Preparation And Electrical Properties Of The Sm₂Zr₂O₇ Codoped With Alkaline-earth And Rare-Earth Metallic Cations

In this article, four zirconate system is sintered using the solid state reaction method: (1) Sm₂Zr₂O₇ ceramics codoped with Yb³⁺ and Mg²⁺ cations, (2) Sm₂Zr₂O₇ ceramics codoped with Yb³⁺ and Ca²⁺ cations, (3) Sm₂Zr₂O₇ ceramics codoped with Dy³⁺ and Mg²⁺ cations, (4) Sm₂Zr₂O₇ ceramics codoped with Dy³⁺ and Ca²⁺ cations. The X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to determine the crystal structure and morphology of the sintered ceramics. An impedance/gain-phase analyzer was used to measure electrical conductivity of the sintered ceramics.In these four zirconate system, only SmYb_1-xMg_xZr₂O_7-x/2 ceramics form complete solid solutions. However, a small amount of second phases are found in SmYb_1-xCa_xZr₂O_7-x/2, SmDy_1-xMg_xZr₂O_7-x/2 and SmDy_1-xCa_xZr₂O_7-x/2 ceramics. MgO is found in the SmDy_1-xMg_xZr₂O_7-x/2 ceramics, while CaZrO3 is formed in the SmYb_1-xCa_xZr₂O_7-x/2 and SmDy_1-xCa_xZr₂O_7-x/2 ceramics. It suggests that the solubility of Mg in the SmDyZr₂O₇ is small, and the doped CaO could react with ZrO2. The sintered Sm₂Zr₂O₇ ceramics is dense, and the relative densities of the sintered ceramics are more than 92%.For A₂Zr₂O₇ zirconate system, the crystal structure is determined by the ratio of average ionic radius rÏ…(An+) at the An+ site to the ionic radius r(Zr⁴⁺) at the Zr⁴⁺ site. The prepared ceramics exhibit a defect fluorite structure when rÏ…(An+)/r(Zr⁴⁺)<1.46, however, they have a pyrochlore structure when rÏ…(An+)/r(Zr⁴⁺)>1.46. The SmYb_1-xMg_xZr₂O_7-x/2 ceramics exhibit a defect fluorite structure, since both the r(Yb³⁺) and r(Mg²⁺) are small. Although the ionic radius of Ca²⁺ is large, the SmYb_1-xCa_xZr₂O_7-x/2 ceramics have a pyrochlore structure, since SmYb_1-xCa_xZr₂O_7-x/2 does not form complete solid solution. since the ionic radius of Dy³⁺ is large, the SmYb_1-xMg_xZr₂O_7-x/2 and SmYb_1-xCa_xZr₂O_7-x/2 ceramics have a pyrochlore structure.The electrical conductivity of the Sm₂Zr₂O₇ ceramics codoped with alkaline-earth and rare-earth metallic cations for each composition gradually increases with increasing temperature from 673 to 1173 K. At identical temperature levels, the electrical conductivity of SmYb_1-xMg_xZr₂O₇-x/2 ceramics slightly increases with increasing magnesia content, but the electrical conductivity of SmYb_1-xMg_xZr₂O₇-x/2 ceramics decreases with increasing magnesia content. The electrical conductivity of SmYb_1-xCa_xZr₂O₇-x/2 ceramics almost remains unchanged when the content of calcia is below 0.10, and then decreases with increasing calcia content. With increasing temperature, the changes of the electrical conductivity of the sintered ceramics becomes less. The SmDyZr₂O₇ ceramic exhibits the highest electrical conductivity of 8-10^–3S·cm^–1 at 1173K in this work. It is confirmed that SmYb_1-xMg_xZr₂O_7-x/2 and SmDy_1-xCa_xZr₂O_7-x/2 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.