Oxygen interstitials and vacancies in LaSrGa3O7-based melilites

The Sr-rich composition of the layered tetrahedral meililite, La0.8Sr1.2Ga3O6.9, was synthesized and a structural investigation on La0.8Sr1.2Ga3O6.9 using neutron powder diffraction revealed a site preference of oxygen vacancies on the bridging oxygen sites of the 4-linked GaO4 tetrahedra. Impedance measurement revealed limited ionic conduction in the oxygen-deficient La0.8Sr1.2Ga3O6.9, presumably associated with oxygen vacancies, which is 2 orders of magnitude higher than the parent material LaSrGa3O7 but similar to 3-4 orders of magnitude lower than the interstitial oxide ionic conductivity in La-rich composition, La1.54Sr0.46Ga3O7.27. Low temperature neutron powder diffraction characterization was performed for the oxygen-excess, La-rich composition, La1.54Sr0.46Ga3O7.27, which confirmed the position near the center of the pentagonal tunnels for the oxygen interstitials identified previously using the room temperature data. Solid state Ga-71 NMR data collected on these LaSrGa3O7-based materials with stoichometric, excess, and deficient oxygen contents was found not able to distinguish these three compositions. A metastability temperature gap within 850-1280 degrees C was identified for the oxygen interstitial-conducting La1.54Sr0.46Ga3O7.27. The structures of these oxygen excess and deficient gallate melilites further demonstrate the structural flexibility of the LaSrGa3O7-based layer tetrahedral network. (C) 2015 Elsevier Inc. All rights reserved.