Preparation And Electrical Properties Of Gd₂ (Ar_1-x M_x) ₂O_7+δ (M=Ce,Nb,Mo) Solid Electrolytes

Gd₂(Zr_1-xCe_x)2O7 (0≤x≤0.3), Gd₂(Zr_1-xNb_x)₂O_7+x (0≤x≤0.3) and Gd₂(Zr1–xMox)2O7+2x (0≤x≤0.2) ceramic materials were synthesized by pressureless sintering process using the powders of Gd₂O₃, m-ZrO₂ and doping metallic oxides (CeO₂, Nb₂O₅ and MoO₃) as starting materials. The microstructure and phase transformation mechanisms of different rare-earth zirconate solid solutions were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The measured electrical conductivity of different rare-earth zirconate solid solutions was further studied by AC impedance spectroscopy to identify their conduction mechanisms.Gd₂(Zr_1-xCe_x)2O7 (0≤x≤0.3), Gd₂(Zr_1-xNb_x)₂O_7+x (0.1≤x≤0.3) and Gd₂(Zr1–xMox)2O7+2x (0.1≤x≤0.2) zirconates, which are synthesized by doping different metallic cations of Ce⁴⁺, Nb5+ and Mo⁶⁺ at the B4+ site of Gd₂B2O7, are complete solid solutions. All of these rare-earth zirconates exhibit a high relative density of about 95%.The crystal structures of Gd₂(Zr_1-xCe_x)2O7, Gd₂(Zr_1-xNb_x)₂O_7+x and Gd₂(Zr1–xMox)2O7+2x zirconates, which are synthesized by doping different metallic cations of Ce⁴⁺, Nb5+ and Mo⁶⁺ at the B4+ site of Gd₂B2O7, are mainly governed by the cation radius ratio of r(Gd3+)/r(B4+) in Gd₂B2O7 system. The cation radius ratio of Gd₂Zr2O7, r(Gd3+)/r(Zr4+) = 1.46, resides just at the phase boundary between pyrochlore-type and defect fluorite-type structures. As the sintered temperature used in this study is higher than the order–disorder transition temperature, Gd₂Zr2O7 exhibits only a defect fluorite structure. The Gd₂(Zr_1-xCe_x)2O7(0.1≤x≤0.3) ceramics exhibit a defect fluorite-type structure for r(Gd3+)/r(B4+) < 1.46, however, the Gd₂(Zr_1-xNb_x)₂O_7+x (0.1≤x≤0.3) and Gd₂(Zr1–xMox)2O7+2x (0.1≤x≤0.2) ceramics have pyrochlore-type structures for r(Gd3+)/r(B4+)>1.46.The electrical properties of Gd₂(Zr_1-xCe_x)2O7 (0≤x≤0.3), Gd₂(Zr_1-xNb_x)₂O_7+x (0.1≤x≤0.3) and Gd₂(Zr1–xMox)2O7+2x (0.1≤x≤0.2) solid solutions are investigated by electrochemical impedance spectroscopy. For the Gd₂(Zr_1-xCe_x)2O7 (0≤x≤0.3) and Gd₂(Zr1–xMox)2O7+2x (0.1≤x≤0.2) solid solutions, the measured electrical conductivity decreases with incresaing the contents of doping metallic cations of Ce⁴⁺ and Mo⁶⁺. However, for the Gd₂(Zr_1-xNb_x)₂O_7+x (0.1≤x≤0.3) solid solutions, the measured electrical conductivity first increases at x = 0.1, and then decreases with further increasing the content of metallic cations of Nb5+. Gd₂Zr1.8Nb0.2O7.1 exhibits the highest conductivity of 4.43×10^-3S·cm^-1 at 1073K.For each composition of rare-earth zirconates, the electrical conductivity obeys the Arrhenius equation. The measured conductivity of these zirconate solid solutions increases with temperture. However, at identical temperature levels, the measured conductivity values of these zirconates solid solutions are closely related to their own crystal structures. The conductivity of rare-earth zirconates exhibits a maximum when the cation radius ratio of r(Gd3+)/r(B4+) is close to 1.48 in Gd₂B2O7 system. Gd₂(Zr_1-xNb_x)₂O_7+x solid solutions with the smallest difference in cation radius at the B sites exhibits the highest conductivity.The electrical conductivity of Gd₂Zr1.8Ce0.2O7, Gd₂Zr1.8Nb0.2O7.1 and Gd₂Zr1.8Mo0.2O7.2 (x = 0.1) solid solutions was measured at different oxygen partial pressures, e.g., 1.0×10^-4 to 1.0 atm, in a closed tube furnace from 1073 to 673 K during cooling. These zirconates are almost independent of oxygen partial pressure from 1.0×10^-4 to 1.0 atm, which indicates that the conduction is purely ionic with negligible electronic contribution.