Study On Order-Disorder Transformation And Electrical Property Of Cation-doped Rare-Earth Zirconates

Rare-earth zirconates of (Sm_1–xY_x)₂Zr₂O₇, (Sm_1–xDy_x)₂Zr₂O₇, (Gd_1–xEu_x)₂Zr₂O₇, Gd₂(Zr_1–xNb_x)₂O7+x, (Sm_1–xCa_x)₂Zr₂O₇–x and Sm₂(Zr1–xTix)₂O7 ceramics doped with different cations were successfully prepared by a solid state reaction process. The microstructure and order-disorder transformation of doped rare-earth zirconates were characterized by X-ray diffraction, Laser Raman spectroscopy, scanning electron microscopy and high-resolution transmission electron microscopy. The electrical conductivity of doped rare-earth zirconates was investigated by AC impedance spectroscopy. The mechanisms of electrical conduction of doped rare-earth zirconates were investigated by making the oxygen concentration cells or by comparing the electrical conductivity obtained at different oxygen partial pressures. In addition, the chemical compatibility between the doped rare-earth zirconates and electrode materials (including the anode and cathode) was also evaluated.The degree of structural order of undoped rare-earth zirconates A2Zr₂O₇ ceramics gradually decreases with decreasing ionic radius of rare-earth cations. The Sm₂Zr₂O₇ and Eu₂Zr₂O₇ ceramics exhibit an ordered pyrochlore phase, while Gd₂Zr₂O₇ and Dy2Zr₂O₇ ceramics have a disordered fluorite phase. When the ionic radius ratio of r(A³⁺)/r(Zr⁴⁺) is lower than 1.46, and the width of the characteristic Raman mode associated with the movement of the 48f oxygen ions is distinctly broadened, the undoped rare-earth zirconates exhibit a disordered defective fluorite phase. Pyrochlore-type Eu₂Zr₂O₇ ceramic with a relatively low structural order degree shows the maximum grain conductivity of 1.03×10–2S·cm–1 at 1173K, as compared with that of other undoped rare-earth zirconates.The order–disorder structural transition takes place in the doped rare-earth zirconate solid solutions of (Sm_1–xY_x)₂Zr₂O₇, (Sm_1–xDy_x)₂Zr₂O₇ and (Gd1–xEux)₂Zr₂O₇ with increasing ionic contents of doping cations such as Y³⁺, Dy³⁺ and Eu³⁺. The crystal structures of these solid solutions are closely related to the ionic radius ratio of r(A³⁺)/r(Zr⁴⁺). The degree of structural order of these solid solutions decreases with decreasing difference in the ionic radii of the A³⁺ and Zr⁴⁺ cations. When the ionic radius ratio of r(A³⁺)/r(Zr⁴⁺) is beyond 1.46, and the width of the characteristic Raman mode associated with the movement of the 48f oxygen ions is clearly sharpened, the doped rare-earth zirconates exhibit an ordered pyrochlore phase. However, the ionic radius ratio of r(A³⁺)/r(Zr⁴⁺) is below 1.46, and the width of the characteristic Raman mode associated with the movement of the 48f oxygen ions is distinctly broadened, the rare-earth zirconates doped with various ionic radius cations transforms to a disordered defective fluorite structure. HRTEM and Raman spectra confirm that the short-range disorder is observed in the ordered pyrochlore-type solid solutions. In addition, the short-range order phenomenon of the defective fluorite-type structure is observed at the phase boundary.As the solubilities of Ca²⁺ and Ti⁴⁺ in the pyrochlore phase Sm₂Zr₂O₇ are very low, the secondary phase occurs at the grain boundary of the pyrochlore phase with increasing doping contents of different valent cations. When the Ca²⁺ content in the (Sm_1–xCa_x)₂Zr₂O₇–x is beyond 0.025, a secondary phase of perovskite-like CaZrO₃ is also observed as well as the Ca²⁺-doped pyrochlore phase solid solution. However, when the Ti⁴⁺ content in the Sm₂(Zr1–xTix)₂O7 is beyond 0.3, a secondary phase of orthorhombic-like Sm₂TiO5 is formed at grain boundaries of the Ti⁴⁺-doped