Raman scattering study of Ge and Sn compounds with type-I clathrate hydrate crystal structure

Raman scattering spectra of polycrystalline ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30},$ ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30},$ ${\mathrm{Cs}}_8{\mathrm{Ga}}_8{\mathrm{Sn}}_{38},$ and ${\mathrm{Cs}}_8{\mathrm{Zn}}_4{\mathrm{Sn}}_{37}{\mathrm{Ge}}_5,$ all with the type-I clathrate crystal structure, were studied at room temperature and 10 K. Several of the Raman-active vibrational modes in these compounds have been identified. The Ge and Sn clathrate spectra are similar, with the vibrational modes of the Sn clathrates shifting to lower frequencies as compared to Ge clathrates. Polarization measurements were used to identify the $A_g$ modes. The lower-frequency mode of the two Raman-active modes corresponding to the “rattling” vibrations of the “guest” atoms inside the tetrakaidecahedral “cages” have been identified in these compounds. This mode is at a frequency that is within the acoustic phonon branch of these compounds. The experimental data are compared to theoretical predictions.