Rietveld analysis of structure transformation between perovskite-type and B-type rare earth structures

Abstract

  LaLnO₃ (Ln = Dy, Ho, Y, Er, and Yb) and La(Ln, Ln')O₃ (Ln, Ln' = Dy, Ho, Er, and Yb) systems were synthesized by solid state reaction method and characterized by X-ray diffraction and Rietveld analysis. In order to investigate the size effect of Ln site ion in ABO₃-type compound, the phase relationship in the LaLnO₃ system. When Ln = Er or Yb which has smaller ionic radius than that of Y³⁺ (0.900 Å), the LaLnO₃ showed an orthorhombic perovskite-type structure, while when Ln = Dy or Ho which has larger ionic radius than that of Y³⁺, it showed a monoclinic B-type rare earth structure. Next, the solid solution system of LaHo_xYb_1-xO₃ was investigated in order to clarify the crystallochemical factor affecting the structural transformation. The XRD experiments revealed that the samples with x = 0.90 (r_av.=0.8977Å) showed the orthorhombic perovskite-type structure, changed to the mixed phases of monoclinic B-type rare earth, and orthorhombic perovskite-type structures with increasing x, and then the samples with x ≧ 0.95 (r_av.=0.8994Å) showed the monoclinic B-type rare earth structures, where r_av. represents the average ionic radii of Ln and Ln'. Therefore, in order to search into process of phase transformation from orthorhombic perovskite-type structure to monoclinic B-type rare earth structure, LaHo_xYb_1-xO₃ (0.90≦x≦0.95) system was performed Rietveld analysis.