The discovery of oxygen-ion conductivity and changes in the crystal-structure model of nondoped lanthanum silicate oxyapatites are reviewed from the researches which led to the discovery to current work. The oxygen-ion conductivity of lanthanoid silicate oxyapatites was discovered by Nakayama et al., during development of new oxide lithium-ion conductors. Although samples with compositions of LiRESiO 4 (RE = La, Nd, Sm, Gd, Dy) were initially reported to be single phases with the same crystal structure, the accurate crystal structure of LiRESiO 4 was not clarified until later. The crystal structure determined from X-ray diffraction patterns of LiLnSiO 4 was revised as oxyapatite by another group. The chemical composition of the crystal phases in LiRESiO 4 was also revised to RE 10 Si 6 O 25 and/or RE 9.33 Si 6 O 24 . The discovery of oxygen-ion conductivity in these materials was confirmed when researchers noted that samples of RE 10 Si 6 O 25 , specifically, the samples without lithium, are electrically conducting. Furthermore, very high oxygen-ion conductivity in the direction parallel to c-axis was discovered after single crystals of RE 9.33 (SiO 4 ) 6 O 2 (RE = Nd, Pr, and Sm) were successfully grown. The crystal structure and defect models were also altered after the discovery of oxygen-ion conductivity. Although numerous reports related to the electrical conductivity of La 9.33+x (SiO 4 ) 6 O 2+3x/2 ceramics have appeared in the literature, clear dependences of the total conductivity on the cation nonstoichiometry (x) as well as the sintering temperature are still unclear because of large discrepancies in the reported data. Furthermore, the composition region, where the lanthanum silicate oxyapatite single phase is formed, is also still unclear because of the inconsistencies in the reported results.
