Abstract
Solid oxide fuel cells (SOFCs) are a major candidate technology for clean energy conversion because of their high efficiency and fuel flexibility.1 The development of intermediate-temperature (500–750 °C) SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10−2 S cm−1 assuming an electrolyte thickness of 15 μm1). This conductivity, in turn, necessitates enhanced understanding of the mechanisms of oxide ion charge carrier creation and mobility at an atomic level. The charge carriers are most commonly oxygen vacancies in fluorites2, 3 and perovskites.3, 4 There are fewer examples of interstitial-oxygen-based conductors such as the apatites5, 6 and La2Mo2O9-based materials,7–9 so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.
Highlights
The A2B3O7 melilite structure consists of anionic layers of five-membered rings of two totally condensed and three partially condensed BO4 tetrahedra, separated by sheets of A cations located above the five-ring centers (Figure 1 a, and Figure S1.1 in the Supporting Information)
Selected area electron diffraction and high-resolution TEM imaging require further doubling of c to give a body-centered supercell with no other systematic absences (Figure S3.2-3 in the Supporting Information), consistent with space groups Imm[2], Im, and I1
The ionic conductivity of the distorted phase is lower than that of the less doped La1.54Sr0.46Ga3O7.27, which retains the small-cell tetragonal structure at all temperatures, suggesting that the structural phase transition is produced by static ordering of the interstitial oxide charge carriers
Summary
The A2B3O7 melilite structure consists of anionic layers of five-membered rings of two totally condensed (four neighboring tetrahedra linked by B-O-B bonds) and three partially condensed (three neighboring tetrahedra) BO4 tetrahedra, separated by sheets of A cations located above the five-ring centers (Figure 1 a, and Figure S1.1 in the Supporting Information). The refined average structure defines the O interstitial and La dopant occupancies relative to the symmetry-distinct centers of the five-rings of the Ga3O7 layer, a more chemically acceptable locally relaxed model is needed to give quantitatively reasonable bond valence sums (BVS)[15,16] at the under-bonded interstitial oxide sites and at the isolated three-connected GaO4 tetrahedra that are closest to the interstitial oxygen atoms (BVS % 3.30, Table S5.5 in the Supporting Information).
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