Synthesis and properties of complex oxides, Ln(2)Mn(2/3)Ta(4/3)O(7) phases (Ln equals rare earth or Y)

The complex oxides with a pyrochlore-related structure, Ln(2)Mn(2/3)Ta(4/3)O(7), have been synthesized by the usual ceramic method in a reducing atmosphere at 1523 K. The X-ray diffraction patterns of these compounds can be indexed by a hexagonal symmetry. The lattice parameters of the hexagonal crystal are linearly dependent on the ionic radii of Ln(3+). The chemical stability range of Ln(2)Mn(2/3)Ta(4/3)O(7) phases was determined by the isothermal thermogravimetric analysis at 1273 K. The temperature dependence of the magnetic susceptibility of Eu2Mn2/3Ta4/3O7 obeys the Curie-Weiss law in the temperature range above 77 K, and the valence numbers of rare earth, manganese and tantalum are 3(+), 2(+) and 5(+) respectively. The results of electrical conductivity exhibit a semiconducting property in the measured temperature range.