Synthesis and phase transition of the orthorhombic BaGd 2Mn 2O 7 phase

Abstract

Each of the single phases of orthorhombic and tetragonal BaGd 2Mn 2O 7 has been distinctly synthesized. The lattice parameters of the orthorhombic phase are a = 0.3921 ± 0.0005, b = 0.3899 ± 0.0005, and c = 2.016 ± 0.002 nm. The orthorhombic phase has an isostructure with BaTb 2Mn 2O 7 and a different distortion from another group of the BaLn 2Mn 2O 7 family (Ln = rare earth) such as Ln = Sm or Eu. Therefore two kinds of distortions of BaLn 2Mn 2O 7 are clearly classified. The tetragonal form has the same structure in all rare earth ions. The orthorhombic BaGd 2Mn 2O 7 has a phase transition at 663 K and 1.99 kJ mol −1 for the enthalpy of the transition, which was found by DSC measurement and is considered to be the same transition as in BaTb 2Mn 2O 7. The transition temperature increases with the atomic number of rare earth ions in two groups of orthorhombic structures of the BaLn 2Mn 2O 7 family. The formation of a single phase of orthorhombic BaGd 2Mn 2O 7 is very difficult and it is considered that the orthorhombic phase might be rather a kind of metastable one.