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 has been prepared at 1525 °C from sintered ceramics by a seeding method without their melt.•The La9.33Ge6O26 single crystal showed little anisotropy in oxide conductivity, in contrast to the La9.33Si6O26 single crystal.•In order to discuss the difference between conduction properties of the La9.33Ge6O26 and La9.33Si6O26 single crystals, their crystal structures have been compared.

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.