic disordered structure for the oxide with x = 0.9 was defined as 1263 K [35].

The oxygen content in the Ln_1-xSr_xCoO_3-δ (Ln = Sm, Gd, Dy, Y, Ho) oxides slightly decreases with decreasing lanthanide radius and much more significantly with increasing strontium content [26,27,29,30,37,38].

The main disadvantage of Sr-substituted rare earth cobaltites in practical application is their weak thermal and Po₂-stability [22,39,40]. It is generally accepted that the partial substitution of Co ions by Fe improves the stability of oxides without significantly deteriorating important functional properties.

This work focuses on the influence of Fe-substitution degree on the homogeneity range, crystal structure, oxygen nonstoichiometry, and electrical properties of Sr-enriched Gd_1-xSr_xCo_1-yFe_yO_3-δ with x = 0.8 and 0.9.

2. Results and Discussion

2.1. Crystal Structure of Gd_1-xSr_xCo_1-yFe_yO_3-δ

In order to determine the homogeneity range of iron substituted gadolinium strontium cobaltate, a series of samples with the overall composition Gd_1-xSr_xCo_1-yFe_yO_3-δ, where x = 0.8; 0.9 and 0.1 ≤ y ≤ 0.9 in a step of y = 0.1, was prepared in air by the glycerol-nitrate technique. According to the results of XRD analysis all samples were identified as single-phase. As an example, Figure 2 demonstrates XRD patterns for the selected Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ oxides.

The crystal structure of quenched Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ oxides with 0.1 ≤ y ≤ 0.5 is similar to the Fe-undoped cobaltate Gd_0.2Sr_0.8CoO_3-δ [30]. The XRD patterns for all of them contain weak diffraction peaks in vicinity of 2θ ≈ 21° and 2θ ≈ 39° (corresponding to the interlayered distances d ≈ 4.26 (hkl = 103) and d ≈ 2.29 (hkl = 215)) that could be attributed to the tetragonal superstructure [27,28,29,30,31]. The latter is formed due to the ordered location of Ln and Sr cations in the A-sublattice of perovskite structure and the ordering of the oxygen vacancies. Thus, the introduction of iron into the cobalt sublattice in Gd_0.2Sr_0.8CoO_3-δ to some extent (up to y = 0.5) does not disturb the tetragonal superstructure, i.e., Sr and Gd ordering in the A-sublattice.

The XRD patterns of single-phase samples Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ with 0.1 ≤ y ≤ 0.5 were refined by the Rietveld method within the tetragonal structure 2a_p × 2a_p × 4a_p (SG I4/mmm). The typical Rietveld refinement profiles for Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ with y = 0.1 and 0.5 are shown in Figure 3, as an example. The values of the refined unit cell parameters and atom coordinates for the Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ (0.1 ≤ y ≤ 0.5) solid solutions are presented in Table 1 and Table 2, respectively.

Further introduction of iron into the cobalt sublattice at x = 0.8 or a decrease in the gadolinium content, regardless of the Co/Fe ratio, leads to a change in the structure from tetragonal with an ordered location of Gd³⁺ and Sr²⁺ in the A-sublattice to cubic with a statistical distribution of cations. The XRD patterns of Gd_1-xSr_xCo_1-yFe_yO_3-δ with x = 0.8, 0.6 ≤ y ≤ 0.9 and x = 0.9, 0.1 ≤ y ≤ 0.9 were refined in the ideal cubic perovskite structure (SG Pm-3m), similar to a Co-free gadolinium strontium ferrite Gd_1-xSr_xFeO_3-δ [36,37]. This result correlates well with the fact that Sr_0.9Gd_0.1CoO_3-δ quenched from 1373 K in air possesses a disordered cubic perovskite structure, and the structural transition from tetragonal to cubic structure occurs at 1263 K [29,30,32,35].

