J.C. Waerenborgh

Redox behavior and transport properties of La0.5−xSr0.5−xFe0.4Ti0.6O3−δ (0<x<0.1) validated by Mössbauer spectroscopy

The range of perovskite-type solid solution formation in the system La0.5−xSr0.5−xTi0.6Fe0.4O3−δ in oxidizing conditions is determined from X-ray diffraction and Mossbauer spectroscopy data to correspond to approximately 0–10% of the A-site concentration, similar to other numerous perovskite systems. Reduction and subsequent reoxidation of the oxides leads to a narrowing of this range and the segregation of the hematite phase at x=0.05. Increasing A-site deficiency results in the formation of oxygen vacancies and decreasing stability in reducing environments. The total conductivity of La0.5−xSr0.5−xTi0.6Fe0.4O3−δ (x=0.02–0.10) ceramics is essentially independent of composition in air and increases with increasing x in reducing atmospheres, due to increasing concentrations of n-type charge carriers and the formation of metallic iron. Partial decomposition of the perovskite phase in hydrogen, resulting in metal Fe formation, was found to be reversible when the A-site deficiency is small (x<0.05). Mossbauer spectroscopic data showed that, contrary to other perovskite-type titanates–ferrites, the concentration of Fe4+ cations in the perovskite lattice of oxidized La0.5−xSr0.5−xTi0.6Fe0.4O3−δ is negligible.

Synthesis of Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}} electrocatalysts by mechanical activation

Potassium-substituted SrFeO{sub 3-{delta}} for possible application as oxygen evolution electrode in alkaline or molten salt media was prepared by mechanical activation and characterized by X-ray diffraction, dilatometric and thermogravimetric analysis, Moessbauer spectroscopy, and electrical conductivity measurements. Room temperature mechanical activation of a mixture of oxide precursors with subsequent thermal treatments at 700-900 Degree-Sign C results in the formation of Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}} with tetragonal perovskite-like structure. Such allows to decrease the synthesis temperature, if compared to the conventional solid-state route, and to prevent possible volatilization of potassium. The results of Moessbauer spectroscopy studies indicate that the oxygen nonstoichiometry in the samples annealed in air at 900-1100 Degree-Sign C with subsequent rapid cooling vary in the range {delta}=0.30-0.32. The electrical conductivity in air exhibits a metal-like behaviour at temperatures above 400 Degree-Sign C and semiconductor behaviour in the low-temperature range, reaching 13-30 S/cm under prospective operation conditions for alkaline electrolyzers ({<=}90 Degree-Sign C). - graphical abstract: XRD patterns of Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}} powders, as-prepared and after annealing at different temperatures. Log({sigma}{center_dot}T) vs. 1000/T plot of the electrical conductivity of Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}}. The inset shows the thermal variation of {sigma}. Ceramics used were prepared by mechanical activation followedmorexa0» by a two-step sintering process at 900 Degree-Sign C for 1 h and 1000 Degree-Sign C for 5 h (82% densification). Highlights: Black-Right-Pointing-Pointer Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}} was successfully obtained by mechanical activation of oxide precursors. Black-Right-Pointing-Pointer Synthesis temperature is significantly lower when compared to a conventional solid-state route. Black-Right-Pointing-Pointer Oxygen nonstoichiometry of annealed samples at 900-1100 Degree-Sign C vary in the range {delta}=0.30-0.32. Black-Right-Pointing-Pointer Sr{sub 0.9}K{sub 0.1}FeO{sub 3-{delta}} shows metal and semiconductor behaviour above and below 400 Degree-Sign C, respectively.«xa0less