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Crystal structures of oxyapatite-type lanthanum silicate (LSO), La9.667Si6O26.5, and Al- and Mg-doped LSOs; La9.667(Si5.8Al0.2)O26.4 and La9.667(Si5.5Mg0.5)O26.0, were refined using synchrotron powder X-ray diffraction (SXRD) data to investigate the origin of enhanced oxide ion (O2 −) conducting behavior with Si-site doping. It was found that temperature factors of oxygen atoms largely increased with slight Al- or Mg-doping into the Si-site of LSO. Especially, that for the 2a-site oxygen located on the c axis (O4) was enhanced by doping. In addition, that for the 12i-site oxygen, O3, the constituent of an SiO4 tetrahedron in the LSO lattice increased with doping anisotropically toward O4. It was suggested that the ionic movement perpendicular to the c axis via SiO4 tetrahedron was activated by cation (M) doping, and that contributed to the ion conduction in addition to the fast ion channel along c axis.

An oxygen-excess-type lanthanum silicate (LSO) solid electrolyte with high chemical stability as well as high ion conductivity was developed by optimizing the chemical composition, La 9.333 + x (Si 6 − y Al y )O 26 + 1.5 x − 0.5 y . It was found that La 10 (Si 5.8 Al 0.2 )O 26.9 ( x = 0.667, y = 0.2) exhibited the highest conductivity among them, and addition of a small amount of iron was effective for the improvement of its chemical stability. The LSO specimen of x = 0.667 and y = 0.2 with iron addition (0.5 mol%) was chemically stable and had a conductivity of 5.6 × 10 − 2 S cm − 1 under air at 1073 K, while its electronic conductivity was less than 2.0 × 10 − 4 S cm − 1 at the same temperature. The oxygen tracer diffusion coefficient, D O * of this material was determined as 2.0 × 10 − 8 cm 2 s − 1 at 1073 K.

Apatite-type lanthanum silicate based films have attracted significant interests to use as an electrolyte of solid oxide fuel cells (SOFCs) working at intermediate temperature. We have prepared Mg doped lanthanum silicate (MDLS) films on NiO–MDLS cermet substrates by spin coating and sintering of nano-sized printable paste made by beads milling. Changes in crystal structure and microstructure of the paste films with the sintering temperature have been investigated to show that porous network structure with a grain growth evolves up to 1300 °C, whereas densification occurred above 1400 °C. Anode supported SOFCs using the pasted MDLS films were successfully fabricated: an open circuit voltage of 0.91 V and a maximum power density of 150 mW cm −2 measured at 800 °C were obtained with the electrolyte film sintered at 1500 °C.

Electrochemical oxygen reduction behavior for a half cell composed of a mixed ionic and electronic conducting (MIEC) oxide and an oxygen excess-type solid electrolyte was investigated. A perovskite-type MIEC oxide, (La 0.6 Sr 0.4 )(Co 1− y Fe y )O 3− δ (LSCF), was employed as a base material for an electrode catalyst of oxygen reduction reaction. As an electrolyte, Al-doped lanthanum silicate [La 10 (Si 5.8 Al 0.2 )O 26.9 (ALSO)] was used, which is a new class of oxide ion conductors with apatite-type structure. The evaluated polarization resistance, R pol , for the reaction largely depended on y , and the minimum resistance was obtained for y = 0.2. The R pol value was 0.3 Ω cm 2 at 1073 K in ambient air [ P (O 2 ) = 2.1 × 10 4 Pa] for y = 0.2. To improve the performance, introduction of silver nano-particles onto the LSCF particles was studied. With the Ag-modification, R pol at 1073 K could be minimized toward 0.08 Ω cm 2 . The cathodic overpotential of the electrode evaluated from DC polarization at 1073 K was 18 mV at 0.1 A cm −2 .

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