Pressure Effects on Transition-Metal Compounds near Insulator-Metal Phase Boundaries

A number of transition-metal compounds exhibit metal–insulator transitions under physical pressure as well as under chemical pressure. An overview is given on the effect of “pressure” on the metallic versus insulating behavior of transition-metal compounds including typical examples of Ni chalcogenides and perovskite-type transition-metal oxides. Systematic variations in the electronic structure of transition-metal compounds revealed by photoemission spectroscopy provide us with the ground to understand that behavior. Correspondence between physical pressure and chemical pressures is examined.