Patent ID: 7682528

Claim:
An oxide ion conductor comprising a composite oxide which has constituent elements of a trivalent element A, a tetravalent element B, and oxygen O, said composite oxide has a composition formula represented by A X B 6 O 1.5X+12 provided that 8≦X≦10, and said composite oxide has an apatite type crystalline structure; wherein said crystal of said composite oxide has a hexagonal system, wherein said hexagonal system has magnetic susceptibilities in the a-axis direction and in the b-axis direction that are different from the magnetic susceptibility in the c-axis direction; said oxide ion conductor has anisotropy in the oxide ion conductivity; said oxide ion conductor is formed of a sintered product which is formed from excessively advanced grain growth, and directions of orientation of a crystal in said sintered product are oriented in a c-axis direction, wherein the oxide ion conductivity along the c-axis direction is greater than the oxide ion conductivity along the a-axis direction and the oxide ion conductivity along the b-axis direction.