Structure and properties of the layered perovskites in Sm-Ba-Co-Fe-O System
March 10-14, 2019
Perovskite oxide materials with the general formula of ABO3, where A is an alkali earth or rare earth metals and B is transition metals, have attracted much attention as cathodes for solid oxide fuel cells because of their high electronic conductivity and fast mobility of oxygen ions. The introduction of Ba2+ and Ln3+ ions with significantly different radii into the A-sites leads to a formation of layered perovskite-type structures which have formed due to the cations’ ordering in the alternating layers. Depending on the nature of rare earth and 3d metal, it was possible to obtain double LnBaM2O6-δ, triple LnBa2M3O9-δ, or quintuple Ln2Ba3M5O15-δ perovskites. The aim of the present work was studying the effect of Ln/Ba and Fe/Co ratio for the crystal and defect structure and properties of oxides in the Sm-Ba-Co-Fe-O system.
Polycrystalline samples of SmBaCo2−xFexO6-δ and Sm2-εBa3+εFe5-yCoyO15-δ were prepared by the glycine–nitrate synthesis. Final annealing was performed at 1100°С in air during 120h with intermediate grindings, followed by slow cooling down to room temperature at a rate of about 100°/h. The structural parameters were refined by the Rietveld method using the Fullprof-2008 software. Transmission electron microscopy studies were performed using FEI Tecnai G2 30 UT microscope operated at 300kV. The changes of oxygen content in complex oxides were measured by coulometric titration method as a function of temperature and oxygen partial pressure. The absolute value of oxygen content in the samples was determined using a direct reduction in the TG cell by hydrogen flow and red-ox titration. Thermal expansion of samples was studied using Netzsch DIL 402C dilatometer within the temperature range 25 – 1100°С in air. Total conductivity and Seebeck coefficient were measured simultaneously using a 4-probe technique.
The crystal structure of SmBaCo2−xFexO6-δ (0≤x≤0.5) was described by the orthorhombic ap×2ap×2ap cell (Pmmm sp. gr.), while SmBaCo2−xFexO6-δ (0.6≤x≤1.1) crystallized in the tetragonal structure, ap×ap×2ap cell (P4/mmm sp. gr.). The crystal structure of single-phase Sm2-εBa3+εFe5-yCoyO15-δ (ε = 0, y = 0.5–1.5; ε = 0.125, y = 0) determined by XRD was described as cubic (sp. gr. Pm3m). However, transmission electronic microscopy revealed that oxides possess tetragonal structure with 5-fold c parameter.
The defect structure of oxides with double perovskite structure was described using the model based on the simple cubic perovskite SmMeO3 (Me = Co, Fe) as a reference state. Equilibrium constants and enthalpies of the point defects formation were refined. The concentrations of all defect species were calculated as functions of temperature and oxygen nonstoichiometry.
The temperature dependencies of total conductivity for SmBaCo2−xFexO6-δ and Sm2-εBa3+εFe5-yCoyO15-δ possess maxima at approximately 300–350ºC in air. The partial substitution of iron for cobalt leads to a decrease in the conductivity value. Seebeck coefficient for all compounds reveals positive values within the entire temperature and oxygen partial pressure ranges that indicate predominant p-type conductivity.
The dependencies of electrical conductivity and Seebeck coefficient versus oxygen nonstoichiometry were discussed on the basis of the defect structure models. The values of activation energy for fixed oxygen content values were calculated.
This work was supported by the Russian Science Foundation (Grant № 18-73-00159)
Nadezhda E. Volkova, Vladimir A. Cherepanov, Albert A. Akhmadeev, Ivan B. Golovachev, and Mikhail Yu Mychinko, "Structure and properties of the layered perovskites in Sm-Ba-Co-Fe-O System" in "Nonstoichiometric Compounds VII", ECI Symposium Series, (2019). https://dc.engconfintl.org/nonstoichiometric_vii/9