Enhancement of ionic conductivity of apatite-type lanthanum silicates doped with cations

The oxide ion conductivity of apatite-type lanthanum silicates has been enhanced by cation doping into both La and Si sites. The Sr and Nd fill vacancies on the La site by creating excess oxide ions and increasing the conductivity. Al and Mg, which substitute for Si at a fixed oxygen content, result in an enhancement of conductivity, accompanied by lattice enlargement. The Mg-doped lanthanum silicate, La-9.533(Si5.7Mg0.3)O-26, has been shown to have the highest conductivity of 4.4 x 10(-2) S/cm at 800 degrees C. The conduction channel consists of La triangles and O triangles, and the channel sizes have been calculated from the Rietveld structure analysis of the powder X-ray diffraction patterns. Expansions of the O triangles have been observed when introducing excess oxide ions by filling vacancies on the La site or by the substitution of Si using dopants.