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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-x)Gd(x))O(2-x/2) samples (x = 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 ≤ T ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at x ∼ 0.2 between a CeO2-based solid solution (for x ≤ 0.2) and a structure where Gd2O3 microdomains grow within the CeO2 matrix, taking advantage of the similarity between Gd(3+) and Ce(4+) sizes; the existence of the boundary at x ∼ 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.

A new model potential for van der Waals atomic pairs is proposed and shown to be in excellent agreement with ab initio calculations and experimental potentials. The model allows the potential energy of a given pair to be estimated with good accuracy from ab initio density functional calculations of the free-atom electron densities.

The potential of modern methods for X-ray diffraction Line Profile Analysis can be fully exploited with data collected at synchrotron radiation beamlines, provided that optics and experimental set-up are suitably designed and characterized. The Material Characterization by X-ray Diffraction beamline, MCX, at Elettra-Sincrotrone Trieste, * Dr. L. Rebuffi E-Mail: luca.rebuffi@elettra.eu [a] Elettra-Sincrotrone Trieste S.C.p.A