Preparation of apatite-type La9.33Ge6O26 single-crystal from sintered ceramics by a seeding method and its oxide ionic conduction

Apatite-type La9.33Ge6O26 single crystal as an oxide ionic conductor has been successfully prepared by a single crystallization of the sintered polycrystalline La9.33Ge6O26 ceramics on the surface of seed single crystal without their melt. The crystal has a hexagonal crystal system with a space group of P63/m. Conductivity measurements revealed that the La9.33Ge6O26 single crystal did not exhibit an anisotropy, in contrast with the previously reported La9.33Si6O26 single crystal (hexagonal crystal system with a space group of P63/m), which apparently exhibited the anisotropy: conductivity component parallel to the c-axis was ca. 100 times higher than the perpendicular component. In order to discuss the difference between these apatite-type single crystals, their crystal structures were compared. Results implied that the conduction pathway perpendicular to the c-axis is rather important in the germanate apatite.