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Journal of Structural Chemistry

2016 year, number 7


A. P. Menushenkov1, V. V. Popov1, Ya. V. Zubavichus2, A. A. Yaroslavtsev1
1National Research Nuclear University MEPhI, Moscow
2NRC "Kurchatov Institute", Moscow
Keywords: флюорит, пирохлор, EXAFS спектроскопия, PDF-анализ, аномальная дифракция, fluorite, pyrochlore, EXAFS spectroscopy, PDF analysis, anomalous X-ray diffractionvv


The crystal, local and electronic structure of complex oxides Ln2Hf2O7 (Ln = Gd, Tb, Dy) formed as result of annealing of amorphous mixed hydroxides LnHf(OH)7x n H2O (precursors) have been studied using a set of modern local-sensitive techniques of structural analysis based on the interaction of synchrotron radiation with condensed matter (XAFS spectroscopy, PDF analysis, anomalous X-ray diffraction). It was shown that the thermal treatment of precursors in the temperature range 600-700°C/3 h induces the formation of nanocrystallites with the fluorite structure characterized by the considerable inequivalence of local surrounding of Ln3+ and Hf4+ cations. It was found that the cation ordering of the pyrochlore type takes place in nanocrystals of gadolinium hafnate Gd2Hf2O7 and terbium hafnate Tb2Hf2O7. The phase transition fluorite - pyrochlore goes through the formation of the pyrochlore nanodomains in the matrix of well-crystallized fluorite. In the case of Gd2Hf2O7 the formation of the pyrochlore local structure is completed at annealing temperature ~1000-1200°C/3 h and at > 1300°C/3 h the pyrochlore phase is detected by the diffraction techniques. In Tb2Hf2O7 the process of the pyrochlore phase formation exhibits the more complex behavior. Nanocrystals of dysprosium hafnate Dy2Hf2O7 retain the structure of defect fluorite in the whole temperature range of the heat treatment (up to 1600°C).