Low-Dimensional and Extended Metal-Metal Bonded Networks in Transition Metal Compounds: Ba2Nb5O9, Ba2-xYxNb5O9

The crystal structure, oxidation state, electronic properties of unusual niobates which contain niobium clusters condensed low dimensionally, were investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), electrical resistivity measurements, and Raman spectroscopy. The results of XPS experiment indicate that there are three niobium oxidation states (Nb⁴⁺, Nb³⁺, Nb²⁺) in Ba₂Nb₅O₉ and these oxidation states are expected to be stable considering their atomic coordinates. Pure BaNbO₃ phase was synthesized and its niobium oxidation state was examined in order to compare with those of niobium in Ba₂Nb₅O₉. Electrical resistivity measurements on Ba₂Nb₅O₉ show negative dρ/dT behavior between 160 K and 600 K. Similar behavior was observed in yttrium substituted niobates, Ba_2-xY_xNb₅O₉. The possibility of relationship between low dimensionality and anomalous properties of these niobates is discussed.