Thermogravimetric analysis showed a very low level of O deficiency (δ) in CaFe2O4−δ, varying in the range 0.003–0.006 at O partial pressures from 10− 5 to 0.21atm and 295–1223K, in agreement with Mössbauer spectroscopy. This resulted in substantial dimensional stability on reducing PO2 as revealed by the controlled-atmosphere dilatometry, but also in low concentrations of the ionic and electronic charge carriers. The steady-state O permeability of dense CaFe2O4−δ ceramics was determined by both bulk ionic conduction and surface exchange kinetics. The O-ion transference numbers calculated from the faradaic efficiency and permeation data, were (0.2–7.2) x 10−4 at 1123 to 1273K, increasing with temperature. The atomistic computer simulations indicate a significant energetic affinity for the defect cluster formation involving O vacancies and Fe2+ cations, and unfavorable vacancy location in the O sites forming one-dimensional pathways with minimum migration energy. As a consequence, the partial O-ionic conductivity of CaFe2O4−δ was as low as 3–210μS/cm at 1123 to 1273K in air. The average thermal expansion coefficients of calcium ferrite ceramics lie in the range (12.0–13.9) x 10−6/K.

Oxygen Ionic Conductivity, Mössbauer Spectra and Thermal Expansion  of CaFe2O4-δ. E.V.Tsipis, Y.V.Pivak, J.C.Waerenborgh, V.A.Kolotygin, A.P.Viskup, V.V.Kharton: Solid State Ionics, 2007, 178[25-26], 1428-36