The role of Si-O, Al-O, and Si-N bonds on the boson peak of silicate glasses has been investigated from a study of amorphous Si, SiO_2, and two calcium aluminosilicates with 0 (Ca28-O) and 4.4 (Ca28-N) mol % Si _3 N_4. The low-frequency part of the vibrational density of states g (ω) has been calculated from inversion of literature data and new heat capacity measurements. As defined by g (ω) / ω~2, the boson peak correlates with the excess heat capacity observed with respect to Debye T~3 limiting law. That libration of SiO_4 tetrahedra represents the main source of low-frequency excitations in silica glass is illustrated by the strong difference between the anomalies of amorphous Si and SiO_2 glass and the marked decrease observed for SiO _2 phases of increasing density. When Al substitutes for Si, libration of AlO_4 tetrahedra appears hampered by the presence of a charge-compensating cation. Rigidification of the silicate network resulting from substitution of N for O causes the boson peak of Ca28-N to be smaller than that of Ca28-O and shifted toward higher frequencies as increased cross-linking hinders libration of SiO_4 or AlO_4 tetrahedra. In agreement with their universal phenomenology, the calorimetric boson anomalies of Ca28-O and Ca28-N plot on the master curve defined previously by SiO_2 and alkali silicate glasses.
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