Sol-gel formation, Moessbauer studies, optical absorption and impedance characteristics of Ba_(0.5)Sr_(0.5)Zn_(0.2)Fe_(0.8)O_(3-ζ)powder

摘要

ABO_(3-ζ)-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 Ba_(0.5)Sr_(0.5)Zn_(0.2)Fe_(0.8)O_(3-ζ) 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 ℃ for 24 h, and decomposition of oxalate at 950 ℃ for 15 h in air. The product is shown to exhibit (i) a perovskite-type cubic phase with a = 3.975 ± 0.002 A, Z = 1, and space group Pm3m, (ii) Fe~(~(3+))_(0.5) and Fe~(4+)n.3 ions, (iii) oxygen deficiency parameter ζ ～ 0.45, and (iv) optical absorption at ～370 nm (～3.4 eV) and ～797 nm (～1.56 eV) - arising due to charge transfer transition from O~2～(2p) to Fe~(~(3+))(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.