Lanthanum substituted CeNbO4+? scheelites: mixed conductivity and structure at elevated temperatures

Abstract

CeNbO_4+δ is a room temperature monoclinic material exhibiting a phase change to a tetragonal scheelite polymorph at 1023 K. The material can accommodate oxygen excess up to a composition of CeNbO_4.25 in air which is believed to be incorporated on interstitial sites and hence is of interest for solid oxide fuel cell applications. Recent research however pointed to poor stability at low p(O₂) making optimisation a necessity. To this end the effect of substituting lanthanum for cerium in CeNbO_4+δ has been investigated in terms of redox, structural and mixed conductive behaviour. On increasing lanthanum content the overall oxygen hyperstoichiometry of the as-prepared substituted phases was found to decrease. A reduction in the temperature of the monoclinic fergusonite to tetragonal scheelite phase transformation with increasing lanthanum content was also observed. However, the extent of oxidation with increasing temperature was found to decrease. Four probe dc conductivity measurements in the temperature range 923 K to 1223 K showed that the total conductivity decreased and that the p(O₂) dependence of the total ionic conductivity increased with increasing lanthanum content. The ionic transference number, t_i, was found to increase to a maximum of approximately 0.75 with lanthanum substitution but at low p(O₂) the stability of the tetragonal phase was found to decrease.