透過型電子顕微鏡によるLn_2Mn_xM_<2-x>O_7(Ln=rare earth or Y, M=Mo, Ta, Nb or W)の構造解析

概要

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Ln_2Mn_xM_<2-x>O_7 (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: Ho_2Mn_<2/3>Nb_<4/3>O_7 and Er_2Mn_<2/3>Mo_<4/3>O_7 possessed a monoclinic structure, while Ho_2Mn_<2/3>Ta_<4/3>O_7 and Ho_2MnTaO_7 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 Er_2MnWO_7, 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. [Received April 23, 2003; Accepted August 29, 2003]
2003-11-01