Abstract
Sm2−xCaxZr2O7−x/2 (x = 0, 0.05, 0.1) and Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore–fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100–900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700–950 °C. The Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen–ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) is ~ 1 × 10−4 S/cm. The fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) to fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2−xCax)Zr2O7−δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2−xCaxZr2O7−x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.
Highlights
An extremely important subject of alternative energy is the development of materials for proton-conducting solid oxide fuel cells (PC-SOFCs)
There are numerous experimental studies discussing the effect of chemistry and/or disorder on ionic conductivity of pyrochlores [2,3,4,5,6,7,8,9,10,11,12,13,14], there are only a few that examine the impact of grain boundaries (GBs) on oxygen and proton diffusion in pyrochlores [15,16,17,18]
The purpose of this work is to assess the ratio of oxygen-ion conductivity to proton conductivity in undoped and Ca-doped Sm2 Zr2 O7 and Gd2 Zr2 O7 pyrochlores, the compositions of which lie at the pyrochlore–fluorite morphotropic phase boundary
Summary
An extremely important subject of alternative energy is the development of materials for proton-conducting solid oxide fuel cells (PC-SOFCs). It is probably for this reason that Kutty et al [28], who synthesized Gd2−x Srx Zr2 O7−x/2 solid solutions at 1400 ◦ C (within the stability range of ordered pyrochlore Gd2 Zr2 O7 [27]), found that the oxygen-ion conductivity of. A Ca-doped 3+/5+ pyrochlore series, which has a pyrochlore–fluorite morphotropic phase boundary, was shown to have proton conductivity [32], which increases in going from. The purpose of this work is to assess the ratio of oxygen-ion conductivity to proton conductivity in undoped and Ca-doped Sm2 Zr2 O7 and Gd2 Zr2 O7 pyrochlores, the compositions of which lie at the pyrochlore–fluorite morphotropic phase boundary. It is of interest to examine how bulk and grain-boundary oxygen-ion conductivity varies in going from the pyrochlores to fluorites and the proton conductivity of the rare-earth zirconate solid solutions gradually disappears. We obtained disordered Gd2 Zr2 O7 -based solid solutions, more similar in structure to fluorite, whereas the Sm2 Zr2 O7 -based solid solutions had the pyrochlore structure
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