Résumé
The vibrational properties of a series of binary as well as ternary aluminosilicate and borosilicate glasses have been investigated in the framework of Density Functional Theory. The vibrational density of states as well as the IR spectrum have been calculated for all systems and the vibrational modes have been assigned to specific atoms or structural units. It is shown that the modifier content as well as the content of the second network-former (B or Al) affect several vibrational features as the position and intensity of the R band, the mixing of the rocking and bending atomic motions of the Si-O-X bridges (X=Si, B, Al) or the vibrational modes of the main structural units (SiO4 , AlO4 , BO3 or BO4 ). For some compositions, the Raman spectra have been calculated as well and have been found to agree with experimental ones. Their decomposition indicates the dominant character of the non-bridging oxygen contribution, in particular for the high-frequency band, above 800 cm−1. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e. Qn units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple gaussians as frequently assumed in the treatment of experimental spectra.