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

Abstract

The crystal structures of various synthetic ABO 4 compounds, in which A is one of the rare earths La, Ce, Nd, Sin, Gd, Dy, Y or Sc, and B is either Nb or Ta, have been examined. Most of the compounds are isotypic with the monoclinically distorted scheelite-like structure, identified by Ferguson for a high-temperature-fired specimen of YTaO4. The lattice parameters are recorded. High-temperature X-ray study of niobium compounds with this structure proves that the mono-clinic distortion is progressively reduced with increasing temperature, and a reversible transition to a tetragonal structure occurs below 800 °C. With tantalum compounds, no structure transition takes place up to 1300 °C, though at 1300 °C a marked reduction of the monoclinic distortion is evident. The two scandium compounds are exceptional and have a monoclinic wolframite-like crystal structure. This difference in structure appears to be dictated by the relatively small radius of the scandium cation. With the largest rare-earth cation, La 3+, the niobium compound has the mono-clinically distorted seheelite-like structure, but the tantalum compound has a different and unidentified structure.