Effect of the oxidation states on the electrical properties of Fe-doped Pr2Zr2O7 pyrochlore

Abstract

Pyrochlores with formula Pr2Zr2-xFexO7±δ (x = 0.00, 0.05, 0.10 and 0.15) were synthesized by the coprecipitation method. Pellets of these compositions sintered at 1400 °C for 12 h were characterized by XRD, Rietveld Refining Method, SEM/EDX, XPS, TGA, and Impedance Spectroscopy (IS). The solid solution limit was x ≤ 0.10. All the compositions exhibited a p-type electronic conductivity mechanism, and the total conductivity increased with the presence of iron. A mixture of Pr(IV)/Pr(III) and Fe(II)/Fe(III) ions was observed by XPS measurements in all samples sintered in air. Moreover, when samples were submitted to different atmospheres and temperatures, the IS technique combined with XPS and TGA was used to detect changes in the oxidation state of the elements present. A reduction of Pr(IV) to Pr(III), accompanied by a significant change in conductivity, was observed at temperatures around 500 °C for samples measured in N2. For the iron-doped compositions, a delay in the praseodymium reduction of 50 °C was observed associated with the effect of oxygen retention in the samples. These compositions showed a higher proportion of Pr(IV) in both oxygen and nitrogen atmospheres. The presence of Pr(IV) implies a greater retention of oxygen in the sample. The results presented in this study allow the electrical properties of the pyrochlore to be tailored depending on the oxidation states of the ions present in it and provide information for future applications of this material, for example, as a component in solid-oxide fuel cells, the generation of clean energy, a heterogeneous catalyst, or an oxygen storage material.