LAMB-MOSSBAUER FACTORS AS A LOCAL PROBE OF FLOPPY MODES IN NETWORK GLASSES

The temperature dependence of the Lamb-Mossbauer factor, f(T), in a solid provides the first inverse, [1/omega], and second inverse, [1/omega(2)], moments of the vibrational density of states. In network glasses, these moments serve as local probes of low-frequency vibrational excitations, such as floppy modes, and provide a means to establish the rigidity percolation threshold. Lamb-Mossbauer results on prototypical chalcogenide glasses (GexSe1-x) correlate well with those of Raman scattering, inelastic neutron scattering and Mossbauer hyperfine structure experiments in indicating that the rigidity percolation threshold occurs near [r](c) = 2.46(4). These observations provide experimental support for predictions of the Phillips-Thorpe constraint theory, when provision is made far a small but finite concentration of broken bond-bending constraints around chalcogen sites.