Metal-insulator transition in NiS2-xSex: chemical vs external pressure effects

The Se alloying (x)- and the pressure (P)-induced metal-insulator transitions on the strongly correlated NiS2-xSex system have been investigated through Raman and infrared (IR) spectroscopies. Raman and IR responses of NiS2 to lattice compression are correlated to a metallization transition, occurring at 4GPa. This result suggests a strong interaction between lattice and electronic degrees of freedom. In particular, IR measurements carried out by applying P on NiS2 (i.e. lattice contraction) and on Se alloying (i.e. lattice expansion) reveal that in both cases a metallic state is obtained. Our optical spectroscopy results deviate from the idea of a simple scaling factor between P and x previously claimed by transport measurements, but, on the contrary, point out the substantially different microscopic origin of the two transitions.