FLUORITE-RELATED PHASES LN3MO7,LN= RARE-EARTH, Y, OR SC, M = NB, SB, OR TA .1. CRYSTAL-CHEMISTRY

Compounds Ln3MO7, where Ln = La, Nd, Gd, Ho, Er, Y, or Sc, and M = Nb, Ta, or Sb have been examined by powder X-ray diffraction, electron diffraction, and electron microscopy. For large Ln cations, an orthorhombic fluorite-related superstructure is formed, of probable space group Cmcm for Ln = La and C2221 for Ln = Nd, Gd, Ho, or Y, while for the smaller Ln cations, Er, and under some conditions, Ho and Y, the structure is defect fluorite containing microdomains of ordered, but undetermined, structure. The composition Sc3MO7 was not single phase under the conditions used. Compounds of the type Ln2ScMO7 have the pyrochlore structure. © 1979.