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

ABO3−ξ-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−ξ 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 °C for 24 h, and decomposition of oxalate at 950 °C for 15 h in air. The product is shown to exhibit (i) a perovskite-type cubic phase with a = 3.975 ± 0.002 Å, Z = 1, and space group Pm3m, (ii) Fe3+0.5 and Fe4+0.3 ions, (iii) oxygen deficiency parameter ξ ∼ 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 O2−(2p) to Fe3+(3d) and octahedral crystal field splitting of iron t2g and eg 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.

► Synthesis of Ba0.5Sr0.5Zn0.2Fe0.8O3−ξ (ξ∼0.45) with a novel oxalate sol–gel route. ► Existence of a stable cubic phase (a ∼ 3.975 Å) with Fe3+ and Fe4+ ions in 5:3 ratio. ► Optical absorption via O2−(2p) → Fe3+(3d) transition and crystal field splitting. ► Observation of high impedance below 10 kHz due to space charge polarization. ► Conduction via small polaron, i.e., electron hopping with lattice distortion.