Studying processes of crystallization and cation ordering in Eu2Hf2O7
Methods of local X-ray anomalous diffraction and X-ray absorption spectroscopy based on the interaction of synchrotron radiation with condensed matter have been used to study the processes of formation and evolution of the crystal structure in Eu2Hf2O7 powders. It has been shown that the isothermal annealing of the X-ray amorphous mixed hydroxide leads initially to the formation of nanocrystalline powders, which have a defect fluorite structure with a clearly pronounced nonequivalence of the local environment of the Eu3+ and Hf4+ cations. An increase in the temperature of heat treatment to ≥1200°C leads to the nucleation and growth of nanodomains with a superstructural cation ordering of the pyrochlore type inside the coarse crystallites of the defect fluorite and, finally, at T ≥ 1500°C, to the complete phase transition of the fluorite-pyrochlore type in the Eu2Hf2O7 system. In the pyrochlore Eu2Hf2O7 structure, there was for the first time revealed a simultaneous broadening of the shape of the L3 absorption edges of both hafnium and europium, which indicates a partial change in the symmetry of the nearest oxygen environment of both cations.
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