FLUORITE-RELATED PHASES LN3MO7, LN = RARE-EARTH, Y OR SC, M = NB, SB, OR TA .2. 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. © 1979.