One-Phonon Spectra and Effective Phonon Density-of-States in CaGa2S4

Room temperature Raman study of the crystalline and amorphous CaGa 2 S 4 was done. Stokes and Anti-Stokes spectra were obtained in back-scattering geometry using a high rejection double monochromator and the 5 14.5nm line of an argon ion laser. Except for polarization characteristics, the Raman spectra of the polycrystalline CaGa 2 S 4 represent one-phonon spectra at the center of the Brillouin zone. The spectra consist of 18 lines rather than 84 Raman-active lines given by the factor-group analysis. The distribution of the one-phonon density-of-states from ω min =74.7cm -1 to ω max =4 12.7cm -1 exhibits a frequency gap between 199.8 cm -1 and 284.7cm -1 . The effective phonon density-of-states across the Brillouin zone, which was derived from the Raman spectrum of the amorphous CaGa 2 S 4 , exhibits somewhat different distribution of the peak densities compared to one-phonon spectra. The upper limit for Debye approximation in CaGa 2 S 4 is found at 21.7 cm -1 . The applicability of the molecular approach to the Raman spectra of CaGa 2 S 4 is discussed. It is proposed that two different structural units generate the most intense 284.7cm -1 - and 360.7cm -1 -vibrations above the frequency gap.