Combined Raman and infrared investigation of the insulator-to-metal transition in NiS(2−x)Se(x) compounds

Ambient-condition Raman spectra were collected in the strongly correlated NiS 1-xSe x pyrite (0≤x≤1.2). Two samples (x=0 and x=0.55) were studied as a function of pressure up to 10 GPa, and for the x=0.55 sample the pressure dependence of the infrared reflectivity was also measured (0-10 GPa). This gave a complete picture of the optical response of that system on approaching the metallic state both by application of pres-sure and/or by Se alloying, which corresponds to a volume expansion. A peculiar nonmonotonic (V-shaped) volume dependence was found for the quasiparticle spectral weight of both pure and Se-doped compounds. In the x=0.55 sample the vibrational frequencies of the chalcogen dimer show an anomalous volume dependence on entering the metallic phase. The abrupt softening observed, particularly significant for the Se-Se pair, indicates the relevant role of the softness of the Se-Se bond as previously suggested by theoretical calculations. © 2011 American Physical Society.