Structural and electrical charge transport properties in oxygen-deficient PbTiO3−δ ceramics

Oxygen-deficient perovskites (PbTi_1−xFe_xLi_yO_3−δ, x = 0–0.25, y = 0.15) were prepared by gel combustion technique. This study presents the effect of Fe³⁺ and Li⁺ on the morpho-structural and electrical properties of ceramics. Two structural behaviors have been identified, depending on the Fe³⁺ amount. At low Fe³⁺ concentrations (3.7%), the variation of the cell parameters is given by the tetragonal change toward cubic phase while at higher levels (>15%) the variation of the cell parameters comes mainly from Fe³⁺/Ti⁴⁺ ionic radii differences. Fe³⁺ doping at Ti sites creates oxygen and titan vacancies in order to compensate the Ti⁴⁺ charge. Introduction of oxygen vacancies reduces progressively band gap energy from 3.28 to 2.63 eV. ICP-OES measurements show that Pb and Ti are lower than theoretical formula which it generates supplementary contributions to the oxygen deficiency. Addition of Fe³⁺ and Li⁺ leads to an increase of lattice micro-strains from 17.07·10⁻⁴ up to 24.44·10⁻⁴, improving ionic conduction. Moreover, DFT calculation shows that the lattice distortion tends to decrease with the increase of the Fe concentration, in agreement with the XRD. Based on BET analysis, the pore diameter decreases from 56.9 to 16.6 nm with the increase of iron amount and is correlated with the relative densities that increase from 82.0 to 91.27%. According to EIS investigations, activation energy varies between 0.632 and 0.950 eV, showing that the conduction in perovskite ceramics is based on double ionized oxygen vacancies. The highest conductivity at 500 °C was obtained for samples doped with 15% Fe and 25% Fe (2.4 × 10⁻³ S·cm⁻¹), sintered at 750 °C.

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