Structure of Ln2MnxM2-xO7 (Ln = rare earth or Y, M = Mo, Ta, Nb or W) studied by transmission electron microscopy

Abstract

Ln2MnxM2-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: Ho2Mn2/3Nb4/3O7 and Er2Mn2/3Mo4/3O7 possessed a monoclinic structure, while Ho2Mn2/3Ta4/3O7 and Ho2MnTaO7 possessed a trigonal structure with an incommensurate 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.