Pressure Induced Insulator–Metal Transition in Hexagonal BaTiO_3−δ

Abstract

The dc electrical resistivity ρ in the reduced hexagonal BaTiO_2.98 single crystal was measured under hydrostatic pressures up to 3.1 GPa in the temperature range of 5 K to 300 K. The ρ along the hexagonal c axis is somewhat larger than that of the perpendicular direction. The ρ vs T curve changes the behavior at the temperature T₀, where the hexagonal to orthorhombic phase transition takes place: it behaves as a semiconductor and a metal at temperatures below and above T₀, respectively. With increasing pressure T₀ decreases, and the metallic behavior dominates whole of the temperature range at pressures above p_c=1.9 GPa. In the pressure-induced metallic phase, T²-dependence of resistivity appears at low temperatures. This rule suggests that electrons in the Ti 3d band of BaTiO₃ are the same as electrons in a strongly correlated electron system. A band model is proposed to explain such insulator–metal transitions in SrTiO₃, cubic BaTiO₃, and hexagonal BaTiO₃.