Electronic properties of polycrystalline LaFeO3. Part I: Experimental results and the qualitative role of Schottky defects

The isothermal electrical conductivity of La1−yFeO3−δ (y = − 0.003, 0.000 and + 0.003) ceramics has been investigated at 1000 °C in oxygen partial pressures from 1 to 10− 17 atm, and found to be essentially independent of the composition (y). Both La2O3-rich and Fe2O3-rich secondary phases were identified by transmission electron microscopy in the non-stoichiometric materials (y = − 0.003 and + 0.003), and a narrow homogeneity range for La1±yFeO3−δ (y < 3 · 10− 3) was inferred. The materials exhibited n-type conductivity at low oxygen partial pressures due to the reduction of Fe3+ to Fe2+ and formation of oxygen vacancies, consistent with previous reports. The rate of the reduction was controlled by oxygen diffusion and a time independent conductivity was established relatively fast. For oxygen pressures between 10− 11 and 10− 4 atm, the time independent conductivity was essentially independent of variations in the oxygen pressure. This was explained by a fully occupied oxygen lattice, implying that the material could not be further oxidized without formation of new oxygen sites. At the highest oxygen partial pressures (PO2 > 10− 4 atm), p-type conductivity was evident. However, in this regime, the conductivity was time dependent on a time scale of days or weeks, inconsistent with previous reports. Possible mechanisms for the p-type conductivity and the slow change in the conductivity at high oxygen partial pressures are discussed, and a new defect model is put forward, which includes formation of thermally activated Schottky defects. The model accounts qualitatively for the time dependent conductivity and points out the importance of cation vacancies and cation diffusion at high oxygen partial pressures.