Phase transition of BaNd2Mn2O7

Phase transition of BaNd 2 Mn 2 O 7 from orthorhombic (space group Fmmm ) to tetragonal phase ( I 4/ mmm ) was studied by high temperature powder X-ray diffractometry and Rietveld analysis. The transition temperature was identified at 523 K, which is almost the same transition temperature as the compounds with other rare earth ions in this BaLn 2 Mn 2 O 7 family (Ln = Sm and Eu) with Fmmm space group. During the transition an oxygen octahedron of each phase changes a little its form, in which four oxygen atoms perpendicular to c -axis make a rectangle and a square for orthorhombic and tetragonal phases, respectively. Manganese ion is not on the center of the quadrilateral consisting of these four oxygen ions, but a little apart from the center along c -axis in both phases.