Fluorite-pyrochlore phase transition in nanostructured Ln2Hf2O7 (Ln = La-Lu)

Complex oxides of the Ln(2)Hf(2)O(7) (Ln = lanthanide) series undergo a fluorite to pyrochlore phase transformation. We have studied the whole process of the crystal and local atomic structure realignment during the crystallization and the phase transition in the series of Ln(2)Hf(2)O(7) (Ln = La-Lu) samples synthesized by the coprecipitation method with the subsequent annealing of mixed hydroxides (precursors). The study employed a combination of x-ray diffraction (normal and anomalous), x-ray absorption spectroscopy, analysis of atomic pair distribution function and Raman spectroscopy. The starting and ending temperatures of the fluorite-pyrochlore phase transition for Ln(2)Hf(2)O(7) compounds have been determined along the lanthanide series La-Dy. The scheme summarizing structure types (amorphous, fluorite and pyrochlore) for the whole Ln(2)Hf(2)O(7) (Ln = La-Lu) series as a function of the Ln cation radius (or the r(Ln3+) = r(Hf4+) ratio) and the annealing temperature has been refined. (C) 2016 Elsevier B.V. All rights reserved.