La Substitution into the Melilite Derivative Ca₅Ga₆O₁₄: Prediction, Synthesis and Ionic Conductivity

Melilite-type gallates of general formula RE_1+<i>x</i>AE_1-<i>x</i>Ga₃O_7+<i>x</i>/2 are of interest for their ability to host mobile interstitial oxide ions in [Ga₃O_7+<i>x</i>] layers. The crystal structure of Ca₅Ga₆O₁₄ is closely related to melilite, with [Ga₃O₇] layers stacked in a more complex way to accommodate an additional 0.5 interlayer cations per formula unit, suggesting the potential for similar oxide ion conduction behavior. We used a computational approach to identify the most promising routes to interstitial oxide incorporation into Ca₅Ga₆O₁₄, leading to an experimental investigation of the system Ca_5-<i>x</i>La_<i>x</i>Ga₆O_{14+<i>x</i>/2}. Single-phase materials were obtained in the range 0 ≤ <i>x</i> ≤ 0.25 by solid state reactions, producing an ∼40× increase in ionic conductivity at 800 °C. This limited compositional range presents a challenge for characterization of the charge-compensating defects. The La substituents were observed directly by X-ray diffraction and STEM-EDX, and a combination of different structural characterization techniques and DFT calculations indicated the presence of interstitial oxide ions indirectly, explaining the conductivity response. As higher carrier concentrations (<i>x</i> > 0.25) are apparently inaccessible in this system, we conclude that its potential as a useful oxide ion conductor is more limited than that of established melilite materials such as La_1+<i>x</i>Ca_1-<i>x</i>Ga₃O_7+<i>x</i>/2.