Ion conductivity in cubically-stabilized fluorite-like structured Er5CeMoO12.5 and Yb5MMoO12.5 (M = Ce, Zr) solid solutions

Abstract

For the first time, the usually rhombohedral R3¯ (no. 148) defect fluorite structured rare earth molybdenum oxides RE6MoO12 (RE = Er, Yb) could be stabilized in the cubic defect fluorite structure Fm3¯m (no. 225) through partial substitution of Er3+ and Yb3+ by M4+ cations with M4+ = Ce for Er6MoO12 and M4+ = Ce, Zr for Yb6MoO12. The solution combustion (SC) method and the classical solid state reaction are the synthetic approaches that were used (ambient atmosphere and temperatures of 1250 °C). The obtained oxide powders were characterized by energy dispersive X-ray (EDX) analysis, powder X-ray diffraction (PXRD), IR, and UV-Vis spectroscopy. The PXRD-data were used for Rietveld refinements. Electrochemical impedance spectroscopy (EIS) was conducted to identify oxygen vacancies in the cubic structure type, revealing oxygen ion conductivity starting at 500 °C. Additionally, the influence on ion conductivity by the cation substitutions are discussed. Calculation of the contributing activation energies for the bulk (best value for Yb5CeMoO12.5 is 119.8 kJmol-1), and the grain boundary (e.g. Er5CeMoO12.5: 152.1 kJmol−1) analyzed by means of the Arrhenius plot, shows similarities to the conventional stabilization of zirconia with yttrium (8-YSZ) (110 kJ mol−1 and 110–163 kJmol−1 respectively). The best-calculated conductivity values σ = 1.03 × 10−4 Scm−1 obtained for Er5CeMoO12.5 at 1000 °C are comparable to the values published for 8-YSZ with σ = 3.94 × 10−5 Scm−1 at similar temperatures. These promising preliminary results underline the potential of the title compounds for application in solid oxide fuel cells (SOFCs).