Oxygen diffusion and ionic conductivity of samaria-doped ceria (SDC) thin films have been studied as a function of composition using experiment and atomistic simulation. SDC thin films were grown on Al(2)O(3) (0001) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) technique. The experimental results show a peak in electrical conductivity of SDC at 15 mol% SmO(1.5). The ionic conductivity obtained from molecular dynamics simulation of the same system shows a peak at about 13 mol% SmO(1.5). The activation energy for oxygen diffusion was found to be in the range from 0.8 to 1 eV by simulations depending on the SmO(1.5) content, which compares well with the range from 0.6 to 0.9 eV given by the experimental work. The simulations also show that oxygen vacancies prefer Sm(3+) ions as first neighbors over Ce(4+) ions. The present results reveal that the optimum samaria content for ionic conductivity in single crystals of SDC is less than that in polycrystals, which can be related to the preferential segregation of dopant cations to grain boundaries in polycrystals. (C) 2011 Elsevier B.V. All rights reserved.