Direct evidence for a low-frequency phonon mode mechanism in the negative thermal expansion compound ZrW2O8

Accurate lattice parameters have been determined between 2 K and 520 K for the negative thermal expansion compound ZrW2O8, by high-resolution neutron powder diffraction measurements. Two different analyses of the temperature dependence of the lattice constant i) in terms of Debye and Einstein contributions and ii) using MaxEnt techniques to reconstruct a Gruneisen-parameter-weighted phonon density of states both provide direct evidence that the negative thermal expansion behaviour is driven by low-frequency modes with an energy scale of 3-8 meV. Comparison with earlier specific heat measurements indicates that these rigid-unit modes are associated with very large Gruneisen parameters.