The XRD patterns for Gd_1-xSr_xCo_1-yFe_yO_3-δ with x = 0.8, y = 0.7 and x = 0.9, y = 0.3 processed by the Rietveld method are shown in Figure 4 as an example. The refined structural parameters for all single-phase Gd_1-xSr_xCo_1-yFe_yO_3-δ with the cubic structure are listed in Table 3.

The unit cell parameters and unit cell volume of Gd_1-xSr_xCo_1-yFe_yO_3-δ increase linearly with increasing iron content both in tetragonal (Figure 5a) and cubic (Figure 5b) structures due to the relative cation sizes of cations, since the ionic radius of iron (r_Fe³⁺(HS) = 0.645 Å, CN = 6) is greater than that of cobalt (r_Co³⁺(LS) = 0.545 Å, r_Co³⁺(HS) = 0.61 Å, CN = 6) [41].

To compare the unit cell parameters of Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ over the entire range of iron concentrations (0.1 ≤ y ≤ 0.9), the tetragonal supercell parameters were recalculated to the pseudo-cubic cell (a_cub) using the formula:

a_{c u b} = {(V / z)}^{1 / 3},

(1)

where V is the volume of a tetragonal supercell and z is the number of ABO₃ formula units belonging to one supercell (z = 16 for 2a_p × 2a_p × 4a_p supercell) (Figure 6). One can see that the plot of pseudo-cubic cell parameter (a_cub) versus iron content in Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ shows a visible gap between y = 0.5 and y = 0.6. This concentration range corresponds to an “order-disorder” structural transition similar to that observed for Gd_1-xSr_xCoO_3-δ with increasing Sr content [29,30].

2.2. Oxygen Content of Gd_1-xSr_xCo_1-yFe_yO_3-δ

Figure 7 demonstrates the changes in oxygen content for Gd_1-xSr_xCo_1-yFe_yO_3-δ with x = 0.8, 0.1 ≤ y ≤ 0.9 in step of y = 0.2 and x = 0.9, y = 0.7 versus temperature in air measured by TGA. The absolute value of oxygen content at room temperature and at 1373 K are listed in Table 4. As expected, the incorporation of iron into the cobalt sublattice Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ leads to a gradual increase in oxygen content. Since iron is a more electropositive element compared to cobalt (χ_Fe = 1.64 and χ_Co = 1.7 [42]), it acts as an electron donor (

F e_{C o}^{•}

), preventing formation of oxygen vacancies (

V_{O}^{• •}

). On the other hand, the substitution of strontium for gadolinium in Gd_1-xSr_xCo_0.3Fe_0.7O_3-δ decreases the oxygen content, since strontium serves as an acceptor-type defect (

S {r^{'}}_{G d}

) and therefore facilitates oxygen release. An excessive negative charge of acceptor-type defect (

S {r^{'}}_{G d}

) is compensated by both oxygen vacancies (

V_{O}^{• •}

) and electron holes localized on 3d-transition metal ions.

The dependences of the oxygen content in Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ versus iron concentration at room temperature and at 1373 K show a bend between y = 0.5 and y = 0.6 which also confirms the structural transition similar to the pseudo-cubic cell parameter (Figure 8).

The total variation in oxygen content for each oxide within the temperature range 298–1373 K for the tetragonal Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ (y = 0.1–0.5) is obviously lower (Δδ = 0.11–0.16) compared to that observed for oxides (y = 0.7–0.9) with the cubic structure (Δδ = 0.19–0.20). This also supports the existence of a “order-disorder” transition since the tetragonal oxides were characterized not only by a cationic superstructure, but also with the oxygen vacancies ordering [26,27,31]. As a rule, the variation in oxygen content for phases ordered in anionic sublattice is always smaller compared to that for the related disordered oxides.

