Pyrochlore-like\nLocal Structure in Globally Disordered\nY<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>: Evidence and Reasoning by\nCombined Theoretical and Experimental Study

Even though technological relevance for nuclear and oxide\nfuel\ncell application has persuaded a large number of investigations on\nthe Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> system, its local structure\nstill remains ambiguous and debatable. While diffraction-based investigations\nclaim a lack of local ordering, the MAS NMR and Raman spectroscopic\ninvestigations speculate the presence of local ordering. Besides,\na correlation between the local and global structures of Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> is also missing to date. Present work\nattempts to get deeper insights into the local structure of Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> and correlate the local structure\nwith global disordering. Complementary investigation using multiple\nstrategies (XRD, RAMAN, electron microscopy, DFT calculations) revealed\nthe presence of pyrochlore-like features in globally disordered Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub>. Globally distributed “local\nprobe ions” (Sn<sup>4+</sup> by MAS NMR and Eu<sup>3+</sup> by photoluminescence (PL)) were utilized for establishing the correlation\nbetween local and global structures. The <sup>119</sup>Sn MAS NMR\nspectra possess the shoulder peak bearing chemical shift values, which\nare improbable under complete cationic randomization. The emission\nand decay profiles of the Eu<sup>3+</sup> ion recommend positioning\nof few probe ions at the centrosymmetric site, which was further reinforced\nby crystal field splitting and lifetime values. Thus, both NMR and\nPL results reaffirm the existence of a pyrochlore-like atomic arrangement\nin predominantly defective fluorite Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> phase. These discernible pyrochlore-like features have been\nenvisaged to be emanating from unevenness in bond strength and ionicity/covalency\nof Y–O and Zr–O bonds. Cationic and anionic randomization\nenergetically endorses global disordering, but the bonding disparity\ninduces short-range ordering. It is believed that the presented findings\nwill guide future research in regulating the physicochemical properties\nof Y<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> for the target applications.