Pyrochlore-like Local Structure in Globally Disordered Y₂Zr₂O₇: Evidence and Reasoning by Combined Theoretical and Experimental Study

Even though technological relevance for nuclear and oxide fuel cell application has persuaded a large number of investigations on the Y₂Zr₂O₇ system, its local structure still remains ambiguous and debatable. While diffraction-based investigations claim a lack of local ordering, the MAS NMR and Raman spectroscopic investigations speculate the presence of local ordering. Besides, a correlation between the local and global structures of Y₂Zr₂O₇ is also missing to date. Present work attempts to get deeper insights into the local structure of Y₂Zr₂O₇ and correlate the local structure with global disordering. Complementary investigation using multiple strategies (XRD, RAMAN, electron microscopy, DFT calculations) revealed the presence of pyrochlore-like features in globally disordered Y₂Zr₂O₇. Globally distributed "local probe ions" (Sn⁴⁺ by MAS NMR and Eu³⁺ by photoluminescence (PL)) were utilized for establishing the correlation between local and global structures. The ¹¹⁹Sn MAS NMR spectra possess the shoulder peak bearing chemical shift values, which are improbable under complete cationic randomization. The emission and decay profiles of the Eu³⁺ ion recommend positioning of few probe ions at the centrosymmetric site, which was further reinforced by crystal field splitting and lifetime values. Thus, both NMR and PL results reaffirm the existence of a pyrochlore-like atomic arrangement in predominantly defective fluorite Y₂Zr₂O₇ phase. These discernible pyrochlore-like features have been envisaged to be emanating from unevenness in bond strength and ionicity/covalency of Y-O and Zr-O bonds. Cationic and anionic randomization energetically endorses global disordering, but the bonding disparity induces short-range ordering. It is believed that the presented findings will guide future research in regulating the physicochemical properties of Y₂Zr₂O₇ for the target applications.