La₂Ga₃O_7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt

Abstract

The La1+x AE 1–x Ga3O7+x/2 melilite family (AE = Ca, Sr, and Ba and 0 ≤ x ≤ 0.64) demonstrates remarkable oxide ion conductivity due to the ability of its layered tetrahedral [Ga3O7+x/2] network to accommodate and transport interstitial oxide ions (Oint). Compositions of x > 0.65 with very high Oint concentrations (referred to here as “super-excess” 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 crystallization 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, 3 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 × √2a × 2c expansion of the parent melilite cell in the 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 compositional ranges for the development of new solid oxide ion electrolytes with high concentrations of interstitial oxide charge carriers.

Macroscopic Orientation Domains Grown via Aerodynamic Levitation: A Path toward Single Crystals

“…Protocol 1 was reported in detail in Ref. [12]: La2O3 and Ga2O3 powders were weighed and then mixed in an agate mortar using an ethanol suspension, dried at 100 °C and then pressed into pellets using a uniaxial pressure of 20 MPa. Pellet fragments weighing ca.…”

“…with x=1, and contains interstitial oxide defects that are fully ordered into chains within the Ga3O7.5 layers [12]. La2Ga3O7.5 is metastable, with a decomposition temperature of 830°C [12], and cannot be prepared by a conventional high temperature ceramic reaction so far.…”

“…Materials composed of compounds with the general composition La1+xSr1-xGa3O7+x/2 are of interest for solid oxide fuel cell (SOFC) applications due to their anionic, non-stoichiometry inducing oxide ion conductivity [10][11][12]. Due to very thin grain boundaries and the limited birefringence of their melilite structure, these materials can also show a certain transparency when produced via levitation melting [11][12][13].…”

“…The transparency of these materials is essential for optical applications, but currently the process sometimes requires several steps of re-melting (e.g. to remove excess Ga2O3) [12] to produce transparent, crystalline beads. In this manuscript, the growth mechanisms in beads produced via aerodynamic levitation are analyzed in order to gain insight into why some beads are opaque while others are transparent.…”

Rapid solidification synthesis of novel (La,Y)2(Zr,Ti)2O7 pyrochlore-based glass-ceramics for the immobilization of high-level wastes

“…Aerodynamic levitation (ADL) was extensively used in the fabrication of novel/metastable materials [39][40][41]. This synthesis process possesses the advantages of high heating power, no contact and no heterogeneous crystallization, high quenching rate which provides access to metastable phases.…”