Structural analysis of Ln₂Mn_xM_2-xO₇(Ln = rare earth,M = Mo,Ta, Nb or W) by transmission electron microscope

Abstract

Ln₂Mn_xM_2-xO₇ (Ln = rare earth or Y, M = Mo, Ta, Nb or W) was synthesized under ordinary pressure and various atmospheres with the expectation of achieving various symmetries and properties. These compounds are analyzed as similar structures of rhombohedrally distorted pyrochlore structures by X-ray diffraction (XRD). However, transmission electron microscope (TEM) images reveal differences between structures: Ln₂Mn_2/3Nb_4/3O₇ (Ln = Ho, Eu) and Er₂Mn_2/3Mo_4/3O₇possessed a monoclinic structure, while Ho₂MnTaO₇ possessed a trigonal structure. The differences in the structures were caused by the stacking sequence along the c-direction of a pair of A cation and B cation layers. In the case of Er₂MnWO₇, however, different types of structures with a trigonal cell show that no pair of A- cation and B- cation layers existed along the c-direction in the crystal.