A small-box approach to the local crystal structure of Y3NbO7

The local crystal structure of Y3NbO7 was probed using neutron total scattering. Five different structural models were fit to the experimental pair distribution function of this material up to 15 & ANGS;. Fits of defect fluorite (Fm3m), pyrochlore (Fd3m), and orthorhombic C weberite models (C2221) were attempted based on prior structural investigations of Y3NbO7 and related fluorite-based materials. Orthorhombic P weberite (P2221) and monoclinic weberite models (P1121) were derived via symmetry reduction of the orthorhombic C weberite structure. All models except monoclinic weberite failed to reproduce the experimental pair distribution function. Misfits were clearly visible for peaks arising from atom pairs within the first coordination shell (defect fluorite and pyrochlore) and beyond (orthorhombic C and P weberite). By contrast, monoclinic weberite (& gamma; & AP;91.8 degrees) provided an excellent fit, with no major misfits observed. This unit cell features distorted YO8 dodecahedra, YO7 monocapped octahedra, and NbO6 octahedra as building blocks. Comparison between the topologies of the cationic and anionic substructures in monoclinic weberite and its parent orthorhombic C structure revealed displacements of the metal atoms along the [100] direction and a less compact spatial distribution of oxygen atoms along the a axis. The monoclinic weberite model presented herein may serve as a starting point to revisit structural modeling of fluorite-based materials with challenging local structures.