The structures of 1:1 compounds of rare earth oxides with niobia and tantala

The crystal structures of various synthetic ABO/sub 4/ compounds, in which A is one of the rare earths La, Ce, Nd, Sm, Gd, Dy, Y, or Sc, and B is either Nb or Ta, were examined. Most of the compounds are isotypic with the monoclinically distorted scheelite-like structure, identified for a high- temperature-fired specimen of YTaO/sub 4/. The lattice parameters are recorded. A high-temperature xray study of niobium compounds with this structure proved that the monoclinic distortion is progressively reduced with increasing temperature, and a reversible transition to a tetragonal structure occurs below 800 deg C. With tantalum compounds, no structure transition takes place up to 1300 deg C, though at 1300 deg C a marked reduction of the monoclinic distortion is evident. The two scandium compounds are exceptional and have a monoclinic wolframite-like crystal structure. The difference in structure appears to be dictated by the relatively small radius of the scandium cation. With the largest rare-eanth cation, La/sup 3+/, the niobium compound has the monoclinically distorted scheelitelike structure, but the tantalum compound has a different and unidentified structure. (auth)