Synthesis, Rietveld refinement of crystal structure, electron diffraction, and electrical transport properties of Ba 2 (Ca 1– x – y Fe x Nb y )(Nb 1– z Fe z )O 6–δ double perovskites

We report the synthesis, crystal structure, and electrochemical properties of new Fe-doped Ba 2 (Ca 1–x–y Fe x Nb y )(Nb 1–z Fe z )O 6–δ (Fe–BCN). The Fe–BCN was synthesized by a solid-state method in air using corresponding metal oxides and salts at elevated temperature. All of the observed powder X-ray diffraction (PXRD) peaks of the investigated Fe–BCN were indexed as cubic Fm-3m (space group No. 225) double perovskite-type structure with cell constants a ∼ 8.4 Å and is consistent with selected area electron diffraction (SAED). Rietveld analysis suggested the distribution of Fe was at both 4a and 4b sites, supported the proposed chemical formula Ba 2 (Ca 1–x–y Fe x Nb y )(Nb 1–z Fe z )O 6–δ . This was further verified by bond valence sum (BVS) analysis of the cations. The Fe–BCN showed superior chemical stability in pure CO 2 and boiling H 2 O confirmed by PXRD and FTIR. The electrical conductivity was determined by AC impedance spectroscopy. Among the samples investigated, Ba 2 (Ca 0.79 Fe 0.21 )(Nb 0.71 Fe 0.29 )O 6–δ showed the highest total (bulk + grain-boundary) conductivity of 1.1 × 10 −2 S cm –1 in humidified (3%) N 2 at 600 °C with activation energy of 0.43 eV in the temperature range 200–800 °C.