Formation and evolution of crystal and local structures in nanostructured Ln2Ti2O7 (Ln = Gd–Dy)

Abstract

The lanthanide titanates Ln2Ti2O7 (Ln = lanthanide) demonstrate a very broad range of structural, chemical and physical properties. We have studied the whole process of the crystallization and local atomic structure rearrangement in Ln2Ti2O7 (Ln = Gd, Tb, Dy) samples synthesized by combination of sol-gel and coprecipitation methods with the subsequent calcination of Ln-Ti precursors. X-ray absorption spectroscopy (XAFS), X-ray powder diffraction, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and simultaneous thermal analysis were used for the complex analysis of the structural properties. It was shown that crystallization of amorphous precursors directly resulted in the formation of nanocrystalline powders with the pyrochlore structure which is formed in process of ordering of both cationic and anionic sublattices. We have demonstrated that both XAFS and Raman spectroscopy ensure the particularly sensitive markers of changes in local electronic and crystal structure of Ln2Ti2O7 depending on the type of Ln element and the preparation procedure. It was established that splitting of the first Ln-O shell in the FT modulus of L3-Ln EXAFS spectra appears to be the reliable indicator of the ordered pyrochlore structure formation, while the changes in the pre-edge region of the K-Ti XANES spectra reflect the process of the Ln2Ti2O7 electronic structure formation.