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In this work, we have discovered Ca 3 Ga 4 O 9 as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (E b ) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca 3 Ga 4 O 9 as a candidate of oxide-ion conductors, because Ca 3 Ga 4 O 9 had a relatively low E b . Ca 3 Ga 4 O 9 was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca 3 Ga 4 O 9 indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca 18 and (Ga 4 O 9 ) 6 units where the (Ga 4 O 9 ) 6 units form the two-dimensional (2D) corner-sharing GaO 4 tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca 3 Ga 4 O 9 , which indicates that the major carrier of Ca 3 Ga 4 O 9 is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10 −5 S cm −1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca 3 Ga 4 O 9 sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca 3 Ga 4 O 9 indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga 4 O 9 ) 6 unit]. It was found that the structural and transport properties of Ca 3 Ga 4 O 9 are similar to those of LaSrGa 3 O 7 melilite.

TaO x film formation by a photo-CVD method using TaCl5 as a source material is examined. The deposition rate increases with increasing growth temperature and decreasing chamber pressure down to 1 Torr. The leakage current of the formed TaO x film decreases drastically with annealing in the presence of both UV-irradiation and an oxygen ambient after deposition (p-O2 annealing), when the underlying layer contains Si. The leakage current density is 10-8 A/cm2 at the 4 MV/cm electric field. The dielectric constant for MIS structure capacitors decreases with decreasing TaO x thickness, but does not decrease much with p-O2 annealing. In addition, the mechanisms of reduction of the leakage current with p-O2 annealing are discussed.