La 1+x Ba 1-x Ga 3 O 7+0.5x Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites

Substitution of La for Ba in LaBaGa O melilite yields a new interstitial-oxide-ion conducting La Ba Ga O solid solution, which only extends up to x = 0.35, giving a maximum interstitial oxygen content allowed in La Ba Ga O as about half of those allowed in La (Sr/Ca) Ga O . La Ba Ga O ceramic displays bulk conductivity ∼1.9 × 10 S/cm at 600 °C, which is lower than those of La (Sr/Ca) Ga O , showing the reduced mobility for the oxygen interstitials in La Ba Ga O than in La (Sr/Ca) Ga O . Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La Ba Ga O are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the c-axis and stabilized via incorporating into the bonding environment of a three-linked GaO among the five GaO tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa O has the largest formation energy for oxygen interstitial defects among La M Ga O (M = Ba, Sr, Ca), consistent with the large Ba cations favoring interstitial oxygen defects in melilite less than the small cations Sr and Ca . The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La M Ga O (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba -containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels.