Mixed conductivity of garnet phases based on gadolinium ferrite

Incorporation of Pr and Ca into the garnet lattice of Gd3Fe5O12±δ was found to increase the electronic conductivity, and to decrease the thermal expansion coefficients and oxygen ion transference numbers. The ionic conductivity of Gd2.2Pr0.8Fe5O12±δ ceramics is similar to that of undoped gadolinium ferrite, whilst Gd2.5Ca0.5Fe5O12-δ exhibits slightly higher ionic transport, suggesting an enhanced vacancy contribution. The activation energies for ionic conduction in Gd3Fe5O12-based materials vary in the narrow range 211–224 kJ/mol. Due to a significantly lower activation energy for the total conductivity (24–81 kJ/mol), the oxygen ion transference numbers in air increase with temperature, from 4 × 10−5 – 5×10−3. Steady-state oxygen permeation fluxes through dense garnet membranes are limited by bulk ambipolar conductivity. The thermal expansion coefficients of the ferrite ceramics (100–1000 °C) were found linear and in the range 10.36–10.86 × 10−6 K−1.