Gas-tight proton-conducting Nd_2 − xCa _xZr₂O_7 − δ (x = 0, 0.05) ceramics

Gas-tight Nd_2 − xCa_xZr₂O_7 − δ (x = 0, 0.05) pyrochlore materials have been prepared via mechanical activation of Nd₂O₃, ZrO₂, 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 − xCa_xZr₂O_7 − δ (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 − xCa_xZr₂O_7 − δ (x = 0.05) synthesized at 1600 °C, and most of the Ca cations reside on the Zr sites. The 500 °C proton conductivity of Nd₂Zr₂O₇ is 5 × 10⁻⁶ S/cm (~ 1 × 10⁻⁵ S/cm at 600 °C) and that of Nd_2 − xCa_xZr₂O_7 − δ (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 − xCa_xZr₂O_7 − δ (x = 0, 0.05), which is attributable to partial dissolution of Nd and Ca. CaO and, to a considerably lesser extent, Nd₂O₃ 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 Ca²⁺ and Nd³⁺ cations, due to the basicity of their oxides in ambient air and water.