Nonstoichiometry and relaxation kinetics of nanocrystalline mixed praseodymium-cerium oxide Pro_(0.7)Ce_(0.3)O_(2-x)

摘要

The oxygen deficiency and kinetics of oxygen uptake and release of nanocrystalline mixed praseodymium-cerium oxide with composition Pr_(0.7)Ce_(0.3)O_(2-x) were investigated by combining coulometric titration and potentiometric measurements using stabilised zirconia oxygen concentration cells. The P(O_2) versus composition isotherms indicate a two-phase region at high P(O_2) [P(O_2)>0.1 bar at 560 deg C] and a single-phase region at lower P(O_2). The oxygen pressure dependence in the homogeneous region can be described by a power law with an exponent (-1/6), in accordance with doubly charged oxygen vacancies as majority defects. The enthalpy of reduction amounts to (2.9+-0.3) eV. The chemical diffusion coefficients are of the order of 10~(-6) cm~2s~(-1) at 640 deg C with an activation energy of approx approx 0.3eV. The low activation energy for diffusion may be related to the high density of interface sites in the nanocrystalline material.