Pr₂M₂O7 (M=Zr, Hf) local and crystal structure changes in a fluorite-pyrochlore phase transition

The formation of nanocrystal powders of Pr₂M₂O₇ (M=Zr, Hf) compounds upon calcinations up to 1400°C has been investigated by using of X-ray absorption fine structure (XAFS) spectroscopy combined with X-ray powder diffraction (XRD). The structure ordering upon calcination was estimated independently from XRD and EXAFS data. It was found that the phase transition fluorite-pyrochlore occurs at calcination temperature in the range of 900–1000°C. The appearance of the ordered pyrochlore structure at calcination temperatures above 1000°C was manifested as splitting of the first oxygen coordination shell of the local praseodymium environment and supported by a sharp reduction of the Debye-Waller factor values of interatomic bonds. Besides we observed the L₃ -Hf X-ray absorption near edge line shape splitting with increasing of annealing temperature of amorphous precursor due to crystal field influence in high ordered pyrochlore-type structure. (paper)