Fluorite-related phases Ln3MO7, Ln = rare earth,Y or Sc,M = Nb,Sb, or Ta: II. Structure determination

The crystal structures of the orthorhombic fluorite-related compounds La₃NbO₇ (Cmcm), Y₃TaO₇, and Y₂GdSbO₇, (C222₁) have been determined from X-ray powder diffraction intensities and single-crystal electron diffraction data. The structures are basically similar. One third of the Ln cations are 8-coordinated, and lie in 001] rows which alternate with parallel rows of corner-linked MO₆ coordination octahedra within slabs parallel to (100). The remaining Ln cations lie in between these slabs in seven-fold coordination. This interslab cation arrangement distinguishes the structure from the closely related pyrochlore (A₂B₂O₇) and weberite (Na₂MgAlF₇) structures. The compounds YGdScSbO₇ and Nd₂ScNbO₇, also examined, have the pyrochlore structure, with Sc and Sb, or Sc and Nb distributed on the octahedral B sites.