Highly dense pellets of an oxygen excess-type lanthanum silicate ͑La 9.333+x Si 6 O 26+1.5x , x Ͼ ca. 0.3, OE-LSO͒ were successfully fabricated, and their electrical conducting properties were studied. The replacement of Si by Al enhanced its conductivity, and the slightly Al-doped OE-LSO specimen ͓La 9.62 ͑Si 5.79 Al 0.21 ͒O 26.33 ͔ had excellent features as a solid electrolyte; that is, it had high ionic conductivity and was highly stable under reducing as well as oxidizing conditions at 873-1073 K. In addition, the ionic transference number was higher than 0.99. In the fuel cell utilizing this electrolyte ͑0.72 mm thick͒, ͑La 0.6 Sr 0.4 ͒͑Co 0.2 Fe 0.8 ͒O 3−␦ cathode, and Ni-Ce 0.9 Gd 0.1 O 1.95−␦ anode, good performance with the maximum power density of ca. 0.25 W cm −2 was obtained at 1073 K. In addition, this electrolyte also had high compatibility with these conventional mixed conducting electrodes, according to an analysis near the electrode/electrolyte interfaces after the fuel cell test.Solid oxide fuel cells ͑SOFCs͒ attract much attention as commercial or household energy conversion devices capable of producing heat as well as electricity directly from natural gas. Because they exhibit high energy efficiency compared with present energy conversion systems, it is expected that developing practical fuel cells will give us a gift for the next generation, encouraging a sustainable use of resources and reducing carbon dioxide emission. In SOFCs, oxide ion or proton conducting oxide ceramics materials are employed as electrolytes. Well-known zirconia-based oxide ion conductors, such as yttria-stabilized zirconia ͑YSZ͒, have long been used as electrolyte materials for SOFCs, but are just now entering the long-awaited stage of practical application.Cost, however, presents a problem. One of the reasons for this is the necessity of using high cost heat-resistive interconnecting materials available for a high operating temperature of around 1273 K. Fuel cell reliability is also a problem due to poor mechanical strength of the electrolyte during operation at such a high temperature or during startup/shutdown cycles. Because of the high activation energy for the ionic conduction of YSZ ͑ca. 0.9 eV͒, its good ionic conductivity ͑ca. 10 −1 S cm −1 at 1273 K͒ falls to ca. 10 −2 S cm −1 at 973 K. If one could develop a novel solid electrolyte that conducts ions sufficiently ͑above 10 −1 S cm −1 ͒ in a reducing temperature region ͑773-1073 K͒, then less expensive stainless steel could be used for the surrounding materials of fuel cells. Hence, there is an escalating need for developing solid electrolytes for this temperature region to improve reliability as well as reduce cost.Oxide ion conduction through material based on lanthanum silicate ͑La 9.333+x Si 6 O 26+1.5x , LSO͒ with apatite-type structure, shown in Fig. 1, was originally discovered by Nakayama et al. 1,2 in 1995. This material is notable because of its low activation energy for conduction, while the reported ionic conductivity was about 10 −3 S cm −1 at 97...

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Electrical properties of an apatite-type oxide ionic conductor, lanthanide silicates (Ln9.33+xSi6O26+1.5x, LnSO; Ln = La or Nd) which are expected as novel solid electrolytes for the intermediate-temperature solid oxide fuel cell (IT-SOFC) have been investigated in detail. Its total electrical conductivity was almost P(O2)-independent, while a slight dependence of the conductivity on P(O2) was observed under high P(O2) conditions at high temperatures. The conductivity for La10Si6O27 was much higher than that for Nd10Si6O27. This was probably due to a difference in an accepting capacity of excess oxide ions. The excess oxide ions, formed by introducing the excess Ln elements (x), play an important part in oxide ion conduction. In the case of Ln = Nd system, formation of the excess oxide ions was blocked probably due to its smaller lattice volume than that of Ln = La system.