Formation and evolution of crystal and local structures in nanostructured Ln 2Ti2O7 (Ln = Gd–Dy)

Highlights: • Local atomic structure rearrangement of lanthanide titanates is investigated. • XAFS, XRD, IR, Raman spectroscopy and SEM methods were used. • Calcination of amorphous precursors directly results in the pyrochlore structure. • Splitting of Ln-O shell in EXAFS spectra is an indicator of the pyrochlore structure. • Pyrochlore forms due to independent ordering of anionic and cationic sublattices. 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.