Electronic properties of NiS2-xSex single crystals:: From magnetic Mott-Hubbard insulators to normal metals

The NiS2-xSex system represents one of the best examples of a Mott-Hubbard system, i.e. a system in which, under appropriate conditions of concentration, temperature, or pressure, a metal-insulator transition driven by electron-electron interaction takes place. Here, the metallic phase is either antiferromagnet;ic (for 0.44 less than or equal to x less than or equal to 1) or paramagnetic (for x greater than or equal to 1), whereas the insulating phase is as a rule antiferromagnetic (including the spin-canted phases). In this paper we review both the physical properties and outline the basic features of the theoretical approach to those correlated electron systems. Emphasis is placed on a qualitative understanding of the observed transformation of the system from semiconductor (or magnetic insulator) to metal.