Samples of Ba-substituted lanthanum silicate (La10−XBaXSi6O27−X/2) were prepared and the effect of Ba substitution upon the crystal structure and conductivity was studied. The X-ray diffraction results for a series of compositions showed that Ba ions could occupy the La site of an apatite structure, with the composition La10−XBaXSi6O27−X/2 (X = 0 to 1). Rietveld analysis of the synchrotron X-ray diffraction 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 this value was the same as that of 8mol%Y2O3-doped ZrO2 at 750C. On the other hand, the activation energy of about 50kJ/mol for La9.4Ba0.6Si6O26.7 was smaller than that for Y2O3-doped ZrO2. Thus the conductivity of La9.4Ba0.6Si6O26.7 was higher than that of Y2O3-doped ZrO2 below 750C. The conductivity parallel to the c-axis, which was attributed to 2a-site oxide-ion migration was known to be predominant in La10Si6O27. However, Ba substitution appeared to produce oxygen vacancies and create another pathway for oxide ions perpendicular to the c-axis. The increase in the pathways led to an increase in the conductivity.

Ionic Conductivity of Apatite-Type Solid Electrolyte Material, La10−xBaxSi6O27−x/2 (x = 0–1), and its Fuel Cell Performance. Y.Nojiri, S.Tanase, M.Iwasa, H.Yoshioka, Y.Matsumura, T.Sakai: Journal of Power Sources, 2010, 195[13], 4059-64