Synthesis, Rietveld refinement of crystal structure, electron diffraction and electrical transport properties of Ba2(Ca_(1-x-y)Fe_xNb_y)(Nb_(1-z)Fe_z) O_(6-δ) double perovskites

摘要

We report the synthesis, crystal structure, and electrochemical properties of new Fe-doped Ba2(Cai_(1-x-y)Fe_xNb_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 A 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(Ca_(1-x-y)Fe_xNb_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 CO2 and boiling H2O confirmed by PXRD and FTIR. The electrical conductivity was determined by AC impedance spectroscopy. Among the samples investigated, Ba2(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%) N2 at 600 °C with activation energy of 0.43 eV in the temperature range 200-800 °C.