Insulator-Metal Transition in Highly Compressed NiO

The insulator-metal transition was observed experimentally in nickel monoxide (NiO) at very high pressures of similar to 240 GPa. The sample resistance becomes measurable at about 130 GPa and decreases substantially with the pressure increase to similar to 240 GPa. A sharp drop in resistance by about 3 orders of magnitude has been observed at similar to 240 GPa with a concomitant change of the resistance type from semiconducting to metallic. This is the first experimental observation of an insulator-metal transition in NiO, which was anticipated by Mott decades ago. From simple multielectron consideration, the metallic phase of NiO forms when the effective Hubbard energy U-eff is almost equal to the estimated full bandwidth 2W.