Synthesis and electrical conductivity of various melilite-type electrolytes Ln1 + xSr1 − xGa3O7 + x/2

Melilite type ceramics ABC3O7 such as La1.54Sr0.46Ga3O7.27 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 Ln1 + xSr1 − xGa3O7 + 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, La1.54Sr0.46Ga3O7.27 is the most promising ceramic in the Ln1 + xSr1 − xGa3O7+x/2 melilite family since La3+ has the largest ionic radius of the lanthanide elements.

Research Highlights
► Melilite Ln1 + xSr1 – xGa3O7 + x/2 (Ln=lanthanide elements) electrolytes are synthesized.
► The melilite structure can be formed with most lanthanides.
► La1.54Sr0.46Ga3O7.27 shows the highest conductivity in the melilite family.