National Research Nuclear University MEPhI, Moscow, Russia:

V. V. Popov, Senior Researcher, e-mail: victorvpopov@mail.ru
А. А. Yastrebtsev, Postgraduate Student, e-mail: alexyastrebtsev@gmail.com
A. P. Menushenkov, Head of Department, Professor, e-mail: APMenushenkov@mephi.ru

VNIIKhT, Moscow, Russia:

N. A. Tsarenko, Lead Researcher, e-mail: nadatsar@gmail.com

Using X-ray diffraction and Raman spectroscopy, the authors of this paper investigated the formation and evolution of crystalline structure in complex oxides Ln₂Me₂O₇ (Ln = La – Lu; Me = Ti, Zr, Hf), which occur as a result of calcination of amorphous precursors synthesized through coprecipitation from metal salt solutions. It is shown that the crystallization parameters and further phase transformations are largely governed by the cation radius ratio γ = r_Ln³⁺/r_Me⁴⁺. In the case of zirconates and hafnates of rare earth elements (REE), nanocrystalline powders with fcc fluorite are initially formed during amorphous precursor crystallization. Further increase of the annealing temperature to ≥1,000 ^oC in the case of Ln = La – Dy leads to the phase transition fluorite → pyrochlore, the temperature of which rises and the intensity decreases as the REE radius gets smaller. The powders containing REEs with smaller cation radii retain their fluorite structure within the entire tempe rature range studied. For Yb₂Zr₂O₇, it was found that δ-phase was formed. In the case of rare earth titanates (Ln = Sm – Yb), crystallization of amorphous precursors directly resulted in a pyrochlore structure. For titanates of REEs with big cation radii (Ln = La – Nd), generation of layered perovskites was observed. Crystallite growth activation energies were calculated for the above mentioned types of phase transitions, which helped explain the sequence of structural transformations (amorphous phase → fluorite → pyrochlore) observed in rare earth zirconates and hafnates from the energy point of view. A good correlation was found between the X-ray diffraction and Raman spectroscopy data. It was concluded that Raman spectroscopy provides a more sensitive technique (versus X-ray diffraction) for studying the phase transitions occurring in complex rare earth oxides.

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