Since the oxidation states of strontium, gadolinium, and oxygen ions are considered unchanged, unlike 3d-transition metals (Co, Fe), the formula of solid solution can be written as

G d_{1 - x}^{3 +} S r_{x}^{2 +} M e^{z +} O_{3 - δ}

(Me = Co and Fe), where z is mean oxidation state of 3d-transition metal ions. Thus, the electroneutrality equation can be written as follows:

3 \cdot (1 - x) + 2 \cdot x + z = 2 \cdot (3 - δ) o r z = 2 \cdot (3 - δ) + x - 3

(2)

The mean oxidation state of 3d-transition metal ions (z) at room temperature and at 1373 K calculated by Equation (3) (see Table 4) increases with increasing iron content. Considering that Co is a more electronegative cation compared to Fe, it can be assumed that at a mean oxidation state value above 3+, iron ions will be the first to increase their oxidation state to a Fe⁴⁺ form. On the other hand, in the case of the mean oxidation state of 3d-transition metal ions being lower than 3+, parts of cobalt ions will be transformed to a Co²⁺ form. According to the electroneutrality condition and the values of oxygen content, the formulas of the G_0.2Sr_0.8Co_1-yFe_yO_3-δ solid solution can be presented as follows:

at 298 K:	at 1373 K:
$G d_{0.2} S r_{0.8} C o_{0.86}^{3 +} C o_{0.04}^{4 +} F e_{0.1}^{4 +} O_{2.67}$	$G d_{0.2} S r_{0.8} C o_{0.82}^{3 +} C o_{0.08}^{2 +} F e_{0.1}^{3 +} O_{2.56}$
$G d_{0.2} S r_{0.8} C o_{0.7}^{3 +} F e_{0.04}^{3 +} F e_{0.26}^{4 +} O_{2.73}$	$G d_{0.2} S r_{0.8} C o_{0.64}^{3 +} C o_{0.06}^{2 +} F e_{0.3}^{3 +} O_{2.57}$
$G d_{0.2} S r_{0.8} C o_{0.5}^{3 +} F e_{0.2}^{3 +} F e_{0.3}^{4 +} O_{2.75}$	$G d_{0.2} S r_{0.8} C o_{0.48}^{3 +} C o_{0.02}^{2 +} F e_{0.5}^{3 +} O_{2.59}$
$G d_{0.2} S r_{0.8} C o_{0.3}^{3 +} F e_{0.22}^{3 +} F e_{0.48}^{4 +} O_{2.84}$	$G d_{0.2} S r_{0.8} C o_{0.3}^{3 +} F e_{0.62}^{3 +} F e_{0.08}^{4 +} O_{2.64}$
$G d_{0.2} S r_{0.8} C o_{0.1}^{3 +} F e_{0.38}^{3 +} F e_{0.52}^{4 +} O_{2.86}$	$G d_{0.2} S r_{0.8} C o_{0.1}^{3 +} F e_{0.76}^{3 +} F e_{0.14}^{4 +} O_{2.67}$

The obtained results illustrate that the introduction of iron in G_0.2Sr_0.8Co_1-yFe_yO_3-δ significantly changes the ratio of 3d-transition metals in various oxidation states and increases the amount of Fe⁴⁺ ions at least at room temperature in air. An increase in temperature leads to a decrease in the mean oxidation state of 3d-metal ions.

2.3. Electrical Conductivity of Gd_1-xSr_xCo_1-yFe_yO_3-δ

The temperature dependences of the Seebeck coefficient (Q) and total electrical conductivity (σ) for Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ together with Fe-free cobaltite and Co-free ferrite taken from [30,43] are shown in Figure 9a,b, correspondingly.

Positive values of Seebeck coefficient in Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ within the entire temperature range (298–1373 K) indicate a predominant p-type conductivity, which is in good correlation with the results obtained for the related rare earth/strontium cobaltites and ferrites [20,21,23,24,30,38,43,44].

