Structure of Ln₂Mn_xM_2-xO₇ (Ln=rare earth or Y, M=Mo, Ta, Nb or W) Studied by Transmission Electron Microscopy

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: Ho₂Mn_2/3Nb_4/3O₇ and Er₂Mn_2/3Mo_4/3O₇ possessed a monoclinic structure, while Ho₂Mn_2/3Ta_4/3O₇ and Ho₂MnTaO₇ possessed a trigonal structure with an incommesurate nature. 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, where B cations formed a hexagonal tungsten bronze (HTB)-type layer that is a characteristic frame-layer in the pyrochlore-related structure. 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.