Reflection spectra of YbS have been measured from 0.5 to 3.5 eV at pressures up to 400 kbar. The optical response indicates the onset of a 4f-shell instability near 100 kbar, which is further supported by lattice-parameter measurements. The optical reflectivity differs considerably from that of intermediate-valence (IV) compounds of Sm and Tm. A pronounced absorption band in the near-infrared spectral range, which is also observed in YbO, provides evidence for a localized d component in the IV ground state of both YbS and YbO.

• Received 1 April 1985

DOI:https://doi.org/10.1103/PhysRevB.32.8246