“…Beyond the limit of about 0.18 for the La:Ce molar ratio, the decrease of the OSC could be due to an excess density of vacancies, leading to stable vacancy} dopant cation association (24}26) that limits the number of mobile vacancies and therefore the ability of the compound of exchanging oxygen with the environment. Several papers concerning the anionic conductivity in trivalent rare earth (RE)-doped ceria (18,24,27) give experimental evidence that the ionic conductivity actually starts to decrease at about x/(1!x)"0.18 in Ce \V RE V O \V , con"rming that in these systems the mobility of oxygen is crucially dependent on the optimal amount of dopant species. The deviation of the OSC of PdCLAP from the general behavior is not surprising, since the activating action of the precious metal is well documented (2).…”