Anomalous electronic structures of transition-metal oxides with hollandite-type crystal structure

We make the electronic structure calculations of transition-metal oxides with a hollandite-type crystal structure A₂M₈O₁₆ using the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction is taken into account (GGA+U). We first examine the electronic structure of the 3d series (M=Ti, V, Cr, Mn) as well as the 4d series (M=Mo, Ru, Rh) to discuss their generic features. We then discuss in particular the origins of the metal-insulator transition observed in K₂Cr₈O₁₆ and quasi-one-dimensional electron conduction observed in K₂Ru₈O₁₆. We also consider the observed metal-insulator transition in K₂V₈O₁₆ where the effect of electron correlations may play an essential role.