Single crystal growth and oxide ion conductivity of apatite-type rare-earth silicates

Rare-earth silicate with an apatite structure is a new type of oxide ion conductor, the electrical conductivity of which is higher at relatively low temperatures than that of stabilized zirconia. Single crystals are indispensable in order to reveal the intrinsic bulk properties such as anisotropy of electrical conductivity. This paper reviews our recent research work on single crystals growth by the floating zone method and electrical properties of this material, including reinvestigation of phase relations in Nd2O3–SiO2 system and structure refinement using neutron diffraction. A possible mechanism for the oxide ion conduction is proposed based on the existence of cation vacancies.