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

摘要

The metal to insulator transition in the charge-transfer NiS_(2-x)Se_x compound has been investigated through infrared reflectivity. Measurements performed by applying pressure to pure NiS_2 (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.