A XAFS investigation of amorphous-to-crystalline and fluorite-to-pyrochlore phase transitions in Ln2M2O7 (Ln= Gd, Tb, Dy; M= Ti, Zr)

Amorphous-to-crystalline and fluorite-to-pyrochlore phase transitions in complex oxides Ln2M2O7 (Ln = Gd, Tb, Dy; M = Ti, Zr) prepared by sol-gel and coprecipitation methods were studied using x-ray absorption spectroscopy in combination with x-ray diffraction and Raman spectroscopy. The Ln titanates crystallize directly from the amorphous state to the pyrochlore structure, while the Ln zirconates form the intermediate fluorite phase first. We have found that both XANES and EXAFS data reflect the considerable difference between the crystallization processes of Ln titanates and zirconates. In particular the evolution of the K–Ti pre-edge features is a good indicator of the phase transition from amorphous to crystalline state. The splitting of the first Ln-O shell in the FT modulus of L3-Ln edge EXAFS spectra in Ln titanates can be regarded as a fingerprint of the ordered pyrochlore structure formation which is accompanied also by specific changes in the main features of the XANES spectra. For the Ln zirconates these features are less pronounced.