Pressure induced insulator-metal transition in hexagonal BaTiO3-δ

The dc electrical resistivity rho in the reduced hexagonal BaTiO2.98 single crystal was measured under hydrostatic pressures up to 3.1 GPa in the temperature range of 5 K to 300 K. The p along the hexagonal c axis is somewhat larger than that of the perpendicular direction. The rho vs T curve changes the behavior at the temperature To, where the hexagonal to orthorhombic phase transition takes place: it behaves as a semiconductor and a metal at temperatures below and above T-0, respectively. With increasing pressure To 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, T-2-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.