Sequential evolution of different phases in metastable Gd _(2-x)Ce_xZr_(2-x)Al_xO_7 (0.0 ≤ x ≤ 2.0) system: Crucial role of reaction conditions

摘要

The Gd_(2-x)Ce_xZr_(2-x)AlxO _7 (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-x)Ce_xZr_(2-x)AlxO_7 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-x)Ce_xZr_(2-x)AlxO_7 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-x)Ce _xZr_(2-x)AlxO_7 samples on heating under reducing conditions show a phase separation to CeAlO_3 perovskite and a defect-fluorite of Gd_2Zr_2O_7. The extent of metastability for a typical composition of Gd_(1.2)Ce _(0.8)Zr_(1.2)Al_(0.8)O_7 (nano), Gd _(1.2)Ce_(0.8)Zr_(1.2)Al_(0.8)O_(6.6) (heated under reduced conditions), Gd_(1.2)Ce_(0.8)Zr _(1.2)Al_(0.8)O_7 (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., Gd~(3+) and Ce~(3+)) is guided by the crystallographic stability.