La Substitution\ninto the Melilite Derivative Ca<sub>5</sub>Ga<sub>6</sub>O<sub>14</sub>: Prediction, Synthesis and Ionic\nConductivity

Melilite-type gallates of general formula RE<sub>1+<i>x</i></sub>AE<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+<i>x</i>/2</sub> are of interest for their ability\nto\nhost mobile interstitial oxide ions in [Ga<sub>3</sub>O<sub>7+<i>x</i></sub>] layers. The crystal structure of Ca<sub>5</sub>Ga<sub>6</sub>O<sub>14</sub> is closely related to melilite, with\n[Ga<sub>3</sub>O<sub>7</sub>] layers stacked in a more complex way\nto accommodate an additional 0.5 interlayer cations per formula unit,\nsuggesting the potential for similar oxide ion conduction behavior.\nWe used a computational approach to identify the most promising routes\nto interstitial oxide incorporation into Ca<sub>5</sub>Ga<sub>6</sub>O<sub>14</sub>, leading to an experimental investigation of the system\nCa<sub>5–<i>x</i></sub>La<sub><i>x</i></sub>Ga<sub>6</sub>O<sub>14+<i>x</i>/2</sub>. Single-phase materials\nwere obtained in the range 0 ≤ <i>x</i> ≤\n0.25 by solid state reactions, producing an ∼40× increase\nin ionic conductivity at 800 °C. This limited compositional range\npresents a challenge for characterization of the charge-compensating\ndefects. The La substituents were observed directly by X-ray diffraction\nand STEM–EDX, and a combination of different structural characterization\ntechniques and DFT calculations indicated the presence of interstitial\noxide ions indirectly, explaining the conductivity response. As higher\ncarrier concentrations (<i>x</i> > 0.25) are apparently\ninaccessible in this system, we conclude that its potential as a useful\noxide ion conductor is more limited than that of established melilite\nmaterials such as La<sub>1+<i>x</i></sub>Ca<sub>1–<i>x</i></sub>Ga<sub>3</sub>O<sub>7+<i>x</i>/2</sub>.