Synthesis and electrical conductivity of various melilite-type electrolytes Ln_1+xSr_1-xGa₃O_7+x/2

Melilite type ceramics ABC₃O₇ such as La _1.54Sr_0.46Ga₃O_7.27 are a new class of oxide conductors where the conductivity is carried out through interstitial oxygen ions. This work presents the attempt to replace the A-site element La with the other lanthanide elements and Y, resulting in various Ln _{1 + x}Sr_{1 - x}Ga₃O_{7 + x/2} ceramics, in which Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Yb, Y, and 0.1 < x < 0.54. X-ray diffraction analysis shows that the melilite structure could be formed when the replacement is conducted with most lanthanides but not Yb and Y. Impedance spectroscopy demonstrates that the conductivity decreases dramatically with the decreasing of Ln³⁺ size and the charge-carrier concentration. These results suggest that, as an interstitial oxide ion electrolyte, La _1.54Sr_0.46Ga₃O_7.27 is the most promising ceramic in the Ln_{1 + x}Sr_{1 - x}Ga ₃O_7+x/2 melilite family since La³⁺ has the largest ionic radius of the lanthanide elements.