Pressure and alloying effects on the metal to insulator transition in NiS(2-x)Se(x) studied by infrared spectroscopy

The metal to insulator transition in the charge-transfer NiS2-x Se x compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS2 lattice contraction and by Se alloying lattice expansion reveal that in both cases an anomalous metallic state is obtained. We find that optical results are not compatible with the linear Se-alloying vs pressure-scaling relation previously established through transport, thus pointing out the substantially different microscopic origin of the two transitions.