Sol-gel formation, Mossbauer studies, optical absorption and impedance characteristics of Ba0.5Sr0.5Zn0.2Fe0.8O3-ξ powder

ABO(3-xi)-type oxides have gained prominence because of their usefulness in gas separation, solid oxide fuel cell, gas sensor, etc. Of particular interest is barium and zinc substituted strontium ferrite - an alternative cathode material due to its high structural stability, accommodation of considerable anion deficiency, and good oxygen permeability. An attempt has therefore been made to synthesize Ba0.5Sr0.5Zn0.2Fe0.8O3-xi powder by a novel oxalate sol-gel route to investigate its formation, nature of iron species, optical absorption, and impedance behaviour. The synthesis process involves gel formation, digestion for 6 h, drying at 150 degrees C for 24 h, and decomposition of oxalate at 950 degrees C for 15 h in air. The product is shown to exhibit (i) a perovskite-type cubic phase with a = 3.975 +/- 0.002 angstrom, Z = 1, and space group Pm3m, (ii) Fe(3+)0.5 and Fe(4+)0.3 ions, (iii) oxygen deficiency parameter xi similar to 0.45, and (iv) optical absorption at similar to 370 nm ( similar to 3.4 eV) and similar to 797 nm (similar to 1.56 eV) - arising due to charge transfer transition from O2-(2p) to Fe3+(3d) and octahedral crystal field splitting of iron t(2g) and e(g) orbitals, respectively. Moreover, the high impedance values observed below 10 kHz over a temperature range of 303-413 K have been attributed to space charge polarization; activation energy of the relaxation process being 0.2 eV. The motion of induced polarons is possibly responsible for the decrease of impedance with increase of temperature in the range 303-413 K. (c) 2012 Elsevier B.V. All rights reserved.