Structural, magnetic, and transport properties of a novel hollandite-type molybdenum oxide, Rb 1.5 Mo 8 O 16 have been investigated. Whereas a typical hollandite structure is characterized by uniform zigzag ladder chains of edge-shared MO 6 (M=transition metal) octahedra, the zigzag ladders in Rb 1.5 Mo 8 O 16 are strongly distorted, and consequently, "Mo 4 " clusters composed of four edge-shared MoO 6 octahedra are formed. A semiquantitative band calculation based on an extended Hückel method shows that the band structure assumes the characteristics of the molecular orbitals of an isolated Mo 4 cluster. The magnetic susceptibility reveals the existence of spins spread over each Mo 4 cluster rather than localized on each Mo atom. The electrical resistivity and the thermoelectric power indicate that the electric conduction is governed by cluster-to-cluster electron hopping. Rb 1.5 Mo 8 O 16 undergoes a phase transtion at T 1 = 208 K, accompanied by a change in the number of spins and carriers. X-ray diffractions at low temperatures detected a slight breakdown of the tetragonal symmetry below T 1 . The transport and the magnetic properties of related compounds, K 2 Mo 8 O 16 and Ba 1.14 Mo 8 O 16 are also reported.
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Ozawa et al. (2006) studied this question.
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