Raman investigation of orthorhombic MIIGa2(S, Se)4 compounds

Raman investigation of isostructural polycrystalline (MGa2)-Ga-II(S, Se)4 compounds with M-II = Ph, Sr, Eu, Yb and Ca was performed. We observed that the effect of M-II atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the M-II cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates (MGa2S4)-Ga-II exhibit nearly the same phonon energies: about 280 cm(-1) (similar to35 meV) and 360 cm(-1) (similar to45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic (MGa2Se4)-Ga-II compounds present phonon energy at about 185 cm(-1) (similar to23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.