Studies on the Crystal Structure, Electrical Conductivity and Oxygen Transport Parameter of A0.5Sr0.5Co0.2-xBxFe0.8O3- [A = La, Ba; B = Zn, Al; x = 0 – 0.2] Perovskite Oxides

Abstract

La0.5Sr0.5Co0.2Fe0.8O3-δ (LSCF) and Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF) powders were prepared by combined EDTA-citrate combustion route, and both the powders showed a single perovskite phase. The LSCF oxides exhibited rhombohedral crystal structure, while BSCF oxides showed cubic structure. The thermogravimetric study indicated that the mass loss behavior is dominated by the thermal reductions and the oxygen non-stoichiometry in the samples. The room temperature and the high-temperature oxygen non-stoichiometry study revealed that the BSCF oxides contain higher oxygen non-stoichiometry than LSCF. The thermal expansion coefficient (TEC) of LSCF and BSCF samples measured from RT – 800 oC were found to be 17.58 and 17.32×10-6 K-1, respectively. Microstructure and electrical properties were significantly affected by the variation in the sintering condition (temperature and time). The grain size (d50) of the LSCF and BSCF samples was 0.75 μm and 19.5 μm, respectively, when sintered at a temperature of 1150 oC for 4 h. Temperature-dependent conductivity of the samples showed metallic conductivity in the high-temperature region and exhibited conductivity hysteresis while measured on heating and cooling. The conductivity of BSCF samples is lower than that of LSCF samples when sintered under identical conditions. The observed difference is comprehended from the combined effect of the sintered sample's defect chemistry change and grain size. The faster electrical conductivity relaxation kinetics noticed in BSCF oxides compared to LSCF oxides is correlated to the combined contribution of higher oxygen vacancy concentration and mobility of oxygen vacancies. The values of surface exchange kinetics (Kchem) measured at 850 oC was found to be 2.86×10-4 cm s-1 and 7.54×10-2 cm s-1 while the values of chemical bulk diffusion coefficient (Dchem) were 1.6×10-6 cm2 s-1 and 8.62×10-3 cm2 s-1 for LSCF and BSCF oxides respectively. Grain size effect on oxygen exchange kinetics of LSCF and BSCF oxides revealed that both Dchem and Kchem increase with the grain size increase.
A thorough study on zinc substituted A0.5Sr0.5Co0.2-xZnxFe0.8O3-δ (A = La and Ba; x = 0 – 0.2) perovskite oxides has been carried out. All the prepared La-containing powders exhibited the distorted rhombohedral crystal structure, while the Ba-based powders showed cubic symmetry. The lattice volume, oxygen non-stoichiometry, electrical conductivity, and oxygen transport parameters varied drastically with the substitution of zinc in place of cobalt in the system. The microstructural study revealed the grain size enhancement with the increase in the zinc content in La-based samples when identical sintering conditions were maintained. A typical nature showing an increase in the grain size of 11.2 – 14.3 μm from 0.0 – 0.10 zinc content and then decrease is noticed in Ba0.5Sr0.5Co0.2-xZnxFe0.8O3-δ (x = 0 – 0.2) system. The La0.5Sr0.5Co0.1Zn0.1Fe0.8O3-δ (LSCZF10) sample showed the highest electrical conductivity while the minimum for the La0.5Sr0.5Zn0.2Fe0.8O3-δ (LSZF) sample. In Ba-containing zinc substituted oxides, a linear decrease in the electrical conductivity is observed. The conductivity deviation observed with substitution level is a combined effect of the difference in concentration of charge carriers, the oxygen vacancies, the average B-site ionic radius, and the grain size of the samples. Oxides prepared with x = 0.10 (LSCZF10) exhibited the highest Kchem (1.11×10-3 cm s-1), and Dchem (1.19×10-5 cm2
s-1) values measured at 850 oC, compared to others. In the Ba0.5Sr0.5Co0.2-xZnxFe0.8O3-δ (BSCZF) system, the calculated Dchem values were found to vary between 3.9×10-4 to 4.5×10-3 cm2 s-1 while the Kchem values varied between 5.4×10-3 to 9.8×10-3 cm s-1 with increasing substitution level (x) in the range 0 – 0.2.
The electrical transport properties, namely electrical conductivity, chemical bulk diffusion coefficient (Dchem), and surface exchange coefficient (Kchem) of A0.5Sr0.5Co0.2-xAlxFe0.8O3-δ (A = La and Ba; x = 0 – 0.2) perovskite oxides were studied systematically. All the studied compositions exhibited a distorted rhombohedral and cubic crystal structure for lanthanum and barium-based oxides, respectively, not prejudiced by the aluminum substitution level in the system. X-ray photoelectron spectroscopic (XPS) study confirms the variation in the average oxidation state B-site cations and active oxygen vacancy concentration in the samples as a function of aluminum substitution. The lattice volume in La0.5Sr0.5Co0.2-xAlxFe0.8O3-δ (x = 0 – 0.2) (LSCAF) series found to be least (347.0761 Å3), while the oxygen non-stoichiometry () found to be highest (0.12) in the sample prepared with x = 0.1 as compared to other studied ones. The lattice parameters of the Ba0.5Sr0.5Co0.2-xAlxFe0.8O3-δ (BACAF) powders synthesized in the present work were in the range of 3.9557 – 3.9564 Å. The microstructural study revealed grain size refinement with increasing aluminum content in the sample when sintered under identical conditions, irrespective of the change in A-site cation substituent. In the Ba0.5Sr0.5Co0.2-xAlxFe0.8O3-δ (BSCAF) system, with increasing substitution level in the range 0 – 0.2, the Kchem and Dchem values ranged from 5.6×10-3 to 9.4×10-3 cm s-1 and 2.6×10-4 to 1.3×10-3 cm2s-1 respectively. The values of Dchem and Kchem of La0.5Sr0.5Al0.2Fe0.8O3-δ calculated from ECR spectra were found to vary between 2.03×10-5 cm2 s-1 to 1.99×10-6 cm2 s-1 and 1.034×10-3 cm s-1 to 5.26×10-4 cm s-1 respectively, with an increase in the grain size ranging from 0.93 – 5.88 μm.