Sequential evolution of different phases in metastable Gd_2–xCe_xZr_2–xAl_xO₇(0.0 ≤x≤ 2.0) system: crucial role of reaction conditions

Abstract

The Gd_2–xCe_xZr_2–xAl_xO₇ (0.0 ≤ x ≤ 2.0) series was synthesized by the gel combustion method. This system exhibited the presence of a fluorite-type phase, along with a narrow biphasic region, depending upon the Ce/Gd content in the sample. Thermal stability of these new compounds under oxidizing and reducing conditions has been investigated. The products obtained on decomposition of Gd_2–xCe_xZr_2–xAl_xO₇ in oxidizing and reducing conditions were found to be entirely different. It was observed that in air the fluorite-type solid solutions of Gd_2–xCe_xZr_2–xAl_xO₇ composition undergo phase separation into perovskite GdAlO₃ and fluorite-type solid solutions of Gd–Ce–Zr–O or Ce–Zr–Al–O depending upon the extent of Ce and Al substitution. On the other hand, Gd_2–xCe_xZr_2–xAl_xO₇ samples on heating under reducing conditions show a phase separation to CeAlO₃ perovskite and a defect-fluorite of Gd₂Zr₂O₇. The extent of metastability for a typical composition of Gd_1.2Ce_0.8Zr_1.2Al_0.8O₇ (nano), Gd_1.2Ce_0.8Zr_1.2Al_0.8O_6.6 (heated under reduced conditions), Gd_1.2Ce_0.8Zr_1.2Al_0.8O₇ (heated in air at 1200 °C) has been experimentally determined employing a high temperature Calvet calorimeter. On the basis of thermodynamic stability data, it could be inferred that the formation of a more stable compound in the presence of two competing cations (i.e., Gd³⁺ and Ce³⁺) is guided by the crystallographic stability.