Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

Abstract

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

“…The pyrochlore structure A 2 B 2 O 7 is a derivative of the fluorite structure in which half of the cubes are replaced by octahedra (more precisely, it consists of the alternating AO 8 polyhedra and BO 6 trigonal antiprisms). Pyrochlores possessing a highly mixed ionic-electronic conductivity such as doped Pr 2 Zr 2 O 7 , Gd 2 Ti 2 O 7 , Er 2 RuMnO 7 , etc., are used in SOFC cathodes [ 164 , 225 , 226 ], oxygen [ 83 , 227 , 228 ] and hydrogen separation membranes [ 229 , 230 ]. They contain high amounts of oxygen vacancies providing fine oxygen transport characteristics.…”

“…There are two forms of oxygen in the pyrochlore structure (O, O′), of which the content ratio is 6:1. However, according to TPIE C 18 O 2 studies [ 56 , 58 , 164 , 228 , 231 , 232 , 233 ], the oxygen bulk mobility is uniform, or, in the case of its nonuniformity, the ratio of various oxygen forms differing in their mobility differs from 6:1. This is evidence that the oxygen migration mechanism is rather complex and includes the oxygen of both O- and O’-sublattices.…”

“…This is evidence that the oxygen migration mechanism is rather complex and includes the oxygen of both O- and O’-sublattices. It was proposed as well that the oxygen forms differing in their mobility can be associated with A–O–A, A–O–B and B–O–B migration pathways with their fraction depending on the partial disordering of the pyrochlore structure [ 164 , 228 ]. The other feature of some pyrochlores (Mg-doped Sm and Gd zirconates) is the fast oxygen transport along grain boundaries being characterized by a very high mobility ( D* ~10 −7 cm 2 s −1 at 1000 K) [ 164 ].…”

“… Arrhenius plots for oxygen tracer diffusion coefficients for Bi 2 Ce 2 O 7 (1) [ 228 ], Bi 1.6 Y 0.4 Ce 2 O 7 (2) [ 228 ], Bi 1.6 Y 0.4 Ti 2 O 7 (3) [ 228 ], Bi 1.6 Sc 0.2 Ti 2 O 7−δ (4) [ 232 ], Bi 1.6 Mg 0.2 Ti 2 O 7-δ (5) [ 232 ] and Bi 1.6 Zn 0.2 Ti 2 O 7−δ (6) [ 231 ]. Adapted from Ref.…”

“…Adapted from Ref. [228] under the CC BY 4.0 license. Shimura et al [234] studied the proton conductivity of Ln 2 Zr 2 O 7 -based (Ln = La, Nd, Sm, Gd и Er) pyrochlore oxides and found that the conductivity of the Ln 2 Zr 1.8 Y 0.2 O 7−δ (Ln = La, Nd, Sm, Gd и Er) solid solutions in a hydrogen atmosphere at T > 600 • C was comparable to that of perovskites.…”