Improvement of electrical conductivity of trivalent rare-earth cation-doped neodymium zirconate by co-doping gadolinium and ytterbium

Pyrochlore oxides of A₂Zr₂O₇, where A represents trivalent rare-earth elements, have a high electrical conductivity, which makes them suitable for applications as high-temperature solid electrolytes. The influence of Gd and Yb cations co-doping at the Nd site on structure and electrical conductivity of a pyrochlore oxide Nd ₂Zr₂O₇ is investigated using X-ray diffraction and impedance spectra measurements. Different zirconate ceramics of Nd ₂Zr₂O₇, Nd_1.8Gd_0.2Zr ₂O₇, Nd_1.8Gd_0.1Yb _0.1Zr₂O₇, Nd_1.4Gd _0.6Zr₂O₇ and Nd_1.4Gd _0.3Yb_0.3Zr₂O₇ are prepared by pressureless-sintering method at 1,973 K for 10 h in air. Nd₂Zr ₂O₇ doped with Gd and Yb cations at the Nd site exhibit a single phase of pyrochlore-type structure. The measured values of the total conductivity obey the Arrhenius relation. Nd₂Zr₂O ₇ and its doped zirconate ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^-4 to 1.0 atm at all test temperature levels. The total conductivity increases with reducing average ionic radii of A-site rare-earth cations. The dual Yb+Gd intermix doping causes a distinctly enhanced total conductivity as compared to unmodified Nd ₂Zr₂O₇ and singly Gd-doped zirconate ceramics. The highest total conductivity value obtained in this work is 1.02 × 10^-2 S cm × ¹ at 1,173 K for Nd_1.4Gd _0.3Yb_0.3Zr₂O₇ ceramic.