The shape of temperature dependence for total conductivity in mixed Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ is generally the same as for the parent Gd_0.2Sr_0.8MeO_3-δ (Me = Fe, Co) oxides [30,43]. All of them possess a maximum at approximately 600 K. The room temperature value for Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ is noticeably higher than that for parent cobaltite and ferrite since the former has high concentration of charge carriers

G d_{0.2} S r_{0.8} C o_{0.1}^{3 +} F e_{0.38}^{3 +} F e_{0.52}^{4 +} O_{2.86}

, i.e., localized holes, already at room temperature. At relatively low temperatures (T < 600 K), where oxygen exchange between the solid and gas phases is negligible, the rise of total electrical conductivity is caused by increasing the mobility of holes localized on 3d-transition metals (

M e^{•}

) and partly by the increasing of charge carrier concentration due to the disproportionation reaction:

2 M e^{\times} = M e^{•} + M e^{/}

(3)

This disproportionation reaction (3) occurs more easily for Me = Co rather than for Me = Fe. This could be a reason for the more rapid increase of conductivity for Fe-free cobaltite compared to the mixed Co/Fe oxide and even more strongly compared to Co-free ferrite (Figure 9b). A decrease in conductivity with increasing Fe content in mixed Co/Fe oxides is often explained by considering (

F e^{•}

)as a “trap” for the holes (main charge carriers).

At T > 600 K, a decrease of total conductivity is associated with the active release of oxygen from the crystal lattice, which results in the formation of oxygen vacancies (

V_{O}^{• •}

) and a decrease in the concentration of main charge carriers—holes, according to the reaction:

O_{O}^{\times} + 2 M e_{}^{•} = \frac{1}{2} O_{2} + V_{O}^{• •} + 2 M e_{}^{\times}

(4)

where Me denotes Fe or Co atoms. The formation of oxygen vacancies disrupts the transport pathways Me–O–Me for the charge carriers which also impairs conductivity.

The activation energy for conductivity in Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ, calculated from the slope of the

\lg (σ \cdot T) = f (1 / T)

dependency in the temperature range 300–700 K, is equal 0.17 eV, which is typical for the small radius polaron-type mechanism.

Changes in the value of the Seebeck coefficient with temperature for the oxide with y = 0.7 measured in our work are in between the results reported earlier [30,43] for the unsubstituted cobaltite and ferrite and are in good agreement with the proposed conductivity model. At T < 600 the conductivity increases mainly due to a raise in the mobility of charge carriers while their concentration remains unchanged. Thus, according to the Heikes formula [45], the value of the Seebeck coefficient should decrease or remain unchanged. At T > 600 K, the concentration of charge carriers decreases with temperature (Equation (4)) and the Heikes formula [45] predicts an increase in the Seebeck coefficient. It is worth noting that the charge disproportionation process will also affect the concentration of 3d metals in various oxidation states, so a more detailed discussion of this issue at present is not possible.

3. Materials and Methods

Polycrystalline samples of Gd_1-xSr_xCo_1-yFe_yO_3-δ (x = 0.8; 0.9 and 0.1 ≤ y ≤ 0.9) were prepared by a glycerol-nitrate technique from stoichiometric amounts of pre-calcined gadolinium oxide Gd₂O₃ (99.99% purity, Lanhit, Russia), strontium carbonate SrCO₃ (99.99% purity, Lanhit, Russia), iron oxalate dihydrate FeC₂O₄·2H₂O (99.0% purity, Ormet, Russia) and metallic cobalt. Metallic cobalt was prepared by the reduction of cobalt oxide Co₃O₄ (99.0% purity, Ormet, Russia) at 673–873 K in a hydrogen flow during 6 h. Further details of glycerol-nitrate technique were described elsewhere [24,30]. Final anneals of the Gd_1-xSr_xCo_1-yFe_yO_3-δ samples were performed at 1373 K in air for 80 h with intermediate grinding. All samples were quenched to room temperature by removing from a furnace to a cold massive metallic plate (cooling rate of about 500 K/min).

The synthesized oxides were characterized by X-ray diffraction (XRD) analysis using a Shimadzu XRD 7000 instrument, Japan (Cu_Kα–radiation, angle range 2Θ = 10°–80°, scan step 0.02 with the exposure time 5 s). The structural parameters were refined by the Rietveld profile method using the Fullprof-2011 software.

