Synthesis, Rietveld refinement of crystal structure, electron diffraction, and electrical transport properties of Ba2(Ca1–x–yFexNby)(Nb1–zFez)O6–δ double perovskites

Abstract

We report the synthesis, crystal structure, and electrochemical properties of new Fe-doped Ba2(Ca1–x–yFexNby)(Nb1–zFez)O6–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 Ba2(Ca1–x–yFexNby)(Nb1–zFez)O6–δ. This was further verified by bond valence sum (BVS) analysis of the cations. The Fe–BCN showed superior chemical stability in pure CO2 and boiling H2O confirmed by PXRD and FTIR. The electrical conductivity was determined by AC impedance spectroscopy. Among the samples investigated, Ba2(Ca0.79Fe0.21)(Nb0.71Fe0.29)O6–δ showed the highest total (bulk + grain-boundary) conductivity of 1.1 × 10−2 S cm–1 in humidified (3%) N2 at 600 °C with activation energy of 0.43 eV in the temperature range 200–800 °C.