We present the results of comparative inelastic neutron scattering mea
surements of both amorphous and crystalline silicates. We demonstrate
the presence of floppy modes in amorphous silica and potassium disilic
ate over the energy scale 0-5 meV. A peak is also observed at 5 meV in
amorphous silica, which is traditionally associated with the boson pe
ak. By comparison with the spectrum of crystalline alpha-cristobalite,
we show that it, in fact, arises from the transverse acoustic modes.
The same peak in amorphous potassium disilicate cannot be resolved bec
ause it is obscured by a large number of floppy modes.