Local peculiarities of the nanocrystalline structure of ternary oxides Ln2Hf2O7 (Ln = Gd, Tb, Dy)

Abstract

The crystal, local and electronic structure of complex oxides Ln₂Hf₂O₇ (Ln = Gd, Tb, Dy) formed as a result of annealing of amorphous mixed hydroxides LnHf(OH)₇∙nH₂O (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).