Phase Stability Control of Interstitial Oxide Ion Conductivity in the La1+xSr1−xGa3O7+x/2 Melilite Family

The two-dimensional polyhedral anion network melilites La 1+ x Sr 1− x Ga 3 O 7+0.5 x exhibit interstitial oxide ion conductivity. The solid solution is shown to extend to x = 0.64, and the variation of the conductivity with x is investigated. At high temperatures, all of these compounds adopt the typical tetragonal melilite structure. When x > 0.6, cooling below 600 °C results in a reversible phase transition to an orthorhombic structure which reduces the ionic conductivity. Quenching experiments have shown the tetragonal structure has a higher conductivity compared to the orthorhombic structure of the same composition. Short-range order effects associated with this transition exercise an important influence on the composition- and temperature-dependence of the conductivity.