Interstitial Oxide Ion Order and Conductivity in La1.64Ca0.36Ga3O7.32 Melilite**

Solid oxide fuel cells (SOFCs) are a major candidate technology for clean energy conversion because of their high efficiency and fuel flexibility. The development of intermediate-temperature (500–750 8C) SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10 2 Scm 1 assuming an electrolyte thickness of 15 mm). 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 fluorites and perovskites. There are fewer examples of interstitial-oxygenbased conductors such as the apatites and La2Mo2O9-based materials, so information on how these excess anion defects are accommodated and the factors controlling their mobility is important. 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 1a, and Figure S1.1 in the Supporting Information). Previously, we demonstrated that A-site substitution in melilite LaSrGa3O7 (A2=LaSr, B=Ga) affords La1.54Sr0.46Ga3O7.27, in which interstitial oxygen atoms (Oint) are located in the five-rings of the two-dimensional tetrahedral network, gives pure oxide ion conductivity of 0.02– 0.1 Scm 1 over the 600–900 8C temperature range. LaCaGa3O7 also adopts the tetragonal melilite structure (space group P4̄21m) found for the Sr phase. [12,13] When doped to introduce oxide charge carriers in the La1+xCa1 xGa3O7+0.5x series, the tetragonal structure is retained to x= 0.5. Herein, we address the impact on the physical properties of a new lower-symmetry phase formed to accommodate the higher interstitial doping levels 0.55 x 0.64. X-ray powder diffraction from La1.64Ca0.36Ga3O7.32 (Section 2 in the Supporting Information) indicates that the lowering of symmetry at high doping levels occurs by Figure 1. a) View of the average structure of the Ga3O7+x/2 network along the [110] direction for La1.64Ca0.36Ga3O7.32. Four of the eight possible oxygen interstitial sites (Chn, n=1–8, labeled in blue) in the five-rings are occupied (Ch1 (O14a) 58.1(1)%, Ch3 (O14b) 23.9(1)%, Ch5 (O13a) 28.0(1)%, Ch7 (O13b) 17.9(1)%). The local ordering involves coupled occupancy of the Ch1/Ch7 (green) and Ch5/Ch3 (red) sites, which differ in the polarity of the interstitial displacement along the [101] direction towards the single three-connected GaO4 unit within each ring, as indicated with broken lines of different colors. The two orientations of the three-connected Ga2O7 dimers within the fiverings are shown with black Ga-O-Ga bonds (unoccupied ring centroids) and by representing the bridging oxygen atoms (O3a/O3b, O4a/ O4b) of the centroid-occupied rings in pink. b) Arrhenius plot of total conductivity for La1.64Ca0.36Ga3O7.32 compared with that for La1.54Sr0.46Ga3O7.27. The dashed line marks the phase transition shown by variable-temperature NPD (top-right inset) from O (orthorhombic) to T (tetragonal); the bottom-left inset shows the stacking of La1+xCa1 x (blue) and Ga3O7 layers in melilite. [*] Dr. M. R. Li, Dr. X. Kuang, Dr. S. Y. Chong, Dr. Z. Xu, C. I. Thomas, Dr. H. J. Niu, Dr. J. B. Claridge, Prof. Dr. M. J. Rosseinsky Department of Chemistry, University of Liverpool Liverpool, L69 7ZD (UK) Fax: (+44)151-794-3598 E-mail: [email protected] [email protected]