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.
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