The boson peak of silicate glasses: the role of Si-O, Al-O, and Si-N bonds.

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(omega) has been calculated from inversion of literature data and new heat capacity measurements. As defined by g(omega)/omega(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.