LnBaCo(2)O(5+delta) (Ln = Nd, Sm, Gd, and Y) oxides with a cation ordered perovskite structure have been investigated as cathode materials for intermediate-temperature solid oxide fuel cells (SOFCs). The oxygen content 5 + delta, thermal expansion coefficient (TEC), and electrical conductivity (metallic) decrease with decreasing size of the Ln(3+) ions from Ln = La to Y. While the decrease in TEC is due to the decreasing ionicity of the Ln-O bond, the decrease in electrical conductivity is due to the increasing oxide ion vacancies and a bending of the O-Co-O bonds. The power density of single-cell SOFCs fabricated with the LnBaCo(2)O(5+delta) cathodes, La0.8Sr0.2Ga0.8Mg0.2O2.8 electrolyte, and Ni-Ce0.9Gd0.1O1.95 cermet anode decrease with decreasing size of the Ln(3+) ions, partly due to a decreasing electrical conductivity. The LnBaCo(2)O(5+delta) cathodes with an intermediate lanthanide ion such as Sm3+ offer a trade-off between catalytic activity and TEC. (c) 2008 The Electrochemical Society.