Novel melilite-type gallium-oxides are attracting attention as promising new oxide-ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic-scale investigation of the LaSrGa 3 O 7 -based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner-sharing GaO 4 tetrahedra. A major finding is the first indication that oxide-ion conduction in La 1.54 Sr 0.46 Ga 3 O 7.27 occurs through an interstitialcy or cooperative-type mechanism involving the concerted knock-on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.
La 2 Mo 2 O 9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities comparable to those of yttria-sabilized zirconia (YSZ) . LAMOX is subject to a structural phase transition from the non-conductive monoclinic form to the highly-conductive cubic form at about 580°C. The origin of the conductivity in cubic LAMOX has been suggested to be due to a "disorder" in the O sub-lattice without any insight into the real distribution of the oxygen ion. In this paper, thanks to the application of the neutron atomic pair distribution function (PDF) analysis, we provided evidences that the local structure of the cubic polymorph of LAMOX is exactly the same of that of the monoclinic phase thus indicating that the structural phase transition is actually a transition from a static to a dynamic distribution of the oxygen defects. This work represent the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor and clearly indicates that a more reliable and detailed description of their local structure, particularly in the highly conductive phases, and can lead to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.
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