We prepared Ba substituted lanthanum silicate (La10-XBaXSi6O27-X/2) and examined the effect of Ba substitution on the crystal structure and conductivity. The X-ray diffraction (XRD) results for a series of compositions showed that Ba ion can occupy the La site of an apatite structure with the composition La10-XBaXSi6O27-X/2 (X=0-1). Rietveld analysis of the synchrotron XRD profiles revealed Ba occupation in the La 4f site rather than the La 6h site and a decrease in the occupation factors of the oxide ions of the SiO4 tetrahedra. The conductivity of La10-XBaXSi6O27-X/2 exhibited a maximum at X=0.6 and the value was the same as that of YSZ (8 mol% Y2O3 doped ZrO2) at 750 degrees C. On the other hand, the activation energy of about 50 kJ mol(-1) for La9.4Ba0.6Si6O26.7 was smaller than that of YSZ. Thus the conductivity of La9.4Ba0.6Si6O26.7 was a higher than those of YSZ below 750 degrees C. The conductivity parallel to the c-axis which is attributed to the 2a site oxide ions migration is known to be dominant in La10Si6O27. However, Ba substitution seems to produce oxygen vacancies and create another pathway for oxide ions perpendicular to the c-axis. The increase in the pathways leads to an increase in the conductivity. We also reported the solid oxide fuel cell (SOFC) performance of La10-XBaXSi6O27-X/2 with a maximum power density of 65 mW cm(-2) using La0.9Sr0.1CoO3-delta as a cathode and NiO-SDC (Sm doped CeO2) as an anode. (C) 2010 Elsevier B.V. All rights reserved.