Gas-tight proton-conducting Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0, 0.05) ceramics

Gas-tight Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0, 0.05) pyrochlore materials have been prepared via mechanical activation of Nd${}_2$O${}_3$, ZrO${}_2$, and CaO, followed by single-step sintering of green compacts at 1600 °C for 3 or 10 h. Their structure has been studied using Rietveld refinement of X-ray diffraction data; their microstructure has been examined by scanning electron microscopy, and their conductivity has been determined by impedance spectroscopy in dry and wet air. In addition, we have assessed the effect of prolonged hydration on the proton conductivity of the Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0, 0.05) pyrochlores. According to the Rietveld refinement results, Ca cations are incorporated into both Nd- and Zr- sub-lattice in Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0.05) synthesized at 1600 °C, and most of the Ca cations reside on the Zr sites. The 500 °C proton conductivity of Nd${}_2$Zr${}_2$O${}_7$ is 5 × 10${}^{-6}$ S/cm (~ 1 × 10${}^{-5}$ S/cm at 600 °C) and that of Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0.05) is 7 × 10${}^{-5}$ S/cm (~ 2.5 × 10${}^{-4}$ S/cm at 600 °C). Prolonged (5 months) immersion in water at room temperature has been shown to reduce the proton conductivity of Nd${}_{2-x}$Ca${}_x$Zr${}_2$O${}_{7-\delta }$ (x = 0, 0.05), which is attributable to partial dissolution of Nd and Ca. CaO and, to a considerably lesser extent, Nd${}_2$O${}_3$ segregation on polished and thermally etched (1450 °C) sections of the ceramics has been demonstrated. We attribute both effects to the high diffusion mobility of Ca${}^{2+}$ and Nd${}^{3+}$ cations, due to the basicity of their oxides in ambient air and water.