Unusual Low-Energy Phonon Dynamics in the Negative Thermal Expansion Compound ZrW₂O₈

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 state 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.