New insight into the symmetry and the structure of the double perovskites Ba2LnNbO6 (Ln=lanthanides and Y)

Structures of the double perovskites Ba2LnNbO6 (Ln=La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, and Y) at room temperature have been re-examined by Rietveld profile analysis of X-ray diffraction data. It was shown that the correct phase sequence across the lanthanides is I2/m (Ln=La, Pr, Nd, and Sm), I4/m (Ln=Eu, Gd, Tb, and Dy), and image (Ln=Ho and Y), respectively. All phases can be derived from the ideal cubic perovskite by ordering the Ln(III) and Nb(V) ions and by out-of-phase tilting the LnO6/NbO6 octahedra around either the primitive two-fold [110]p-axis (I2/m) or the four-fold [001]p-axis (I4/m). The monoclinic P21/n structure that contains both out-of-phase and in-phase tilt around the primitive [110]p- and [001]p-axis, respectively, has not been observed for this series of compounds.