The pyrochlore to defect fluorite phase transition in Y2Sn2−xZrxO7

Abstract

The system Y2Sn2−xZrxO7 (0.0 ≤ x ≤ 2.0) undergoes a phase transformation from ordered pyrochlore (Fdm) to defect fluorite (Fmm) actuated by the substitution of Zr for Sn. X-ray diffraction patterns map the retention of the pyrochlore structure up to x = 1.2. For samples with x = 1.4–2.0 the structure can be described as defect fluorite in broad terms. Electron diffraction patterns are consistent with this interpretation; however, they also demonstrate that the defect fluorite phase exhibits a strain driven compositional/displacive modulation that changes gradually with increasing Zr content. Raman spectra are consistent with gradual anion disorder up to x = 1.0 and highly disordered anion distributions inferred for x > 1.4, but the spectra also suggest the presence of residual order due to the modulated structure. The phase transformation in this system occurs at a higher Zr content than predicted by classical radius ratio models, consistent with the covalent character of Sn–O bonding. An unusual finding of this work comes from 119Sn MAS NMR and Sn L3-edge XANES analyses, indicating that Sn4+ prefers to occupy lattice sites with a 6-fold local coordination environment throughout the series. These results suggest that the incorporation of Sn or other metal cations having significant covalent bonding or a strong preference for octahedral coordination in pyrochlore-based materials may have a detrimental effect on ionic conductivity.