Thermogravimetric measurements (TGA) were carried out using a simultaneous thermal analyzer, STA 409 PC Luxx, Netzsch, Germany, within the temperature range 298 ≤ T, K ≤ 1373 in air. The measurements were performed with the cooling/heating rate equal to 2 K/min. The absolute values of oxygen content in the sample were determined using a direct reduction of the complex oxides in TG cell by hydrogen (10% N₂–90% H₂) at 1423 K according to the following reaction:

G d_{1 - x} S r_{x} C o_{1 - y} F e_{y} O_{3 - δ} + \frac{1}{2} (3 - 2 δ + x) H_{2} \to \frac{1 - x}{2} G d_{2} O_{3} + x S r O + (1 - y) C o + y F e + \frac{1}{2} (3 - 2 δ + x) H_{2} O

(5)

The phase composition of the samples after reduction was confirmed by XRD analysis. The accuracy of the oxygen index determination was not less than ±0.01.

Total electrical conductivity (σ) was studied using a 4-probe method in the temperature range 298 ≤ T, K ≤ 1373 in air. The sample for measurements was preliminarily compacted into the form of a bar (4 mm × 4 mm × 25 mm) and sintered at 1573 K in air for 24 h, with subsequent slow cooling (1 K/min) to room temperature. The density of the polished ceramic sample was 92% of its theoretical value calculated from the XRD data. The Seebeck coefficient was measured simultaneously with conductivity using two Pt-Pt/Rh thermocouples at both ends of the ceramic sample.

4. Conclusions

It is shown that the crystal structure of Gd_1-xSr_xCo_1-yFe_yO_3-δ complex oxides significantly depends on strontium and iron contents. Fe-poor Gd_1-xSr_xCo_1-yFe_yO_3-δ oxides with x = 0.8, 0.1 ≤ y ≤ 0.5 possess a tetragonal 2a_p × 2a_p × 4a_p superstructure (SG I4/mmm) with the ordered location of Gd³⁺ and Sr²⁺ cations in the A-sublattice, whereas oxides with x = 0.8, 0.6 ≤ y ≤ 0.9 and x = 0.9, 0.1 ≤ y ≤ 0.9 possess a cubic structure (SG Pm-3m) with the statistically distributed cations in the A-sublattice. The unit cell parameters and unit cell volume are gradually increased with increasing iron content. It is shown that the oxygen content decreases with an increase in temperature and an increase in the concentration of strontium and cobalt in the samples. The change in the oxygen content for oxides with the tetragonal layered superstructure is visibly lower than that for oxides with the cubic disordered perovskite structure. The positive value of the Seebeck coefficient confirmed p-type conductivity. Total conductivity of mixed Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ oxide at room temperature exceeds the values for “pure” ferrite and cobaltite. However, at T > 400 K, an increase in the Fe content in Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ results in a decrease of conductivity value.

Author Contributions

Conceptualization, V.A.C. and T.V.A.; methodology, V.A.C. and T.V.A.; software, T.V.A.; validation, D.K.M. and T.V.A.; formal analysis, D.K.M. and T.V.A.; investigation, D.K.M. and T.V.A.; resources, V.A.C.; data curation, V.A.C. and T.V.A.; writing—original draft preparation, T.V.A.; writing—review and editing, V.A.C.; visualization, V.A.C. and T.V.A.; supervision, V.A.C.; project administration, V.A.C.; funding acquisition, V.A.C. All authors have read and agreed to the published version of the manuscript.

Funding

Ministry of Science and Higher Education of Russian Federation (State Task number AAAA-A20-120061990010-7).

Acknowledgments

The work was financially supported by the Ministry of Science and Higher Education of Russian Federation (State Task number AAAA-A20-120061990010-7).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structural model of the unit cell for a tetragonal Ln_0.25Sr_0.75CoO_3-δ complex oxide.

Figure 2. XRD patterns of the Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ solid solutions with various iron content.

