Pressure-induced metal-insulator transition in orthorhombic Nb₁₂O₂₉

In orthorhombic Nb₁₂O₂₉ reported as the first example of a magnetically ordered ( T_N = 12 K) system in niobium-based oxides, the metallic conduction even below T_N suggests that Nb₁₂O₂₉ could be an itinerant antiferromagnet. In order to investigate the pressure effects on these novel magnetic and metallic behaviors, we have carried out resistivity measurements up to 2.5 GPa. The resistivity-temperature curves show metallic behavior below P_c ∼ 1 GPa. On the other hand, when pressure is applied over ∼1 GPa, the resistivity increases with decreasing temperature from 300 K. These results indicate metal-insulator (MI) transition at P_c ∼ 1 GPa. We will discuss the origin of the MI transition in Nb₁₂O₂₉ in terms of charge ordering and pressure-induced amorphization as have been suggested in other niobium oxides.