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

• The scheme of structure types in lanthanide hafnates Ln2Hf2O7 was refined.
• Complementary experimental methods XRD, XAFS, PDF and Raman spectroscopy were used.
• The structure realignment during the fluorite-pyrochlore transition was captured.
• The Ln and Hf local environment is highly nonequivalent in disordered fluorite.
• The pyrochlore phase emerges as nanodomains in a fluorite matrix for Ln = La-Dy.

Complex oxides of the Ln2Hf2O7 (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 Ln2Hf2O7 (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 Ln2Hf2O7 compounds have been determined along the lanthanide series La-Dy. The scheme summarizing structure types (amorphous, fluorite and pyrochlore) for the whole Ln2Hf2O7 (Ln = La-Lu) series as a function of the Ln cation radius (or the rLn3+/rHf4+rLn3+/rHf4+ ratio) and the annealing temperature has been refined.

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