Figure 3. XRD patterns for Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ: y = 0.1 (a) and y = 0.5 (b) refined by the Rietveld method. Points represent the experimental data and the solid curve is the calculated profile. A difference curve is plotted at the bottom. Vertical lines mark the positions of allowed Bragg reflections. Arrows indicate super-structural reflections for the 2a_p × 2a_p × 4a_p tetragonal cell.

Figure 4. XRD patterns for Gd_1-xSr_xCo_1-yFe_yO_3-δ: (a) x = 0.8, y = 0.7 and (b) x = 0.9, y = 0.3 refined by the Rietveld method. Points represent the experimental data and the solid curve is the calculated profile. A difference curve is plotted at the bottom. Vertical lines represent the positions of allowed Bragg reflections.

Figure 5. The unit cell parameters of Gd_1-xSr_xCo_1-yFe_yO_3-δ versus iron content (y): (a) x = 0.8, 0.1 ≤ y ≤ 0.5; (b) x = 0.8, 0.6 ≤ y ≤ 0.9 and x = 0.9, 0.1 ≤ y ≤ 0.9.

Figure 6. The pseudo-cubic cell parameters of the quenched Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ oxides versus iron content (y).

Figure 7. Temperature dependencies of oxygen content in Gd_1-xSr_xCo_1-yFe_yO_3-δ in air.

Figure 8. The oxygen content in Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ versus iron content.

Figure 9. Temperature dependencies of the Seebeck coefficient (a) and total electrical conductivity (b) for Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ (y = 0.0; 0.7; 1.0) in air.

Table 1. The unit cell parameters (SG I4/mmm) of Gd_0.2Sr_0.8Co_1-yFe_yO_3-δ (0.1 ≤ y ≤ 0.5) quenched from 1373 K in air and R-factors refined by the Rietveld method.

y	a, Å	c, Å	V, (Å)³	R-Factors, %
y	a, Å	c, Å	V, (Å)³	R_Br	R_f	R_p
0.1	7.674(1)	15.401(1)	907.15(2)	6.32	11.3	8.14
0.2	7.685(1)	15.403(1)	909.79(2)	5.61	9.66	7.53
0.3	7.693(1)	15.408(1)	912.13(1)	5.85	11.6	9.67
0.4	7.697(1)	15.412(1)	913.09(3)	6.24	11.5	7.19
0.5	7.704(1)	15.426(1)	915.59(3)	6.13	9.25	7.82

Table 2. Atomic coordinates in the tetragonal cell of Gd_0.2Sr_0.8Co_0.8Fe_0.2O_3-δ refined by the Rietveld analysis.

Atom	x	y	z
Co1/Fe1	0.245(1)	0.245(1)	0
Co2/Fe2	0.250	0.250	0.250
Sr1/Gd1	0	0	0.138(1)
Sr2	0	0	0.627(1)
Sr3	0	0.5	0.130(1)
O1	0.221(2)	0.221(2)	0.117(1)
O2	0.179(1)	0	0
O3	0.211(3)	0.5	0
O4	0	0.238(4)	0.251(1)

Table 3. The structural parameters for the Gd_1-xSr_xCo_1-yFe_yO_3-δ solid solution quenched from 1373 K in air (SG Pm-3m) and R-factors refined by the Rietveld method.

SG Pm-3m: Gd/Sr (0.5;0.5;0.5); Fe/Co (0; 0; 0); O (0.5; 0; 0)							R-Factors, %
x	y	a, Å	V, (Å)³	d_Fe/Co-O, Å	d_Gd/Sr-O, Å	d_Gd/Sr-Fe/Co, Å	R_Br	R_f	R_p
0.8	0.6	3.851(1)	57.11(1)	1.925(1)	2.723(1)	3.335(1)	5.06	4.84	8.46
	0.7	3.857(1)	57.40(1)	1.928(1)	2.727(1)	3.340(1)	3.79	4.85	7.04
	0.8	3.860(1)	57.52(1)	1.930(1)	2.729(1)	3.343(1)	5.35	4.19	11.6
	0.9	3.864(1)	57.72(1)	1.932(1)	2.732(1)	3.346(1)	4.52	3.93	9.60
0.9	0.1	3.849(1)	57.03(1)	1.924(1)	2.721(1)	3.333(1)	5.46	4.24	9.58
	0.3	3.856(1)	57.36(1)	1.928(1)	2.727(1)	3.340(1)	2.49	2.21	7.24
	0.5	3.861(1)	57.59(1)	1.930(1)	2.730(1)	3.344(1)	5.95	3.93	7.88
	0.7	3.864(1)	57.70(1)	1.932(1)	2.732(1)	3.346(1)	3.33	2.41	9.59
	0.9	3.870(1)	57.99(1)	1.934(1)	2.734(1)	3.348(1)	1.11	2.98	7.90

