High Temperature Structural Study of Gd-Doped Ceria by Synchrotron X‑ray Diffraction (673 K ≤ <i>T</i> ≤ 1073 K)

The crystallographic features of Gd-doped ceria were investigated at the operating temperature of solid oxides fuel cells, where these materials are used as solid electrolytes. (Ce_1–<i>x</i>Gd_<i>x</i>)­O_{2‑<i>x</i>/2} samples (<i>x</i> = 0.1, 0.3, 0.5, 0.7) were prepared by coprecipitation of mixed oxalates, treated at 1473 K in air, and analyzed by synchrotron X-ray diffraction in the temperature range 673 K ≤ <i>T</i> ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at <i>x</i> ∼ 0.2 between a CeO₂-based solid solution (for <i>x</i> ≤ 0.2) and a structure where Gd₂O₃ microdomains grow within the CeO₂ matrix, taking advantage of the similarity between Gd³⁺ and Ce⁴⁺ sizes; the existence of the boundary at <i>x</i> ∼ 0.2 was confirmed also by measurements of ionic conductivity performed by impedance spectroscopy. Similar to what observed at room temperature, the trend of the cell parameter shows the presence of a maximum; with increasing temperature, the composition corresponding to the maximum moves toward lower Gd content. This evidence can be explained by analyzing the behavior of the coefficient of thermal expansion as a function of composition