Oxygen Nonstoichiometry and Ionic Conductivity of Sr₃Fe_2-xSc_xO_7-δ

Abstract

The substitution of scandium for iron in the Ruddlesden−Popper Sr3Fe2 - x Sc x O7 - δ (x = 0−0.3) system increases tetragonal unit-cell volume and oxygen nonstoichiometry and decreases partial p- and n-type electronic conductivities studied in the oxygen partial pressure range from 1 × 10-20 to 0.7 atm at 973−1223 K. The solubility of Sc3+ corresponds to approximately x ≈ 0.35. The relatively low, temperature-activated hole mobility indicates a small-polaron mechanism of the electronic transport, as for undoped Sr3Fe2O7 - δ. The atomistic computer simulations showed that major contribution to the ionic conductivity is provided by the oxygen sites surrounded by iron cations in the perovskite-type layers of Sr3(Fe,Sc)2O7- δ structure, whereas stable ScO6 octahedra are essentially excluded from the oxygen diffusion processes. Minimum migration energy, 0.9−1.4 eV, was found for nonlinear pathways formed by the apical O1 sites linking iron−oxygen polyhedra along the c-axis and equatorial O3 positions in the perovskite-type planes. The direct O3 → O3 jumps are characterized with higher energetic barrier, 1.5−2.2 eV. Because of the increasing concentration of vacant O3 sites induced by scandium doping, the apparent activation energy for oxygen−ionic transport decreases from about 2 eV, as observed for undoped strontium ferrite at 1123−1223 K, down to 0.95−1.15 eV for Sr3Fe2 - x Sc x O7 - δ (x = 0.2−0.3). As a result, the partial ionic conductivity of Sr3Fe1.7Sc0.3O7- δ at temperatures below 1000 K becomes higher than that in Sr3Fe2O7 - δ.

Oxygen Ion‐Conducting Materials

“…Ruddlesden-Popper series, the partial electronic and ionic conductivities tend to decrease when the concentration of rock salt-type (Ln,A) 2 O 2 layers increases; the general trends observed on acceptor doping are similar to those in the perovskite systems (e.g., Refs [134,165,167,168]). If compared to manganite electrode materials, one important disadvantage of perovskite-related ferrites relates to a high chemical expansion, which provides a critical contribution to the apparent TECs due to oxygen losses at elevated temperatures, and may lead to a thermomechanical incompatibility with common solid electrolytes [117,162,176,177].…”

“…These primarily include perovskite-like (Ln,A)FeO 3Àd and their derivatives existing in all Ln-A-Fe-O systems, A 2 Fe 2 O 5AEd brownmillerites, (Ln,A) 3 Fe 5 O 12AEd garnets in the systems with relatively small Ln 3 þ cations, Ruddlesden-Popper series (Ln,A) n þ 1 Fe n O z , and a variety of other intergrowth compounds such as Sr 4 Fe 6 O 13AEd (see Refs [4,8,17,98,134,152,[157][158][159][160][161][162][163][164][165][166][167][168] and references cited therein). However, due to structural constrains and defect chemistry features limiting both ionic and electronic transport, in most cases an extensive iron substitution is necessary to achieve the total conductivity higher than 10-30 S cm À1 and partial ionic conductivity higher than 0.1 S cm À1 at temperatures above 700 K. For B-site-undoped ferrites, the maximum conductivity is characteristic of perovskite-related solid solutions, such as Ln 1Àx Sr x FeO 3Àd , where the highest level of electronic and ionic transport is known for Ln ¼ La and x % 0.5.…”

“…If compared to manganite electrode materials, one important disadvantage of perovskite-related ferrites relates to a high chemical expansion, which provides a critical contribution to the apparent TECs due to oxygen losses at elevated temperatures, and may lead to a thermomechanical incompatibility with common solid electrolytes [117,162,176,177]. The layered ferrite phases -particularly the Ruddlesden-Popper series and brownmillerites -display modest stoichiometry changes and better thermomechanical properties compared to the perovskite compounds, although their compatibility with electrolyte ceramics is still limited [133,134,165,167,168,178]. For the membrane reactor materials, the thermomechanical and thermodynamic instabilities typical for undoped ferrite ceramics require extensive compositional modifications, such as the substitution of more than 20-30% iron with other cations (e.g., Ti, Cr, Ga, Al) and/or the fabrication of ferrite-based composites containing dimensionally stable components [4, 7, 8, 17, 102, 114-117, 152, 157, 162-166].…”

Design and experimental investigation of oxide ceramic dual-phase membranes

“…Among the developed materials, cobalt-doped Ln x (Ba,Sr,Ca) 1−x Co y Fe 1−y O 3−ı (Ln: rare earth metal, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1) usually have high oxygen permeation fluxes but are unstable under reducing environments and long-term operation at high temperatures [11][12][13][14][15][16][17][18][19][20]. Ln x (Ba,Sr,Ca) 1−x Fe y (Zn,Al,Ga,Ti,Zr,Ce) 1−y O 3−ı (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) usually possesses high stability under reducing environments but poor permeation fluxes [21][22][23][24][25][26][27][28][29][30][31]. Researchers have found that it is difficult to obtain a membrane material with high stability and permeation flux based on perovskite or perovskite-related structures [32,33].…”

Oxygen nonstoichiometry, chemical expansion, mixed conductivity, and anodic behavior of Mo-substituted Sr3Fe2O7-δ

“…An attractive combination of properties important for these applications, namely a relatively high conductivity, moderate thermal expansion coefficients (TECs), and substantial stability in reducing atmospheres, is known for the Ruddlesden-Popper (RP) type Sr 3 Fe 2 O 7-δ [5][6][7][8][9][10][11][12]. The intergrowth structure of this ferrite ( Fig.…”