pyrochlore phase solid solution. In consideration of the defective fluorite phase Gd₂Zr₂O₇, the doping of small amounts of Nb⁵⁺ causes the structural transition from the defective fluorite phase to pyrochlore phase, however; no secondary phase is found.The measured grain conductivities of (Sm_1–xY_x)₂Zr₂O₇, (Sm_1–xDy_x)₂Zr₂O₇ and (Gd_1–xEu_x)₂Zr₂O₇ solid solutions are closely related to the unit cell free volume and the degree of structural order. The grain conductivity increases with increasing unit cell free volume when the crystal structure is a disordered defective fluorite phase. The grain conductivity increases with increasing unit cell free volume and degree of structural order simultaneously when the crystal structure exhibits a relatively lowly-ordered pyrochlore phase around the fluorite–pyrochlore phase boundary. However, the grain conductivity is controlled by the degree of structural order, which decreases with increasing degree of structural order when the crystal structure exhibits a relatively highly-ordered pyrochlore-type phase. Therefore, the doped rare-earth zirconates with relatively lowly-ordered pyrochlore-type phase at the vicinity of the phase boundary between fluorite and pyrochlore phases shows the highest grain conductivity. The (Sm0.7Dy0.3)₂Zr₂O₇ solid solution shows the maximum grain conductivity of 1.10×10–2S·cm^–1 at temperature of 1173K.The grain conductivities of rare-earth zirconates of (Sm_1–xCa_x)₂Zr₂O₇–x and Sm₂(Zr_1–xTi_x)₂O7 decrease with increasing contents of the doping cations due to the formation of poor conductive perovskite-like CaZrO3 and orthorhombic-like Sm₂TiO5 at grain boundaries of the pyrochlore phase. The grain conductivity of Gd₂(Zr0.9Nb0.1)₂O7.1 is slightly higher than that of pure Gd₂Zr₂O₇, which is mainly ascribed to the structural transition from a defective fluorite phase to a pyrochlore phase. However, the grain conductivity gradually decreases with further increasing Nb⁵⁺ content, which is attributed to the increase in the degree of structural order and the decrease in the content of mobile oxygen vacancies.The grain conductivities of (Sm_1–xY_x)₂Zr₂O₇ and (Sm_1–xDy_x)₂Zr₂O₇ solid solutions in a hydrogen atmosphere are almost the same as the values obtained in air, which indicates the conduction of these solid solutions is purely ionic. The grain conductivity of (Gd0.4Eu0.6)₂Zr₂O₇ is almost independent of oxygen partial pressure from 1.0×10?4 to 1.0 atm. This suggests that oxide ion conduction is dominant for (Gd0.4Eu0.6)₂Zr₂O₇ solid solution. The oxide-ionic transference number of (Gd0.4Eu0.6)₂Zr₂O₇ solid solution calculated by the EMF method is as high as 99%. The grain conductivity of Gd₂(Zr0.9Nb0.1)₂O7.1 in a humidified hydrogen atmosphere is slightly higher than that obtained in air, which indicates that the conduction of Gd₂(Zr0.9Nb0.1)₂O7.1 may not be a pure oxide ionic conductor. However, no electronic contribution is introduced. The doping of Nb⁵⁺ for Zr⁴⁺ increases the proton-type conduction in a humidified hydrogen atmosphere.The average thermal expansion coefficients of (Sm0.9Y0.1)₂Zr₂O₇ and (Gd0.4Eu0.6)₂Zr₂O₇ solid solutions are 10.85×10–6K–1 and 10.62×10–6K–1 in the temperature range of 323–1473K, respectively. These doped rare-earth zirconates do not react with the anode material of NiO in the temperature range of 1673–1873K, and also do not react with the cathode material of La0.7Sr0.3MnO3 in the temperature range of 1173–1473K. (Sm_0.9Y_0.1)₂Zr₂O₇ and (Gd_0.4Eu_0.6)₂Zr₂O₇ solid solutions do not react with the cathode material of La0.6Sr0.4Co0.2Fe0.8O3–δat 1073K. However, these doped rare-earth zirconates react with La_0.6Sr_0.4Co_0.2Fe_0.8O_{3–δ} to form SrZrO₃ above 1173K.