Conductivity of iron-doped strontium titanate in the quenched and degraded states

The electrical behavior of iron-doped strontium titanate (Fe:SrTiO ₃ ) single crystals equilibrated at 900°C and quenched below 400°C at various oxygen partial pressures (PO ₂ ) was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO ₃ annealed and quenched between 1.2 × 10 ⁻¹⁴ and 2.0 × 10 ⁻⁴  Pa PO ₂ exhibits a conduction activation energy (E _A ) around 0.6 eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in E _A are found to either side of this range; a transition from 0.6 to 1 eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23 eV is found in reducing PO ₂ These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO ₃ provides insight into the origins of the conductivity activation energies observed in those samples.