  An infrared study of the phonon spectra of ZrW2O8 as a function of
temperature which includes the low energy (2-10 meV) region relevant to
negative thermal expansion is reported and discussed in the context of specific
heat and neutron density of states results. The prevalence of infrared active
phonons at low energy and their observed temperature dependence are highly
unusual and indicative of exotic low-energy lattice dynamics. Eigenvector
calculations indicate a mixing of librational and translational motion within
each low-frequency IR mode. The role of the underconstrained structure in
establishing the nature of these modes, and the relationship between the IR
spectra and the large negative thermal expansion in ZrW2O8 are discussed.

Materials Science