Study of Mg doping on the zirconium and lanthanide sites of Ln2Zr2O7 (Ln = Sm, Gd) pyrochlores

Abstract

Ln2Zr2O7 (Ln = Sm, Gd) zirconate solid solutions doped with Mg on the Ln and Zr sites have been prepared via mechanical activation of oxide mixtures, followed by heat treatment of green compacts at 1500 and 1600 °C for 4 h. The ceramics prepared at 1600 °C have been characterized by X-ray diffraction and scanning electron microscopy. Their electrical conductivity has been determined in dry and wet air using impedance spectroscopy, and the total conductivity of the best samples has been measured from 700 to 1000 °C in a wide range of oxygen partial pressures. The conductivity data obtained in dry and wet air for the samarium zirconate-based solid solutions doped with Mg on the Zr site demonstrate that raising the Mg content on the Zr site in the Sm2Zr2−xMgxO7−δ (x = 0, 0.05, 0.1, 0.2) pyrochlore series leads to a decrease in the total conductivity of the materials. However, the Sm2Zr2−xMgxO7−δ (x = 0.05, 0.1) materials have a proton component of conductivity in the range of 300–600 °C, which also decreases with increasing Mg content. Among the samarium zirconates studied here, high proton conductivity (1 × 10−4 S/cm at 500°С) is offered by the Sm2Zr1.95Mg0.05O7−δ solid solution. Sm1.9Mg0.1Zr2O7−δ, containing Mg on the Sm site, has the highest oxygen ion conductivity: 5 × 10−3 S/cm at 750°С. The total conductivity of the Gd2Zr2−xMgxO7−δ (x = 0, 0.05, 0.1, 0.2) series also decreases with increasing Mg content, and no proton conduction has been detected in these zirconates. Mg doping of Gd2Zr2O7 on the Zr site reduces the total conductivity of the material, whereas Mg doping on the Gd site (Gd1.9Mg0.1Zr2O7−δ) raises it to 7.5 × 10−3 S/cm at 750 °C.