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La2Ga3O7.5 : A metastable ternary melilite with a super-excess of interstitial oxide ions synthesized by direct crystallization of the melt

DOI: 10.1021/acs.chemmater.0c03441 DOI Help

Authors: Jintai Fan (CNRS, CEMHTI UPR3079; Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science) , Vincent Sarou-kanian (CNRS, CEMHTI UPR3079) , Xiaoyan Yang (Guilin University of Technology) , Maria Diaz-lopez (ISIS Facility; Diamond Light Source) , Franck Fayon (CNRS, CEMHTI UPR3079) , Xiaojun Kuang (Guilin University of Technology) , Michael J. Pitcher (CNRS, CEMHTI UPR3079) , Mathieu Allix (CNRS, CEMHTI UPR3079)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry Of Materials

State: Published (Approved)
Published: October 2020

Abstract: The La1+xAE1–xGa3O7+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.

Journal Keywords: Chemical structure; Oxides; Electrical conductivity; Layers; Ions

Subject Areas: Materials, Chemistry, Energy

Instruments: I15-1-X-ray Pair Distribution Function (XPDF)

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