Oxygen ionic conductivity, Mössbauer spectra and thermal expansion of CaFe2O4 - δ

Thermogravimetric analysis showed a very low level of oxygen deficiency (δ) in CaFe2O4 − δ, varying in the range 0.003–0.006 at oxygen partial pressures from 10− 5 to 0.21 atm and 295–1223 K, in agreement with Mössbauer spectroscopy. This results in substantial dimensional stability on reducing p(O2) as revealed by the controlled-atmosphere dilatometry, but also in low concentrations of the ionic and electronic charge carriers. The steady-state oxygen permeability of dense CaFe2O4 − δ ceramics is determined by both bulk ionic conduction and surface exchange kinetics. The oxygen-ion transference numbers calculated from the faradaic efficiency and permeation data, are (0.2–7.2) × 10− 4 at 1123–1273 K, increasing with temperature. The atomistic computer simulations indicate a significant energetic affinity for the defect cluster formation involving oxygen vacancies and Fe2+ cations, and unfavorable vacancy location in the oxygen sites forming one-dimensional pathways with minimum migration energy. As a consequence, the partial oxygen-ionic conductivity of CaFe2O4 − δ is as low as 3–210 μS/cm at 1123–1273 K in air. The average thermal expansion coefficients of calcium ferrite ceramics lie in the range (12.0–13.9) × 10− 6 K− 1.