Effect of Pr3+/Pr4+ ratio on the oxygen ion transport and thermomechanical properties of the pyrochlore and fluorite phases in the ZrO2-Pr2O3 system

Abstract

This paper examines the effect of the Pr3+/Pr4+ ratio on the mechanism of ionic and electronic transport in the (Pr2−xZrx)Zr2O7+x/2 (x = 0.15), Pr2Zr2O7, and Pr2(Zr2−xPrx)O7−x/2 (x = 0.1) pyrochlore phases and Pr3ZrOx with the fluorite structure and on the behavior of their thermal expansion coefficient (TEC). The solid solutions were prepared through coprecipitation followed by firing of the green compacts in air at a high temperature of 1550 °C for 4 h. The Pr3+/Pr4+ ratio was shown to decrease in going from the (Pr2−xZrx)Zr2O7+x/2 (x = 0.15), Pr2Zr2O7, and Pr2(Zr2−xPrx)O7−x/2 (x = 0.1) pyrochlores to the Pr3ZrOx fluorite, leading to changes in the conductivity type from mixed (ionic–electronic) to electronic and in the color of the materials from beige to black and to an anomalous deviation of the TEC from linearity in fluorite Pr3ZrOx, i.e. at the highest Pr4+ content. According to impedance spectroscopy results, (Pr2−xZrx)Zr2O7+x/2 with x = 0.15 has purely oxide-ion conductivity (3 × 10−3 S/cm at 1000 °C) in a wide range of oxygen partial pressures: from 10−10 to 102 Pa. With increasing Pr content, p-type electronic conductivity becomes significant, reaching a maximum in fluorite Pr3ZrOx: ∼0.5 S/cm at 1000 °C. According to XPS data, all pyrochlore samples (Pr2−xZrx)Zr2O7+x/2 (x = 0.15), Pr2Zr2O7 and Pr2(Zr2−xPrx)O7−x/2 (x = 0.1) contain only Pr3+ at room temperature, whereas Pr3ZrOx contains both Pr3+ and Pr4+. The considerable deviation of the TEC of Pr3ZrOx from linearity above 500 °C is due to partial reduction of Pr4+. The reduction process Pr4+ + e′ → Pr3+ followed by oxygen release in the range 500–1100 °C has been identified in Pr3ZrOx by thermal analysis and mass spectrometry in a He atmosphere.