Synthesis and Electrical Properties of Dense Ce0.9Gd0.1O1.95 Ceramics

Uniform spherical powders of Ce_0.9Gd_0.1O_1.95 with an average diameter of 250 nm were obtained at 700°C from a sol-gel process of mixing nitrates and ethylene glycol. Broadening of the X-ray peaks of the fluorite structure reveals a small crystallite size within the powders. Sintering in air of pressed pellets of the powders at 1585°C gives ceramics of 99% theoretical density with grain sizes 1-10 µm and a cubic unit cell a = 5.422 ± 0.034 Å. The electrical conductivity σ=σ_o+σ_e has two components. In air or argon, the electronic component σ_e is negligible and the oxide-ion conductivity σ_o is not described by a classical Arrhenius equation; a pronounced curvature at T similar/congruent T * has been observed in the Arrhenius plot of the bulk conductivity. The system could be modeled by a condensation of mobile oxygen vacancies into ordered clusters below a temperature T * similar/congruent 583 ± 45°C and a motional enthalpy Δ H _m= 0.63 ± 0.01 eV for the vacancies. A measured trapping energy Δ H _t(1 - T/T *) has Δ H _t= 0.19 ± 0.01 eV = 2.57 kT *. In a reducing atmosphere, σ_e exhibits a small-polaron motional enthalpy Δ H _p= 0.40 ± 0.08 eV for transfer of a 4 f electron from a Ce³⁺ to a Ce⁴⁺ ion and a Δ H _p+Δ H _pt= 0.51 ± 0.04 eV at T < T *.