KCS

A langasite-type La₃Ga₅GeO₁₄ has been studied primarily as a piezoelectric material. The crystal structure of La₃Ga₅GeO₁₄ is very similar with the melilite, La_1+xSr_1-xGa₃O_7+x/2, which has been known to show high oxygen ion conductor. In the melilite structure, lanthanum was equivalently substituted with strontium to add oxygen in the interstitial site, leading to the high oxygen ion conductivity. In this study, we synthesized La₃Ga_5+xGe_1-xO_14+x/2 (x = 0 ~ 0.5) by solid state reaction to introduce interstitial oxygen in the lattice. The crystal structure of La₃Ga_5+xGe_1-xO_14+x/2 (x = 0 ~ 0.5) was characterized by synchrotron powder X-ray diffraction. The La₃Ga_5+xGe_1-xO_14+x/2 (x = 0 ~ 0.5) crystallize hexagonal symmetry with space group P 321. The oxygen ion conductivity was measured up to 900℃(± 5), the highest ion conductivity was 2.13×10^-3S/cm(x = 0.4) and the activation energy was 1.12 eV (x = 0.4). The position of interstitial oxygen was confirmed by Maximum Entropy Method (MEM). In order to identify the possible pathways for oxygen diffusion, Bond Valence Energy Landscape (BVEL) maps were calculated.