Phonons in GdS—Raman scattering of an fcc metal

Metallic, fcc GdS was investigated by means of Raman scattering. First- and second-order Raman scattering was observed and identified by the dependence on temperature and defect concentration. Particular consideration was given to the effect of sample stoichiometry on the Raman spectra. The symmetry-forbidden first-order scattering is found to be defect induced, in contrast to the defect-independent, allowed, second-order scattering. The observed defect-induced one-phonon density of states was assigned to acoustical and optical branches. Raman scattering of GdS provides information about the phonon spectrum which cannot be obtained from neutron scattering or infrared spectroscopy. The antiferromagnetic order of GdS is found to have no effect on the Raman scattering, which is attributed to the nonresonant excitation of the $4f^7$ states at about 7 eV below the Fermi level.