La2Ga3O7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt

Abstract

The La1+xAE1-xGa3O7+x/2 melilite family (AE = Ca, Sr, Ba, and 0 0.65 with very high Oint concentrations (referred to here as compositions) have the potential to support correspondingly high ionic conductivities, but have never before been accessed due to the limitations of conventional solid-state ceramic synthesis. Here we report that fully-substituted La2Ga3O7.5 (x = 1) melilite ceramics can be synthesized by direct crys-tallization of an under-cooled melt, demonstrating that super-excess compositions are accessible under suitable nonequilibrium reaction conditions. La2Ga3O7.5 is stable up to 830 °C and exhibits an ionic conductivity of 0.01 S.cm-1 at 800 °C, three orders of magnitude higher than the corresponding x = 0 end-member LaSrGa3O7, and close to the range exhibited by the current best-in-class La1.54Sr0.46Ga3O7.23 (0.1 S.cm-1). It crystallizes in an orthorhombic √2a x √2a x 2c expansion of the parent melilite cell, in space group Ima2, with full long-range ordering of Oint into chains within the [Ga3O7.5] layers. The emergence of this chain-like (1D) ordering within the 2D melilite framework, which appears to be an incipient feature of previously reported partially-ordered melilites, is explained in terms of the underlying hexagonal topology of the structure. These results will enable the exploration of extended com-positional ranges for the development of new solid oxide ion electrolytes with high concentrations of interstitial oxide charge carriers.