The pyrochlore to defect fluorite phase transition in Y2Sn2−xZrxO7

Abstract

The system Y₂Sn_2−xZr_xO₇ (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.