Table 4. Oxygen content and mean oxidation state of 3d-transition metals in Gd_1-xSr_xCo_1-yFe_yO_3-δ in air.

Composition	T, K	Oxygen Content 3-δ	Mean Oxidation State of 3d-Transition Metals
Gd_0.2Sr_0.8Co_0.9Fe_0.1O_3-δ	298 1373	2.67 ± 0.01 2.56 ± 0.01	3.14 2.92
Gd_0.2Sr_0.8Co_0.7Fe_0.3O_3-δ	298 1373	2.73 ± 0.01 2.57 ± 0.01	3.26 2.94
Gd_0.2Sr_0.8Co_0.5Fe_0.5O_3-δ	298 1373	2.75 ± 0.01 2.59 ± 0.01	3.30 2.98
Gd_0.2Sr_0.8Co_0.3Fe_0.7O_3-δ	298 1373	2.84 ± 0.01 2.64 ± 0.01	3.48 3.08
Gd_0.2Sr_0.8Co_0.1Fe_0.9O_3-δ	298 1373	2.86 ± 0.01 2.67 ± 0.01	3.52 3.14
Gd_0.1Sr_0.9Co_0.3Fe_0.7O_3-δ	298 1373	2.81 ± 0.01 2.58 ± 0.01	3.52 3.06

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Aksenova, T.V.; Mysik, D.K.; Cherepanov, V.A. Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application. Catalysts 2022, 12, 1344. https://doi.org/10.3390/catal12111344

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Aksenova TV, Mysik DK, Cherepanov VA. Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application. Catalysts. 2022; 12(11):1344. https://doi.org/10.3390/catal12111344

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Aksenova, Tatiana V., Darya K. Mysik, and Vladimir A. Cherepanov. 2022. "Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application" Catalysts 12, no. 11: 1344. https://doi.org/10.3390/catal12111344

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Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application

Abstract

1. Introduction

2. Results and Discussion

2.1. Crystal Structure of Gd_1-xSr_xCo_1-yFe_yO_3-δ

2.2. Oxygen Content of Gd_1-xSr_xCo_1-yFe_yO_3-δ

2.3. Electrical Conductivity of Gd_1-xSr_xCo_1-yFe_yO_3-δ

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Crystal Structure and Properties of Gd1-xSrxCo1-yFeyO3-δ Oxides as Promising Materials for Catalytic and SOFC Application

Abstract

1. Introduction

2. Results and Discussion

2.1. Crystal Structure of Gd1-xSrxCo1-yFeyO3-δ

2.2. Oxygen Content of Gd1-xSrxCo1-yFeyO3-δ

2.3. Electrical Conductivity of Gd1-xSrxCo1-yFeyO3-δ

3. Materials and Methods

4. Conclusions

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Crystal Structure and Properties of Gd_1-xSr_xCo_1-yFe_yO_3-δ Oxides as Promising Materials for Catalytic and SOFC Application

2.1. Crystal Structure of Gd_1-xSr_xCo_1-yFe_yO_3-δ

2.2. Oxygen Content of Gd_1-xSr_xCo_1-yFe_yO_3-δ

2.3. Electrical Conductivity of Gd_1-xSr_xCo_1-yFe_yO_3-δ