Fluorite-related phases Ln3MO 7, Ln = rare earth, Y, or Sc, M = Nb, Sb, or Ta. I. Crystal chemistry

Abstract

Compounds Ln 3 MO 7, 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 C222 1 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 Sc 3 MO 7 was not single phase under the conditions used. Compounds of the type Ln 2Sc MO 7 have the pyrochlore structure.