Abstract
Bismuth-deficient sodium bismuth titanate (nominally Na0.5Bi0.49TiO2.985, NB0.49T) is a good oxide-ion conductor. Here we report the influence of A-site divalent ions, M2+=Ca2+, Sr2+ and Ba2+, on the electrical properties of NB0.49T. A-site divalent doping for Bi3+ enhances the bulk (grain) conductivity by ~one order of magnitude without changing the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on the doping mechanism Bi3++½ O2−→M2+. Among these three dopants, Sr2+ is the most effective in increasing the bulk conductivity due to a combination of its smaller mismatch in ion size with Bi3+, its intermediate polarisability and lower bond strength to oxygen compared to Ca2+ and Ba2+. Doping strategies for further improvements to bulk conductivity of NBT materials are discussed based on these results. Comparison with other oxide-ion conductors and initial stability test under reducing atmosphere show the doped non-stoichiometric NBT materials are promising for low and intermediate temperature applications.
Highlights
Oxide-ion conductors are widely used in various important technological devices such as solid oxide fuel cells (SOFCs), oxygen sensors, oxygen pumps, oxygen separation membranes, etc. [1,2,3,4]
High ionic conductivity was found in bismuth-deficient NBT, i.e., Na0.5Bi0.49TiO2.985, NB0.49T, which originates from oxygen vacancies generated according to the following Kroger-Vink equation
From a structural view point, ionic conductivity of perovskite-type oxides is often dependent on the tolerance factor, t, and specific free volumep, Vffiffisffif. t is determined by the ionic radii of the various ions as t 1⁄4 ðrA þ rOÞ= 2ðrB þ rOÞ, where rA, rB are the mean ionic radii for A and B site cations and rO is the ionic radius of oxygen ion
Summary
Oxide-ion conductors are widely used in various important technological devices such as solid oxide fuel cells (SOFCs), oxygen sensors, oxygen pumps, oxygen separation membranes, etc. [1,2,3,4]. A new family of oxide-ion conductors based on the ferroelectric perovskite sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) was reported [6,7]. High ionic conductivity was found in bismuth-deficient NBT, i.e., Na0.5Bi0.49TiO2.985, NB0.49T, which originates from oxygen vacancies generated according to the following Kroger-Vink equation ð1Þ. [8,9,10] reveal the lowest migration barriers for oxygen vacancies occur in saddle points between two A-sites and a Ti-site when both Asites are Bi ions, whereas higher barriers are observed for A-site combinations of Na and Bi or 2 Na ions.
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