The oxygen-permeable ceramic membrane (OPCM), made of the mixed-conductive ceramic oxide Gd0.2Ce0.8O2-delta (GCO20), has been successfully fabricated on a porous CeO2 tubular support by means of the slurry-coating and co-sintering techniques. In this asymmetric membrane, the GCO20 membrane (10-20 mu m thick) is intercalated in the surface layers of a porous CeO2 tube. It gives an O-2 permeation flux of 0.45 sccm/cm(2) from air feed at 900 degrees C and I bar with exclusive permselectivity. To promote oxygen flux at relatively lower separation temperatures, a thin layer of La0.2Sr0.8CoO3-delta (LSCO80) was deposited on top of GCO20/CeO2 to generate a dual-layer membrane. Nevertheless, raising the calcination temperature to consolidate the outer LSCO80 layer will suppress the oxygen flux. Based on SEM and XRD investigations, this phenomenon is due to removal of fractal surface features and distortion of LSCO80 crystalline phase. Furthermore, the temperature leverages on oxygen ionic conduction were assessed using impedance analysis. The electric measurement results are in agreement with the oxygen permeation testing results. Finally, the density function theory (DFT) was applied to perform simulations in order to find out the dependence of the equilibrium dimension of lattice cell on the oxygen vacancy concentration in the perovskite LSCO80 structure at the ground state of the crystal. The outcome shows that the lattice undergoes expansion upon losing oxygen, which provides a theoretical explanation for the crystalline distortion induced by high calcination temperature. (c) 2005 Elsevier B.V All rights reserved.