Phase changes and lattice distortion in fluorite-related phases of R₃TaO₇ (3R₂O₃·Ta₂O₅, R=rare earth)

The crystal structuresof the compounds R₃TaO₇ (3R₂O₃·Ta₂O₅, RLaYb, Y) were studied by X-ray diffraction method. According to the powder X-ray patterns, R₃TaO₇ prepared by solid state reaction at 1700°C for for 4 h showed several polymorphic modifications based on fluorite-related structures. The diffraction patterns are of cubic fluorite-like type for smaller R³⁺ ions. For larger R³⁺ ions the superlattice lines appeared and the diffraction peak of the fundamental lines was broadened and/or apparently split, which is of weberite orthorhombic type. The orthorhombic structure has monoclinic distortion of the fluorite unit cell. With the increase of the ionic radii of R³⁺ ions until Nd³⁺, the distortion of the fluorite unit cell increased. The compounds of R₃TaO₇ with much larger radii for RLa³⁺ and Pr³⁺, however, have a different distortion type in their unit cells. Therefore, two orthorhombic fluorite related superstructures are formed, of space group Cmcm for RLa and Pr, and C222₁ for Nd, Sm, Eu, Gd, Tb and Dy. The smaller ions such as Ho, Er, Yb and Y take the cubic fluorite structure.